sed, a stream editor
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This file documents version 4.1.4 of
GNU sed, a stream editor.
   
Copyright © 1998, 1999, 2001, 2002, 2003, 2004 Free
Software Foundation, Inc.
   
This document is released under the terms of the GNU Free
Documentation License as published by the Free Software Foundation;
either version 1.1, or (at your option) any later version.
   
You should have received a copy of the GNU Free Documentation
License along with 
GNU sed; see the file COPYING.DOC. 
If not, write to the Free Software Foundation, 59 Temple Place - Suite
330, Boston, MA 02111-1307, USA.
   
There are no Cover Texts and no Invariant Sections; this text, along
with its equivalent in the printed manual, constitutes the Title Page.
#IntroductionIntroduction :                Introduction
#Invoking-sedInvoking sed :                Invocation
#sed-Programssed Programs :                sed programs
#ExamplesExamples :                    Some sample scripts
#LimitationsLimitations :                 Limitations and (non-)limitations of GNU sed #Other-ResourcesOther Resources :             Other resources for learning about sed #Reporting-BugsReporting Bugs :              Reporting bugs
#Extended-regexpsExtended regexps :            egrep-style regular expressions
#Concept-IndexConcept Index :               A menu with all the topics in this manual. 
#Command-and-Option-IndexCommand and Option Index :    A menu with all sed commands and
                               command-line options.
--- The detailed node listing ---
sed Programs:
#Execution-CycleExecution Cycle :                  How sed works
#AddressesAddresses :                        Selecting lines with sed #Regular-ExpressionsRegular Expressions :              Overview of regular expression syntax
#Common-CommandsCommon Commands :                  Often used commands
#The-_0022s_0022-CommandThe "s" Command :                  sed's Swiss Army Knife
#Other-CommandsOther Commands :                   Less frequently used commands
#Programming-CommandsProgramming Commands :             Commands for sed gurus
#Extended-CommandsExtended Commands :                Commands specific of GNU sed #EscapesEscapes :                          Specifying special characters
Examples:
#Centering-linesCentering lines #Increment-a-numberIncrement a number #Rename-files-to-lower-caseRename files to lower case #Print-bash-environmentPrint bash environment #Reverse-chars-of-linesReverse chars of lines #tactac :                              Reverse lines of files
#cat-_002dncat -n :                           Numbering lines
#cat-_002dbcat -b :                           Numbering non-blank lines
#wc-_002dcwc -c :                            Counting chars
#wc-_002dwwc -w :                            Counting words
#wc-_002dlwc -l :                            Counting lines
#headhead :                             Printing the first lines
#tailtail :                             Printing the last lines
#uniquniq :                             Make duplicate lines unique
#uniq-_002dduniq -d :                          Print duplicated lines of input
#uniq-_002duuniq -u :                          Remove all duplicated lines
#cat-_002dscat -s :                           Squeezing blank lines
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#TopTop 1 Introduction
sed is a stream editor. 
A stream editor is used to perform basic text
transformations on an input stream
(a file or input from a pipeline). 
While in some ways similar to an editor which
permits scripted edits (such as 
ed),
sed works by making only one pass over the
input(s), and is consequently more efficient. 
But it is 
sed's ability to filter text in a pipeline
which particularly distinguishes it from other types of
editors.
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#TopTop 2 Invocation
Normally sed is invoked like this:
     sed SCRIPT INPUTFILE...
   
The full format for invoking sed is:
     sed OPTIONS... [SCRIPT] [INPUTFILE...]
   
If you do not specify INPUTFILE, or if INPUTFILE is -,
sed filters the contents of the standard input.  The scriptis actually the first non-option parameter, which 
sed specially
considers a script and not an input file if (and only if) none of the
other 
options specifies a script to be executed, that is if neither
of the 
-e and -f options is specified.
   
sed may be invoked with the following command-line options:
     
--version Print out the version of sed that is being run and a copyright notice,
then exit.
     
--help Print a usage message briefly summarizing these command-line options
and the bug-reporting address,
then exit.
     
-n--quiet--silent By default, sed prints out the pattern space
at the end of each cycle through the script. 
These options disable this automatic printing,
and 
sed only produces output when explicitly told to
via the 
p command.
     
-i[SUFFIX]--in-place[=SUFFIX] This option specifies that files are to be edited in-place. 
GNU sed does this by creating a temporary file and
sending output to this file rather than to the standard
output.
#fn-11 .
     
This option implies -s.
     
When the end of the file is reached, the temporary file is
renamed to the output file's original name.  The extension,
if supplied, is used to modify the name of the old file
before renaming the temporary file, thereby making a backup
copy
#fn-22 ).
     
This rule is followed: if the extension doesn't contain a *,
then it is appended to the end of the current filename as a
suffix; if the extension does contain one or more 
*characters, then 
each asterisk is replaced with the
current filename.  This allows you to add a prefix to the
backup file, instead of (or in addition to) a suffix, or
even to place backup copies of the original files into another
directory (provided the directory already exists).
     
If no extension is supplied, the original file is
overwritten without making a backup.
     
-l N--line-length=N Specify the default line-wrap length for the l command. 
A length of 0 (zero) means to never wrap long lines.  If
not specified, it is taken to be 70.
     
--posix GNU sed includes several extensions to POSIXsed.  In order to simplify writing portable scripts, this
option disables all the extensions that this manual documents,
including additional commands. 
Most of the extensions accept sed programs that
are outside the syntax mandated by 
POSIX, but some
of them (such as the behavior of the 
N command
described in see 
#Reporting-BugsReporting Bugs ) actually violate the
standard.  If you want to disable only the latter kind of
extension, you can set the 
POSIXLY_CORRECT variable
to a non-empty value.
     
-r--regexp-extended Use extended regular expressions rather than basic
regular expressions.  Extended regexps are those that
egrep accepts; they can be clearer because they
usually have less backslashes, but are a 
GNU extension
and hence scripts that use them are not portable. 
See 
#Extended-regexpsExtended regular expressions .
     
-s--separate By default, sed will consider the files specified on the
command line as a single continuous long stream.  This 
GNU sedextension allows the user to consider them as separate files:
range addresses (such as `
/abc/,/def/') are not allowed
to span several files, line numbers are relative to the start
of each file, 
$ refers to the last line of each file,
and files invoked from the 
R commands are rewound at the
start of each file.
     
-u--unbuffered Buffer both input and output as minimally as practical. 
(This is particularly useful if the input is coming from
the likes of `
tail -f', and you wish to see the transformed
output as soon as possible.)
     
-e script--expression=script Add the commands in script to the set of commands to be
run while processing the input.
     
-f script-file--file=script-file Add the commands contained in the file script-fileto the set of commands to be run while processing the input.
   
   
If no -e, -f, --expression, or --fileoptions are given on the command-line,
then the first non-option argument on the command line is
taken to be the 
script to be executed.
   
If any command-line parameters remain after processing the above,
these parameters are interpreted as the names of input files to
be processed. 
A file name of `-' refers to the standard input stream. 
The standard input will be processed if no file names are specified.
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#TopTop 3 sed Programs
A sed program consists of one or more sed commands,
passed in by one or more of the
-e, -f, --expression, and --fileoptions, or the first non-option argument if zero of these
options are used. 
This document will refer to “the” 
sed script;
this is understood to mean the in-order catenation
of all of the 
scripts and script-files passed in.
   
Each sed command consists of an optional address or
address range, followed by a one-character command name
and any additional command-specific code.
#Execution-CycleExecution Cycle :           How sed works
#AddressesAddresses :                 Selecting lines with sed #Regular-ExpressionsRegular Expressions :       Overview of regular expression syntax
#Common-CommandsCommon Commands :           Often used commands
#The-_0022s_0022-CommandThe "s" Command :           sed's Swiss Army Knife
#Other-CommandsOther Commands :            Less frequently used commands
#Programming-CommandsProgramming Commands :      Commands for sed gurus
#Extended-CommandsExtended Commands :         Commands specific of GNU sed #EscapesEscapes :                   Specifying special characters
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#sed-Programssed Programs 3.1 How sed Works
sed maintains two data buffers: the active pattern space,
and the auxiliary 
hold space. Both are initially empty.
   
sed operates by performing the following cycle on each
lines of input: first, 
sed reads one line from the input
stream, removes any trailing newline, and places it in the pattern space. 
Then commands are executed; each command can have an address associated
to it: addresses are a kind of condition code, and a command is only
executed if the condition is verified before the command is to be
executed.
   
When the end of the script is reached, unless the -n option
is in use, the contents of pattern space are printed out to the output
stream, adding back the trailing newline if it was removed.
#fn-33  Then the next cycle starts for the next
input line.
   
Unless special commands (like `D') are used, the pattern space is
deleted between two cycles. The hold space, on the other hand, keeps
its data between cycles (see commands `
h', `H', `x',
`
g', `G' to move data between both buffers).
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#sed-Programssed Programs 3.2 Selecting lines with sed
Addresses in a 
sed script can be in any of the following forms:
     
number Specifying a line number will match only that line in the input. 
(Note that 
sed counts lines continuously across all input files
unless 
-i or -s options are specified.)
     
first~step This GNU extension matches every stepth line
starting with line 
first. 
In particular, lines will be selected when there exists
a non-negative 
n such that the current line-number equals
first + (n * step). 
Thus, to select the odd-numbered lines,
one would use 
1~2;
to pick every third line starting with the second, `
2~3' would be used;
to pick every fifth line starting with the tenth, use `
10~5';
and `
50~0' is just an obscure way of saying 50.
     
$ This address matches the last line of the last file of input, or
the last line of each file when the 
-i or -s options
are specified.
     
/regexp/ This will select any line which matches the regular expression regexp. 
If 
regexp itself includes any / characters,
each must be escaped by a backslash (
\).
     
The empty regular expression `//' repeats the last regular
expression match (the same holds if the empty regular expression is
passed to the 
s command).  Note that modifiers to regular expressions
are evaluated when the regular expression is compiled, thus it is invalid to
specify them together with the empty regular expression.
     
\%regexp%(The % may be replaced by any other single character.)
     
This also matches the regular expression regexp,
but allows one to use a different delimiter than 
/. 
This is particularly useful if the 
regexp itself contains
a lot of slashes, since it avoids the tedious escaping of every 
/. 
If 
regexp itself includes any delimiter characters,
each must be escaped by a backslash (
\).
     
/regexp/I\%regexp%I The I modifier to regular-expression matching is a GNUextension which causes the 
regexp to be matched in
a case-insensitive manner.
     
/regexp/M\%regexp%M The M modifier to regular-expression matching is a GNU sedextension which causes 
^ and $ to match respectively
(in addition to the normal behavior) the empty string after a newline,
and the empty string before a newline.  There are special character
sequences
(
\` and \')
which always match the beginning or the end of the buffer. 
M stands for multi-line.
   
   
If no addresses are given, then all lines are matched;
if one address is given, then only lines matching that
address are matched.
   
An address range can be specified by specifying two addresses
separated by a comma (
,).  An address range matches lines
starting from where the first address matches, and continues
until the second address matches (inclusively).
   
If the second address is a regexp, then checking for the
ending match will start with the line 
following the
line which matched the first address: a range will always
span at least two lines (except of course if the input stream
ends).
   
If the second address is a number less than (or equal to)
the line matching the first address, then only the one line is
matched.
   
GNU sed also supports some special two-address forms; all these
are 
GNU extensions:
     
0,/regexp/A line number of 0 can be used in an address specification like
0,/regexp/ so that sed will try to match
regexp in the first input line too.  In other words,
0,/regexp/ is similar to 1,/regexp/,
except that if 
addr2 matches the very first line of input the
0,/regexp/ form will consider it to end the range, whereas
the 
1,/regexp/ form will match the beginning of its range and
hence make the range span up to the 
second occurrence of the
regular expression.
     
Note that this is the only place where the 0 address makes
sense; there is no 0-th line and commands which are given the 
0address in any other way will give an error.
     
addr1,+NMatches addr1 and the N lines following addr1.
     
addr1,~NMatches addr1 and the lines following addr1until the next line whose input line number is a multiple of 
N. 
   
Appending the ! character to the end of an address
specification negates the sense of the match. 
That is, if the 
! character follows an address range,
then only lines which do 
not match the address range
will be selected. 
This also works for singleton addresses,
and, perhaps perversely, for the null address.
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#sed-Programssed Programs 3.3 Overview of Regular Expression Syntax
To know how to use sed, people should understand regular
expressions (
regexp for short).  A regular expression
is a pattern that is matched against a
subject string from left to right.  Most characters are
ordinary: they stand for
themselves in a pattern, and match the corresponding characters
in the subject.  As a trivial example, the pattern
          The quick brown fox
   
matches a portion of a subject string that is identical to
itself.  The power of regular expressions comes from the
ability to include alternatives and repetitions in the pattern. 
These are encoded in the pattern by the use of 
special characters,
which do not stand for themselves but instead
are interpreted in some special way.  Here is a brief description
of regular expression syntax as used in 
sed.
     
charA single ordinary character matches itself.
     
* Matches a sequence of zero or more instances of matches for the
preceding regular expression, which must be an ordinary character, a
special character preceded by 
\, a ., a grouped regexp
(see below), or a bracket expression.  As a 
GNU extension, a
postfixed regular expression can also be followed by 
*; for
example, 
a** is equivalent to a*.  POSIX1003.1-2001 says that 
* stands for itself when it appears at
the start of a regular expression or subexpression, but many
non
GNU implementations do not support this and portable
scripts should instead use 
\* in these contexts.
     
\+ As *, but matches one or more.  It is a GNU extension.
     
\? As *, but only matches zero or one.  It is a GNU extension.
     
\{i\}As *, but matches exactly i sequences (i is a
decimal integer; for portability, keep it between 0 and 255
inclusive).
     
\{i,j\}Matches between i and j, inclusive, sequences.
     
\{i,\}Matches more than or equal to i sequences.
     
\(regexp\)Groups the inner regexp as a whole, this is used to:
          
Apply postfix operators, like \(abcd\)*:
this will search for zero or more whole sequences
of `
abcd', while abcd* would search
for `
abc' followed by zero or more occurrences
of `
d'.  Note that support for \(abcd\)* is
required by 
POSIX 1003.1-2001, but many non-GNUimplementations do not support it and hence it is not universally
portable.
          
Use back references (see below). 
     
.Matches any character, including newline.
     
^Matches the null string at beginning of line, i.e. what
appears after the circumflex must appear at the
beginning of line. 
^#include will match only
lines where `
#include' is the first thing on line—if
there are spaces before, for example, the match fails. 
^ acts as a special character only at the beginning
of the regular expression or subexpression (that is,
after 
\( or \|).  Portable scripts should avoid
^ at the beginning of a subexpression, though, as
POSIX allows implementations that treat ^ as
an ordinary character in that context.
     
$It is the same as ^, but refers to end of line. 
$ also acts as a special character only at the end
of the regular expression or subexpression (that is, before 
\)or 
\|), and its use at the end of a subexpression is not
portable.
     
[list][^list]Matches any single character in list: for example,
[aeiou] matches all vowels.  A list may include
sequences like 
char1-char2, which
matches any character between (inclusive) 
char1and 
char2.
     
A leading ^ reverses the meaning of list, so that
it matches any single character 
not in list.  To include
] in the list, make it the first character (after
the 
^ if needed), to include - in the list,
make it the first or last; to include 
^ put
it after the first character.
     
The characters $, *, ., [, and \are normally not special within 
list.  For example, [\*]matches either `
\' or `*', because the \ is not
special here.  However, strings like 
[.ch.], [=a=], and
[:space:] are special within list and represent collating
symbols, equivalence classes, and character classes, respectively, and
[ is therefore special within list when it is followed by
., =, or :.  Also, when not in
POSIXLY_CORRECT mode, special escapes like \n and
\t are recognized within list.  See #EscapesEscapes .
     
regexp1\|regexp2 Matches either regexp1 or regexp2.  Use
parentheses to use complex alternative regular expressions. 
The matching process tries each alternative in turn, from
left to right, and the first one that succeeds is used. 
It is a 
GNU extension.
     
regexp1regexp2Matches the concatenation of regexp1 and regexp2. 
Concatenation binds more tightly than 
\|, ^, and
$, but less tightly than the other regular expression
operators.
     
\digitMatches the digit-th \(...\) parenthesized
subexpression in the regular expression.  This is called a 
back
reference
.  Subexpressions are implicity numbered by counting
occurrences of 
\( left-to-right.
     
\nMatches the newline character.
     
\charMatches char, where char is one of $,
*, ., [, \, or ^. 
Note that the only C-like
backslash sequences that you can portably assume to be
interpreted are 
\n and \\; in particular
\t is not portable, and matches a `t' under most
implementations of 
sed, rather than a tab character.
   
   
Note that the regular expression matcher is greedy, i.e., matches
are attempted from left to right and, if two or more matches are
possible starting at the same character, it selects the longest.
Examples:
     
`abcdef'Matches `abcdef'.
     
`a*b'Matches zero or more `a's followed by a single
`
b'.  For example, `b' or `aaaaab'.
     
`a\?b'Matches `b' or `ab'.
     
`a\+b\+'Matches one or more `a's followed by one or more
`
b's: `ab' is the shortest possible match, but
other examples are `
aaaab' or `abbbbb' or
`
aaaaaabbbbbbb'.
     
`.*'`.\+'These two both match all the characters in a string;
however, the first matches every string (including the empty
string), while the second matches only strings containing
at least one character.
     
`^main.*(.*)'his matches a string starting with `main',
followed by an opening and closing
parenthesis.  The `
n', `(' and `)' need not
be adjacent.
     
`^#'This matches a string beginning with `#'.
     
`\\$'This matches a string ending with a single backslash.  The
regexp contains two backslashes for escaping.
     
`\$'Instead, this matches a string consisting of a single dollar sign,
because it is escaped.
     
`[a-zA-Z0-9]'In the C locale, this matches any ASCII letters or digits.
     
`[^ tab]\+'(Here tab stands for a single tab character.) 
This matches a string of one or more
characters, none of which is a space or a tab. 
Usually this means a word.
     
`^\(.*\)\n\1$'This matches a string consisting of two equal substrings separated by
a newline.
     
`.\{9\}A$'This matches nine characters followed by an `A'.
     
`^.\{15\}A'This matches the start of a string that contains 16 characters,
the last of which is an `
A'.
   
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#sed-Programssed Programs 3.4 Often-Used Commands
If you use sed at all, you will quite likely want to know
these commands.
     
#[No addresses allowed.]
     
The # character begins a comment;
the comment continues until the next newline.
     
If you are concerned about portability, be aware that
some implementations of 
sed (which are not posixconformant) may only support a single one-line comment,
and then only when the very first character of the script is a 
#.
     
Warning: if the first two characters of the sed script
are 
#n, then the -n (no-autoprint) option is forced. 
If you want to put a comment in the first line of your script
and that comment begins with the letter `
n'
and you do not want this behavior,
then be sure to either use a capital `
N',
or place at least one space before the `
n'.
     
q [exit-code]This command only accepts a single address.
     
Exit sed without processing any more commands or input. 
Note that the current pattern space is printed if auto-print is
not disabled with the 
-n options.  The ability to return
an exit code from the 
sed script is a GNU sed extension.
     
d Delete the pattern space;
immediately start next cycle.
     
p Print out the pattern space (to the standard output). 
This command is usually only used in conjunction with the 
-ncommand-line option.
     
n If auto-print is not disabled, print the pattern space,
then, regardless, replace the pattern space with the next line of input. 
If there is no more input then 
sed exits without processing
any more commands.
     
{ commands } A group of commands may be enclosed between
{ and } characters. 
This is particularly useful when you want a group of commands
to be triggered by a single address (or address-range) match.
   
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#sed-Programssed Programs 3.5 The s Command
The syntax of the s (as in substitute) command is
`
s/regexp/replacement/flags'.  The /characters may be uniformly replaced by any other single
character within any given 
s command.  The /character (or whatever other character is used in its stead)
can appear in the 
regexp or replacementonly if it is preceded by a 
\ character.
   
The s command is probably the most important in sedand has a lot of different options.  Its basic concept is simple:
the 
s command attempts to match the pattern
space against the supplied 
regexp; if the match is
successful, then that portion of the pattern
space which was matched is replaced with 
replacement.
   
The replacement can contain \n (n being
a number from 1 to 9, inclusive) references, which refer to
the portion of the match which is contained between the 
nth
\( and its matching \). 
Also, the 
replacement can contain unescaped &characters which reference the whole matched portion
of the pattern space. 
Finally, as a GNU sed extension, you can include a
special sequence made of a backslash and one of the letters
L, l, U, u, or E. 
The meaning is as follows:
     
\LTurn the replacement
to lowercase until a 
\U or \E is found,
     
\lTurn the
next character to lowercase,
     
\UTurn the replacement to uppercase
until a 
\L or \E is found,
     
\uTurn the next character
to uppercase,
     
\EStop case conversion started by \L or \U. 
   
To include a literal \, &, or newline in the final
replacement, be sure to precede the desired 
\, &,
or newline in the 
replacement with a \.
   
The s command can be followed by zero or more of the
following 
flags:
     
g Apply the replacement to all matches to the regexp,
not just the first.
     
number Only replace the numberth match of the regexp.
     
Note: the posix standard does not specify what should happen
when you mix the 
g and number modifiers,
and currently there is no widely agreed upon meaning
across 
sed implementations. 
For 
GNU sed, the interaction is defined to be:
ignore matches before the 
numberth,
and then match and replace all matches from
the 
numberth on.
     
p If the substitution was made, then print the new pattern space.
     
Note: when both the p and e options are specified,
the relative ordering of the two produces very different results. 
In general, 
ep (evaluate then print) is what you want,
but operating the other way round can be useful for debugging. 
For this reason, the current version of 
GNU sed interprets
specially the presence of 
p options both before and after
e, printing the pattern space before and after evaluation,
while in general flags for the 
s command show their
effect just once.  This behavior, although documented, might
change in future versions.
     
w file-name If the substitution was made, then write out the result to the named file. 
As a 
GNU sed extension, two special values of file-name are
supported: 
/dev/stderr, which writes the result to the standard
error, and 
/dev/stdout, which writes to the standard
output.
#fn-44      
e This command allows one to pipe input from a shell command
into pattern space.  If a substitution was made, the command
that is found in pattern space is executed and pattern space
is replaced with its output.  A trailing newline is suppressed;
results are undefined if the command to be executed contains
a 
nul character.  This is a GNU sed extension.
     
Ii The I modifier to regular-expression matching is a GNUextension which makes 
sed match regexp in a
case-insensitive manner.
     
Mm The M modifier to regular-expression matching is a GNU sedextension which causes 
^ and $ to match respectively
(in addition to the normal behavior) the empty string after a newline,
and the empty string before a newline.  There are special character
sequences
(
\` and \')
which always match the beginning or the end of the buffer. 
M stands for multi-line.
   
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#sed-Programssed Programs 3.6 Less Frequently-Used Commands
Though perhaps less frequently used than those in the previous
section, some very small yet useful 
sed scripts can be built with
these commands.
     
y/source-chars/dest-chars/(The / characters may be uniformly replaced by
any other single character within any given 
y command.)
     
Transliterate any characters in the pattern space which match
any of the 
source-chars with the corresponding character
in 
dest-chars.
     
Instances of the / (or whatever other character is used in its stead),
\, or newlines can appear in the source-chars or dest-charslists, provide that each instance is escaped by a 
\. 
The 
source-chars and dest-chars lists mustcontain the same number of characters (after de-escaping).
     
a\text As a GNU extension, this command accepts two addresses.
     
Queue the lines of text which follow this command
(each but the last ending with a 
\,
which are removed from the output)
to be output at the end of the current cycle,
or when the next input line is read.
     
Escape sequences in text are processed, so you should
use 
\\ in text to print a single backslash.
     
As a GNU extension, if between the a and the newline there is
other than a whitespace-
\ sequence, then the text of this line,
starting at the first non-whitespace character after the 
a,
is taken as the first line of the 
text block. 
(This enables a simplification in scripting a one-line add.) 
This extension also works with the 
i and c commands.
     
i\text As a GNU extension, this command accepts two addresses.
     
Immediately output the lines of text which follow this command
(each but the last ending with a 
\,
which are removed from the output).
     
c\text Delete the lines matching the address or address-range,
and output the lines of text which follow this command
(each but the last ending with a 
\,
which are removed from the output)
in place of the last line
(or in place of each line, if no addresses were specified). 
A new cycle is started after this command is done,
since the pattern space will have been deleted.
     
= As a GNU extension, this command accepts two addresses.
     
Print out the current input line number (with a trailing newline).
     
l n Print the pattern space in an unambiguous form:
non-printable characters (and the 
\ character)
are printed in C-style escaped form; long lines are split,
with a trailing 
\ character to indicate the split;
the end of each line is marked with a 
$.
     
n specifies the desired line-wrap length;
a length of 0 (zero) means to never wrap long lines.  If omitted,
the default as specified on the command line is used.  The 
nparameter is a 
GNU sed extension.
     
r filename As a GNU extension, this command accepts two addresses.
     
Queue the contents of filename to be read and
inserted into the output stream at the end of the current cycle,
or when the next input line is read. 
Note that if 
filename cannot be read, it is treated as
if it were an empty file, without any error indication.
     
As a GNU sed extension, the special value /dev/stdinis supported for the file name, which reads the contents of the
standard input.
     
w filename Write the pattern space to filename. 
As a 
GNU sed extension, two special values of file-name are
supported: 
/dev/stderr, which writes the result to the standard
error, and 
/dev/stdout, which writes to the standard
output.
#fn-55      
The file will be created (or truncated) before the
first input line is read; all 
w commands
(including instances of 
w flag on successful s commands)
which refer to the same 
filename are output without
closing and reopening the file.
     
D Delete text in the pattern space up to the first newline. 
If any text is left, restart cycle with the resultant
pattern space (without reading a new line of input),
otherwise start a normal new cycle.
     
N Add a newline to the pattern space,
then append the next line of input to the pattern space. 
If there is no more input then 
sed exits without processing
any more commands.
     
P Print out the portion of the pattern space up to the first newline.
     
h Replace the contents of the hold space with the contents of the pattern space.
     
H Append a newline to the contents of the hold space,
and then append the contents of the pattern space to that of the hold space.
     
g Replace the contents of the pattern space with the contents of the hold space.
     
G Append a newline to the contents of the pattern space,
and then append the contents of the hold space to that of the pattern space.
     
x Exchange the contents of the hold and pattern spaces.
   
Next: 
#Extended-CommandsExtended Commands ,
Previous: 
#Other-CommandsOther Commands ,
Up: 
#sed-Programssed Programs 3.7 Commands for sed gurus
In most cases, use of these commands indicates that you are
probably better off programming in something like 
awkor Perl.  But occasionally one is committed to sticking
with 
sed, and these commands can enable one to write
quite convoluted scripts.
   
     
: label[No addresses allowed.]
     
Specify the location of label for branch commands. 
In all other respects, a no-op.
     
b label Unconditionally branch to label. 
The 
label may be omitted, in which case the next cycle is started.
     
t label Branch to label only if there has been a successful substitution
since the last input line was read or conditional branch was taken. 
The 
label may be omitted, in which case the next cycle is started.
   
Next: 
#EscapesEscapes ,
Previous: 
#Programming-CommandsProgramming Commands ,
Up: 
#sed-Programssed Programs 3.8 Commands Specific to GNU sed
These commands are specific to GNU sed, so you
must use them with care and only when you are sure that
hindering portability is not evil.  They allow you to check
for 
GNU sed extensions or to do tasks that are required
quite often, yet are unsupported by standard 
seds.
     
e [command] This command allows one to pipe input from a shell command
into pattern space.  Without parameters, the 
e command
executes the command that is found in pattern space and
replaces the pattern space with the output; a trailing newline
is suppressed.
     
If a parameter is specified, instead, the e command
interprets it as a command and sends its output to the output stream
(like 
r does).  The command can run across multiple
lines, all but the last ending with a back-slash.
     
In both cases, the results are undefined if the command to be
executed contains a 
nul character.
     
L n This GNU sed extension fills and joins lines in pattern space
to produce output lines of (at most) 
n characters, like
fmt does; if n is omitted, the default as specified
on the command line is used.  This command is considered a failed
experiment and unless there is enough request (which seems unlikely)
will be removed in future versions.
     
Q [exit-code]This command only accepts a single address.
     
This command is the same as q, but will not print the
contents of pattern space.  Like 
q, it provides the
ability to return an exit code to the caller.
     
This command can be useful because the only alternative ways
to accomplish this apparently trivial function are to use
the 
-n option (which can unnecessarily complicate
your script) or resorting to the following snippet, which
wastes time by reading the whole file without any visible effect:
     
          :eat
          $d       
Quit silently on the last line          N        
Read another line, silently          g        
Overwrite pattern space each time to save memory          b eat
     
     
R filename Queue a line of filename to be read and
inserted into the output stream at the end of the current cycle,
or when the next input line is read. 
Note that if 
filename cannot be read, or if its end is
reached, no line is appended, without any error indication.
     
As with the r command, the special value /dev/stdinis supported for the file name, which reads a line from the
standard input.
     
T label Branch to label only if there have been no successful
substitutions since the last input line was read or
conditional branch was taken. The 
label may be omitted,
in which case the next cycle is started.
     
v version This command does nothing, but makes sed fail if
GNU sed extensions are not supported, simply because other
versions of 
sed do not implement it.  In addition, you
can specify the version of 
sed that your script
requires, such as 
4.0.5.  The default is 4.0because that is the first version that implemented this command.
     
This command enables all GNU extensions even if
POSIXLY_CORRECT is set in the environment.
     
W filename Write to the given filename the portion of the pattern space up to
the first newline.  Everything said under the 
w command about
file handling holds here too. 
Previous: 
#Extended-CommandsExtended Commands ,
Up: 
#sed-Programssed Programs 3.9 GNU Extensions for Escapes in Regular Expressions
Until this chapter, we have only encountered escapes of the form
`
\^', which tell sed not to interpret the circumflex
as a special character, but rather to take it literally.  For
example, `
\*' matches a single asterisk rather than zero
or more backslashes.
   
This chapter introduces another kind of escape #fn-66 —that
is, escapes that are applied to a character or sequence of characters
that ordinarily are taken literally, and that 
sed replaces
with a special character.  This provides a way
of encoding non-printable characters in patterns in a visible manner. 
There is no restriction on the appearance of non-printing characters
in a 
sed script but when a script is being prepared in the
shell or by text editing, it is usually easier to use one of
the following escape sequences than the binary character it
represents:
   
The list of these escapes is:
     
\aProduces or matches a bel character, that is an “alert” (ascii 7).
     
\fProduces or matches a form feed (ascii 12).
     
\nProduces or matches a newline (ascii 10).
     
\rProduces or matches a carriage return (ascii 13).
     
\tProduces or matches a horizontal tab (ascii 9).
     
\vProduces or matches a so called “vertical tab” (ascii 11).
     
\cxProduces or matches Control-x, where x is
any character.  The precise effect of `
\cx' is as follows:
if 
x is a lower case letter, it is converted to upper case. 
Then bit 6 of the character (hex 40) is inverted.  Thus `
\cz' becomes
hex 1A, but `
\c{' becomes hex 3B, while `\c;' becomes hex 7B.
     
\dxxxProduces or matches a character whose decimal ascii value is xxx.
     
\oxxxProduces or matches a character whose octal ascii value is xxx.
     
\xxxProduces or matches a character whose hexadecimal ascii value is xx. 
   
`\b' (backspace) was omitted because of the conflict with
the existing “word boundary” meaning.
   
Other escapes match a particular character class and are valid only in
regular expressions:
     
\wMatches any “word” character.  A “word” character is any
letter or digit or the underscore character.
     
\WMatches any “non-word” character.
     
\bMatches a word boundary; that is it matches if the character
to the left is a “word” character and the character to the
right is a “non-word” character, or vice-versa.
     
\BMatches everywhere but on a word boundary; that is it matches
if the character to the left and the character to the right
are either both “word” characters or both “non-word”
characters.
     
\`Matches only at the start of pattern space.  This is different
from 
^ in multi-line mode.
     
\'Matches only at the end of pattern space.  This is different
from 
$ in multi-line mode.
   
Next: 
#LimitationsLimitations ,
Previous: 
#sed-Programssed Programs ,
Up: 
#TopTop 4 Some Sample Scripts
Here are some sed scripts to guide you in the art of mastering
sed.
Some exotic examples:
#Centering-linesCentering lines #Increment-a-numberIncrement a number #Rename-files-to-lower-caseRename files to lower case #Print-bash-environmentPrint bash environment #Reverse-chars-of-linesReverse chars of lines Emulating standard utilities:
#tactac :                              Reverse lines of files
#cat-_002dncat -n :                           Numbering lines
#cat-_002dbcat -b :                           Numbering non-blank lines
#wc-_002dcwc -c :                            Counting chars
#wc-_002dwwc -w :                            Counting words
#wc-_002dlwc -l :                            Counting lines
#headhead :                             Printing the first lines
#tailtail :                             Printing the last lines
#uniquniq :                             Make duplicate lines unique
#uniq-_002dduniq -d :                          Print duplicated lines of input
#uniq-_002duuniq -u :                          Remove all duplicated lines
#cat-_002dscat -s :                           Squeezing blank lines
Next: 
#Increment-a-numberIncrement a number ,
Up: 
#ExamplesExamples 4.1 Centering Lines
This script centers all lines of a file on a 80 columns width. 
To change that width, the number in 
\{...\} must be
replaced, and the number of added spaces also must be changed.
   
Note how the buffer commands are used to separate parts in
the regular expressions to be matched—this is a common
technique.
     #!/usr/bin/sed -f
     
     # Put 80 spaces in the buffer
     1 {
       x
       s/^$/          /
       s/^.*$/&&&&&&&&/
       x
     }
     
     # del leading and trailing spaces
     y/
tab/ /
     s/^ *//
     s/ *$//
     
     # add a newline and 80 spaces to end of line
     G
     
     # keep first 81 chars (80 + a newline)
     s/^\(.\{81\}\).*$/\1/
     
     # \2 matches half of the spaces, which are moved to the beginning
     s/^\(.*\)\n\(.*\)\2/\2\1/
   
Next: 
#Rename-files-to-lower-caseRename files to lower case ,
Previous: 
#Centering-linesCentering lines ,
Up: 
#ExamplesExamples 4.2 Increment a Number
This script is one of a few that demonstrate how to do arithmetic
in 
sed.  This is indeed possible, #fn-77  but must be done manually.
   
To increment one number you just add 1 to last digit, replacing
it by the following digit.  There is one exception: when the digit
is a nine the previous digits must be also incremented until you
don't have a nine.
   
This solution by Bruno Haible is very clever and smart because
it uses a single buffer; if you don't have this limitation, the
algorithm used in 
#cat-_002dnNumbering lines , is faster. 
It works by replacing trailing nines with an underscore, then
using multiple 
s commands to increment the last digit,
and then again substituting underscores with zeros.
     #!/usr/bin/sed -f
     
     /[^0-9]/ d
     
     # replace all leading 9s by _ (any other character except digits, could
     # be used)
     :d
     s/9\(_*\)$/_\1/
     td
     
     # incr last digit only.  The first line adds a most-significant
     # digit of 1 if we have to add a digit.
     #
     # The 
tn commands are not necessary, but make the thing
     # faster
     
     s/^\(_*\)$/1\1/; tn
     s/8\(_*\)$/9\1/; tn
     s/7\(_*\)$/8\1/; tn
     s/6\(_*\)$/7\1/; tn
     s/5\(_*\)$/6\1/; tn
     s/4\(_*\)$/5\1/; tn
     s/3\(_*\)$/4\1/; tn
     s/2\(_*\)$/3\1/; tn
     s/1\(_*\)$/2\1/; tn
     s/0\(_*\)$/1\1/; tn
     
     :n
     y/_/0/
   
Next: 
#Print-bash-environmentPrint bash environment ,
Previous: 
#Increment-a-numberIncrement a number ,
Up: 
#ExamplesExamples 4.3 Rename Files to Lower Case
This is a pretty strange use of sed.  We transform text, and
transform it to be shell commands, then just feed them to shell. 
Don't worry, even worse hacks are done when using 
sed; I have
seen a script converting the output of 
date into a bcprogram!
   
The main body of this is the sed script, which remaps the name
from lower to upper (or vice-versa) and even checks out
if the remapped name is the same as the original name. 
Note how the script is parameterized using shell
variables and proper quoting.
     #! /bin/sh
     # rename files to lower/upper case...
     #
     # usage:
     #    move-to-lower *
     #    move-to-upper *
     # or
     #    move-to-lower -R .
     #    move-to-upper -R .
     #
     
     help()
     {
     	cat << eof
     Usage: $0 [-n] [-r] [-h] files...
     
     -n      do nothing, only see what would be done
     -R      recursive (use find)
     -h      this message
     files   files to remap to lower case
     
     Examples:
            $0 -n *        (see if everything is ok, then...)
            $0 *
     
            $0 -R .
     
     eof
     }
     
     apply_cmd='sh'
     finder='echo "$@" | tr " " "\n"'
     files_only=
     
     while :
     do
         case "$1" in
             -n) apply_cmd='cat' ;;
             -R) finder='find "$@" -type f';;
             -h) help ; exit 1 ;;
             *) break ;;
         esac
         shift
     done
     
     if [ -z "$1" ]; then
             echo Usage: $0 [-h] [-n] [-r] files...
             exit 1
     fi
     
     LOWER='abcdefghijklmnopqrstuvwxyz'
     UPPER='ABCDEFGHIJKLMNOPQRSTUVWXYZ'
     
     case `basename $0` in
             *upper*) TO=$UPPER; FROM=$LOWER ;;
             *)       FROM=$UPPER; TO=$LOWER ;;
     esac
     
     eval $finder | sed -n '
     
     # remove all trailing slashes
     s/\/*$//
     
     # add ./ if there is no path, only a filename
     /\//! s/^/.\//
     
     # save path+filename
     h
     
     # remove path
     s/.*\///
     
     # do conversion only on filename
     y/'$FROM'/'$TO'/
     
     # now line contains original path+file, while
     # hold space contains the new filename
     x
     
     # add converted file name to line, which now contains
     # path/file-name\nconverted-file-name
     G
     
     # check if converted file name is equal to original file name,
     # if it is, do not print nothing
     /^.*\/\(.*\)\n\1/b
     
     # now, transform path/fromfile\n, into
     # mv path/fromfile path/tofile and print it
     s/^\(.*\/\)\(.*\)\n\(.*\)$/mv \1\2 \1\3/p
     
     ' | $apply_cmd
   
Next: 
#Reverse-chars-of-linesReverse chars of lines ,
Previous: 
#Rename-files-to-lower-caseRename files to lower case ,
Up: 
#ExamplesExamples 4.4 Print bash Environment
This script strips the definition of the shell functions
from the output of the 
set Bourne-shell command.
     #!/bin/sh
     
     set | sed -n '
     :x
     
     # if no occurrence of `
=()' print and load next line
     /=()/! { p; b; }
     / () $/! { p; b; }
     
     # possible start of functions section
     # save the line in case this is a var like FOO="() "
     h
     
     # if the next line has a brace, we quit because
     # nothing comes after functions
     n
     /^{/ q
     
     # print the old line
     x; p
     
     # work on the new line now
     x; bx
     '
   
Next: 
#tactac ,
Previous: 
#Print-bash-environmentPrint bash environment ,
Up: 
#ExamplesExamples 4.5 Reverse Characters of Lines
This script can be used to reverse the position of characters
in lines.  The technique moves two characters at a time, hence
it is faster than more intuitive implementations.
   
Note the tx command before the definition of the label. 
This is often needed to reset the flag that is tested by
the 
t command.
   
Imaginative readers will find uses for this script.  An example
is reversing the output of 
banner. #fn-88      #!/usr/bin/sed -f
     
     /../! b
     
     # Reverse a line.  Begin embedding the line between two newlines
     s/^.*$/\
     &\
     /
     
     # Move first character at the end.  The regexp matches until
     # there are zero or one characters between the markers
     tx
     :x
     s/\(\n.\)\(.*\)\(.\n\)/\3\2\1/
     tx
     
     # Remove the newline markers
     s/\n//g
   
Next: 
#cat-_002dncat -n ,
Previous: 
#Reverse-chars-of-linesReverse chars of lines ,
Up: 
#ExamplesExamples 4.6 Reverse Lines of Files
This one begins a series of totally useless (yet interesting)
scripts emulating various Unix commands.  This, in particular,
is a 
tac workalike.
   
Note that on implementations other than GNU sedthis script might easily overflow internal buffers.
     #!/usr/bin/sed -nf
     
     # reverse all lines of input, i.e. first line became last, ...
     
     # from the second line, the buffer (which contains all previous lines)
     # is *appended* to current line, so, the order will be reversed
     1! G
     
     # on the last line we're done -- print everything
     $ p
     
     # store everything on the buffer again
     h
   
Next: 
#cat-_002dbcat -b ,
Previous: 
#tactac ,
Up: 
#ExamplesExamples 4.7 Numbering Lines
This script replaces `cat -n'; in fact it formats its output
exactly like 
GNU cat does.
   
Of course this is completely useless and for two reasons:  first,
because somebody else did it in C, second, because the following
Bourne-shell script could be used for the same purpose and would
be much faster:
     #! /bin/sh
     sed -e "=" $@ | sed -e '
       s/^/      /
       N
       s/^ *\(......\)\n/\1  /
     '
   
It uses sed to print the line number, then groups lines two
by two using 
N.  Of course, this script does not teach as much as
the one presented below.
   
The algorithm used for incrementing uses both buffers, so the line
is printed as soon as possible and then discarded.  The number
is split so that changing digits go in a buffer and unchanged ones go
in the other; the changed digits are modified in a single step
(using a 
y command).  The line number for the next line
is then composed and stored in the hold space, to be used in the
next iteration.
     #!/usr/bin/sed -nf
     
     # Prime the pump on the first line
     x
     /^$/ s/^.*$/1/
     
     # Add the correct line number before the pattern
     G
     h
     
     # Format it and print it
     s/^/      /
     s/^ *\(......\)\n/\1  /p
     
     # Get the line number from hold space; add a zero
     # if we're going to add a digit on the next line
     g
     s/\n.*$//
     /^9*$/ s/^/0/
     
     # separate changing/unchanged digits with an x
     s/.9*$/x&/
     
     # keep changing digits in hold space
     h
     s/^.*x//
     y/0123456789/1234567890/
     x
     
     # keep unchanged digits in pattern space
     s/x.*$//
     
     # compose the new number, remove the newline implicitly added by G
     G
     s/\n//
     h
   
Next: 
#wc-_002dcwc -c ,
Previous: 
#cat-_002dncat -n ,
Up: 
#ExamplesExamples 4.8 Numbering Non-blank Lines
Emulating `cat -b' is almost the same as `cat -n'—we only
have to select which lines are to be numbered and which are not.
   
The part that is common to this script and the previous one is
not commented to show how important it is to comment 
sedscripts properly...
     #!/usr/bin/sed -nf
     
     /^$/ {
       p
       b
     }
     
     # Same as cat -n from now
     x
     /^$/ s/^.*$/1/
     G
     h
     s/^/      /
     s/^ *\(......\)\n/\1  /p
     x
     s/\n.*$//
     /^9*$/ s/^/0/
     s/.9*$/x&/
     h
     s/^.*x//
     y/0123456789/1234567890/
     x
     s/x.*$//
     G
     s/\n//
     h
   
Next: 
#wc-_002dwwc -w ,
Previous: 
#cat-_002dbcat -b ,
Up: 
#ExamplesExamples 4.9 Counting Characters
This script shows another way to do arithmetic with sed. 
In this case we have to add possibly large numbers, so implementing
this by successive increments would not be feasible (and possibly
even more complicated to contrive than this script).
   
The approach is to map numbers to letters, kind of an abacus
implemented with 
sed.  `a's are units, `b's are
tenths and so on: we simply add the number of characters
on the current line as units, and then propagate the carry
to tenths, hundredths, and so on.
   
As usual, running totals are kept in hold space.
   
On the last line, we convert the abacus form back to decimal. 
For the sake of variety, this is done with a loop rather than
with some 80 
s commands #fn-99 : first we
convert units, removing `
a's from the number; then we
rotate letters so that tenths become `
a's, and so on
until no more letters remain.
     #!/usr/bin/sed -nf
     
     # Add n+1 a's to hold space (+1 is for the newline)
     s/./a/g
     H
     x
     s/\n/a/
     
     # Do the carry.  The t's and b's are not necessary,
     # but they do speed up the thing
     t a
     : a;  s/aaaaaaaaaa/b/g; t b; b done
     : b;  s/bbbbbbbbbb/c/g; t c; b done
     : c;  s/cccccccccc/d/g; t d; b done
     : d;  s/dddddddddd/e/g; t e; b done
     : e;  s/eeeeeeeeee/f/g; t f; b done
     : f;  s/ffffffffff/g/g; t g; b done
     : g;  s/gggggggggg/h/g; t h; b done
     : h;  s/hhhhhhhhhh//g
     
     : done
     $! {
       h
       b
     }
     
     # On the last line, convert back to decimal
     
     : loop
     /a/! s/[b-h]*/&0/
     s/aaaaaaaaa/9/
     s/aaaaaaaa/8/
     s/aaaaaaa/7/
     s/aaaaaa/6/
     s/aaaaa/5/
     s/aaaa/4/
     s/aaa/3/
     s/aa/2/
     s/a/1/
     
     : next
     y/bcdefgh/abcdefg/
     /[a-h]/ b loop
     p
   
Next: 
#wc-_002dlwc -l ,
Previous: 
#wc-_002dcwc -c ,
Up: 
#ExamplesExamples 4.10 Counting Words
This script is almost the same as the previous one, once each
of the words on the line is converted to a single `
a'
(in the previous script each letter was changed to an `
a').
   
It is interesting that real wc programs have optimized
loops for `
wc -c', so they are much slower at counting
words rather than characters.  This script's bottleneck,
instead, is arithmetic, and hence the word-counting one
is faster (it has to manage smaller numbers).
   
Again, the common parts are not commented to show the importance
of commenting 
sed scripts.
     #!/usr/bin/sed -nf
     
     # Convert words to a's
     s/[ 
tab][ tab]*/ /g
     s/^/ /
     s/ [^ ][^ ]*/a /g
     s/ //g
     
     # Append them to hold space
     H
     x
     s/\n//
     
     # From here on it is the same as in wc -c.
     /aaaaaaaaaa/! bx;   s/aaaaaaaaaa/b/g
     /bbbbbbbbbb/! bx;   s/bbbbbbbbbb/c/g
     /cccccccccc/! bx;   s/cccccccccc/d/g
     /dddddddddd/! bx;   s/dddddddddd/e/g
     /eeeeeeeeee/! bx;   s/eeeeeeeeee/f/g
     /ffffffffff/! bx;   s/ffffffffff/g/g
     /gggggggggg/! bx;   s/gggggggggg/h/g
     s/hhhhhhhhhh//g
     :x
     $! { h; b; }
     :y
     /a/! s/[b-h]*/&0/
     s/aaaaaaaaa/9/
     s/aaaaaaaa/8/
     s/aaaaaaa/7/
     s/aaaaaa/6/
     s/aaaaa/5/
     s/aaaa/4/
     s/aaa/3/
     s/aa/2/
     s/a/1/
     y/bcdefgh/abcdefg/
     /[a-h]/ by
     p
   
Next: 
#headhead ,
Previous: 
#wc-_002dwwc -w ,
Up: 
#ExamplesExamples 4.11 Counting Lines
No strange things are done now, because sed gives us
`
wc -l' functionality for free!!! Look:
     #!/usr/bin/sed -nf
     $=
   
Next: 
#tailtail ,
Previous: 
#wc-_002dlwc -l ,
Up: 
#ExamplesExamples 4.12 Printing the First Lines
This script is probably the simplest useful sed script. 
It displays the first 10 lines of input; the number of displayed
lines is right before the 
q command.
     #!/usr/bin/sed -f
     10q
   
Next: 
#uniquniq ,
Previous: 
#headhead ,
Up: 
#ExamplesExamples 4.13 Printing the Last Lines
Printing the last n lines rather than the first is more complex
but indeed possible.  
n is encoded in the second line, before
the bang character.
   
This script is similar to the tac script in that it keeps the
final output in the hold space and prints it at the end:
     #!/usr/bin/sed -nf
     
     1! {; H; g; }
     1,10 !s/[^\n]*\n//
     $p
     h
   
Mainly, the scripts keeps a window of 10 lines and slides it
by adding a line and deleting the oldest (the substitution command
on the second line works like a 
D command but does not
restart the loop).
   
The “sliding window” technique is a very powerful way to write
efficient and complex 
sed scripts, because commands like
P would require a lot of work if implemented manually.
   
To introduce the technique, which is fully demonstrated in the
rest of this chapter and is based on the 
N, Pand 
D commands, here is an implementation of tailusing a simple “sliding window.”
   
This looks complicated but in fact the working is the same as
the last script: after we have kicked in the appropriate number
of lines, however, we stop using the hold space to keep inter-line
state, and instead use 
N and D to slide pattern
space by one line:
     #!/usr/bin/sed -f
     
     1h
     2,10 {; H; g; }
     $q
     1,9d
     N
     D
   
Next: 
#uniq-_002dduniq -d ,
Previous: 
#tailtail ,
Up: 
#ExamplesExamples 4.14 Make Duplicate Lines Unique
This is an example of the art of using the N, Pand 
D commands, probably the most difficult to master.
     #!/usr/bin/sed -f
     h
     
     :b
     # On the last line, print and exit
     $b
     N
     /^\(.*\)\n\1$/ {
         # The two lines are identical.  Undo the effect of
         # the n command.
         g
         bb
     }
     
     # If the 
N command had added the last line, print and exit
     $b
     
     # The lines are different; print the first and go
     # back working on the second.
     P
     D
   
As you can see, we mantain a 2-line window using P and D. 
This technique is often used in advanced 
sed scripts.
Next: 
#uniq-_002duuniq -u ,
Previous: 
#uniquniq ,
Up: 
#ExamplesExamples 4.15 Print Duplicated Lines of Input
This script prints only duplicated lines, like `uniq -d'.
     #!/usr/bin/sed -nf
     
     $b
     N
     /^\(.*\)\n\1$/ {
         # Print the first of the duplicated lines
         s/.*\n//
         p
     
         # Loop until we get a different line
         :b
         $b
         N
         /^\(.*\)\n\1$/ {
             s/.*\n//
             bb
         }
     }
     
     # The last line cannot be followed by duplicates
     $b
     
     # Found a different one.  Leave it alone in the pattern space
     # and go back to the top, hunting its duplicates
     D
   
Next: 
#cat-_002dscat -s ,
Previous: 
#uniq-_002dduniq -d ,
Up: 
#ExamplesExamples 4.16 Remove All Duplicated Lines
This script prints only unique lines, like `uniq -u'.
     #!/usr/bin/sed -f
     
     # Search for a duplicate line --- until that, print what you find.
     $b
     N
     /^\(.*\)\n\1$/ ! {
         P
         D
     }
     
     :c
     # Got two equal lines in pattern space.  At the
     # end of the file we simply exit
     $d
     
     # Else, we keep reading lines with 
N until we
     # find a different one
     s/.*\n//
     N
     /^\(.*\)\n\1$/ {
         bc
     }
     
     # Remove the last instance of the duplicate line
     # and go back to the top
     D
   
Previous: 
#uniq-_002duuniq -u ,
Up: 
#ExamplesExamples 4.17 Squeezing Blank Lines
As a final example, here are three scripts, of increasing complexity
and speed, that implement the same function as `
cat -s', that is
squeezing blank lines.
   
The first leaves a blank line at the beginning and end if there are
some already.
     #!/usr/bin/sed -f
     
     # on empty lines, join with next
     # Note there is a star in the regexp
     :x
     /^\n*$/ {
     N
     bx
     }
     
     # now, squeeze all '\n', this can be also done by:
     # s/^\(\n\)*/\1/
     s/\n*/\
     /
   
This one is a bit more complex and removes all empty lines
at the beginning.  It does leave a single blank line at end
if one was there.
     #!/usr/bin/sed -f
     
     # delete all leading empty lines
     1,/^./{
     /./!d
     }
     
     # on an empty line we remove it and all the following
     # empty lines, but one
     :x
     /./!{
     N
     s/^\n$//
     tx
     }
   
This removes leading and trailing blank lines.  It is also the
fastest.  Note that loops are completely done with 
n and
b, without exploting the fact that sed cycles back
to the top of the script automatically at the end of a line.
     #!/usr/bin/sed -nf
     
     # delete all (leading) blanks
     /./!d
     
     # get here: so there is a non empty
     :x
     # print it
     p
     # get next
     n
     # got chars? print it again, etc...
     /./bx
     
     # no, don't have chars: got an empty line
     :z
     # get next, if last line we finish here so no trailing
     # empty lines are written
     n
     # also empty? then ignore it, and get next... this will
     # remove ALL empty lines
     /./!bz
     
     # all empty lines were deleted/ignored, but we have a non empty.  As
     # what we want to do is to squeeze, insert a blank line artificially
     i\
     
     bx
   
Next: 
#Other-ResourcesOther Resources ,
Previous: 
#ExamplesExamples ,
Up: 
#TopTop 5 GNU sed's Limitations and Non-limitations
For those who want to write portable sed scripts,
be aware that some implementations have been known to
limit line lengths (for the pattern and hold spaces)
to be no more than 4000 bytes. 
The 
posix standard specifies that conforming sedimplementations shall support at least 8192 byte line lengths. 
GNU sed has no built-in limit on line length;
as long as it can 
malloc() more (virtual) memory,
you can feed or construct lines as long as you like.
   
However, recursion is used to handle subpatterns and indefinite
repetition.  This means that the available stack space may limit
the size of the buffer that can be processed by certain patterns.
Next: 
#Reporting-BugsReporting Bugs ,
Previous: 
#LimitationsLimitations ,
Up: 
#TopTop 6 Other Resources for Learning About sed
In addition to several books that have been written about sed(either specifically or as chapters in books which discuss
shell programming), one can find out more about 
sed(including suggestions of a few books) from the FAQ
for the 
sed-users mailing list, available from any of:
      http://www.student.northpark.edu/pemente/sed/sedfaq.htmlhttp://www.student.northpark.edu/pemente/sed/sedfaq.html       
http://sed.sf.net/grabbag/tutorials/sedfaq.htmlhttp://sed.sf.net/grabbag/tutorials/sedfaq.html    
Also of interest are
http://www.student.northpark.edu/pemente/sed/index.htmhttp://www.student.northpark.edu/pemente/sed/index.htm and 
http://sed.sf.net/grabbaghttp://sed.sf.net/grabbag ,
which include 
sed tutorials and other sed-related goodies.
   
The sed-users mailing list itself maintained by Sven Guckes. 
To subscribe, visit 
http://groups.yahoo.comhttp://groups.yahoo.com  and search
for the 
sed-users mailing list.
Next: 
#Extended-regexpsExtended regexps ,
Previous: 
#Other-ResourcesOther Resources ,
Up: 
#TopTop 7 Reporting Bugs
Email bug reports to mailto:bonzini@gnu.orgbonzini@gnu.org . 
Be sure to include the word “sed” somewhere in the 
Subject: field. 
Also, please include the output of `
sed --version' in the body
of your report if at all possible.
   
Please do not send a bug report like this:
     while building frobme-1.3.4     $ configure
     error--> sed: file sedscr line 1: Unknown option to 's'
   
If GNU sed doesn't configure your favorite package, take a
few extra minutes to identify the specific problem and make a stand-alone
test case.  Unlike other programs such as C compilers, making such test
cases for 
sed is quite simple.
   
A stand-alone test case includes all the data necessary to perform the
test, and the specific invocation of 
sed that causes the problem. 
The smaller a stand-alone test case is, the better.  A test case should
not involve something as far removed from 
sed as “try to configure
frobme-1.3.4”.  Yes, that is in principle enough information to look
for the bug, but that is not a very practical prospect.
   
Here are a few commonly reported bugs that are not bugs.
     
N command on the last line Most versions of 
sed exit without printing anything when
the 
N command is issued on the last line of a file. 
GNU sed prints pattern space before exiting unless of course
the 
-n command switch has been specified.  This choice is
by design.
     
For example, the behavior of
     
          sed N foo bar
     
     
would depend on whether foo has an even or an odd number of
lines
#fn-1010 .  Or, when writing a script to read the
next few lines following a pattern match, traditional
implementations of 
sed would force you to write
something like
     
          /foo/{ $!N; $!N; $!N; $!N; $!N; $!N; $!N; $!N; $!N }
     
     
instead of just
     
          /foo/{ N;N;N;N;N;N;N;N;N; }
     
     
In any case, the simplest workaround is to use $d;N in
scripts that rely on the traditional behavior, or to set
the 
POSIXLY_CORRECT variable to a non-empty value.
     
Regex syntax clashes (problems with backslashes) sed uses the posix basic regular expression syntax.  According to
the standard, the meaning of some escape sequences is undefined in
this syntax;  notable in the case of 
sed are \|,
\+, \?, \`, \', \<,
\>, \b, \B, \w, and \W.
     
As in all GNU programs that use posix basic regular
expressions, 
sed interprets these escape sequences as special
characters.  So, 
x\+ matches one or more occurrences of `x'. 
abc\|def matches either `abc' or `def'.
     
This syntax may cause problems when running scripts written for other
seds.  Some sed programs have been written with the
assumption that 
\| and \+ match the literal characters
| and +.  Such scripts must be modified by removing the
spurious backslashes if they are to be used with modern implementations
of 
sed, like
GNU sed.
     
On the other hand, some scripts use s|abc\|def||g to remove occurrences
of 
either abc or def.  While this worked until
sed 4.0.x, newer versions interpret this as removing the
string 
abc|def.  This is again undefined behavior according to
POSIX, and this interpretation is arguably more robust: older
seds, for example, required that the regex matcher parsed
\/ as / in the common case of escaping a slash, which is
again undefined behavior; the new behavior avoids this, and this is good
because the regex matcher is only partially under our control.
     
In addition, this version of sed supports several escape characters
(some of which are multi-character) to insert non-printable characters
in scripts (
\a, \c, \d, \o, \r,
\t, \v, \x).  These can cause similar problems
with scripts written for other 
seds.
     
-i clobbers read-only files In short, `
sed -i' will let you delete the contents of
a read-only file, and in general the 
-i option
(see 
#Invoking-sedInvocation ) lets you clobber
protected files.  This is not a bug, but rather a consequence
of how the Unix filesystem works.
     
The permissions on a file say what can happen to the data
in that file, while the permissions on a directory say what can
happen to the list of files in that directory.  `
sed -i'
will not ever open for writing  a file that is already on disk. 
Rather, it will work on a temporary file that is finally renamed
to the original name: if you rename or delete files, you're actually
modifying the contents of the directory, so the operation depends on
the permissions of the directory, not of the file.  For this same
reason, 
sed does not let you use -i on a writeable file
in a read-only directory (but unbelievably nobody reports that as a
bug
...).
     
0a does not work (gives an error)There is no line 0.  0 is a special address that is only used to treat
addresses like 
0,/RE/ as active when the script starts: if
you write 
1,/abc/d and the first line includes the word `abc',
then that match would be ignored because address ranges must span at least
two lines (barring the end of the file); but what you probably wanted is
to delete every line up to the first one including `
abc', and this
is obtained with 
0,/abc/d.
     
[a-z] is case insensitiveYou are encountering problems with locales.  POSIX mandates that [a-z]uses the current locale's collation order – in C parlance, that means using
strcoll(3) instead of strcmp(3).  Some locales have a
case-insensitive collation order, others don't: one of those that have
problems is Estonian.
     
Another problem is that [a-z] tries to use collation symbols. 
This only happens if you are on the 
GNU system, using
GNU libc's regular expression matcher instead of compiling the
one supplied with 
GNU sed.  In a Danish locale, for example,
the regular expression 
^[a-z]$ matches the string `aa',
because this is a single collating symbol that comes after `
a'
and before `
b'; `ll' behaves similarly in Spanish
locales, or `
ij' in Dutch locales.
     
To work around these problems, which may cause bugs in shell scripts, set
the 
LC_COLLATE and LC_CTYPE environment variables to `C'. 
Next: 
#Concept-IndexConcept Index ,
Previous: 
#Reporting-BugsReporting Bugs ,
Up: 
#TopTop Appendix A Extended regular expressions
The only difference between basic and extended regular expressions is in
the behavior of a few characters: `
?', `+', parentheses,
and braces (`
{}').  While basic regular expressions require
these to be escaped if you want them to behave as special characters,
when using extended regular expressions you must escape them if
you want them 
to match a literal character.
Examples:
     
abc?becomes `abc\?' when using extended regular expressions.  It matches
the literal string `
abc?'.
     
c\+becomes `c+' when using extended regular expressions.  It matches
one or more `
c's.
     
a\{3,\}becomes `a{3,}' when using extended regular expressions.  It matches
three or more `
a's.
     
\(abc\)\{2,3\}becomes `(abc){2,3}' when using extended regular expressions.  It
matches either `
abcabc' or `abcabcabc'.
     
\(abc*\)\1becomes `(abc*)\1' when using extended regular expressions. 
Backreferences must still be escaped when using extended regular
expressions. 
Next: 
#Command-and-Option-IndexCommand and Option Index ,
Previous: 
#Extended-regexpsExtended regexps ,
Up: 
#TopTop Concept Index
This is a general index of all issues discussed in this manual, with the
exception of the 
sed commands and command-line options.
#index-Additional-reading-about-_0040command_007bsed_007d-213Additional reading about sed : #Other-ResourcesOther Resources #index-g_t_0040var_007baddr1_007d_002c_002bN-63addr1,+N : #AddressesAddresses #index-g_t_0040var_007baddr1_007d_002c_007eN-64addr1,~N : #AddressesAddresses #index-Address_002c-as-a-regular-expression-51Address, as a regular expression : #AddressesAddresses #index-Address_002c-last-line-48Address, last line : #AddressesAddresses #index-Address_002c-numeric-45Address, numeric : #AddressesAddresses #index-Addresses_002c-in-_0040command_007bsed_007d-scripts-42Addresses, in sed scripts : #AddressesAddresses #index-Append-hold-space-to-pattern-space-166Append hold space to pattern space : #Other-CommandsOther Commands #index-Append-next-input-line-to-pattern-space-151Append next input line to pattern space : #Other-CommandsOther Commands #index-Append-pattern-space-to-hold-space-159Append pattern space to hold space : #Other-CommandsOther Commands #index-Appending-text-after-a-line-122Appending text after a line : #Other-CommandsOther Commands #index-Backreferences_002c-in-regular-expressions-97Backreferences, in regular expressions : #The-_0022s_0022-CommandThe "s" Command #index-Branch-to-a-label_002c-if-_0040code_007bs_002f_002f_002f_007d-failed-201Branch to a label, if s/// failed : #Extended-CommandsExtended Commands #index-Branch-to-a-label_002c-if-_0040code_007bs_002f_002f_002f_007d-succeeded-178Branch to a label, if s/// succeeded : #Programming-CommandsProgramming Commands #index-Branch-to-a-label_002c-unconditionally-175Branch to a label, unconditionally : #Programming-CommandsProgramming Commands #index-Buffer-spaces_002c-pattern-and-hold-38Buffer spaces, pattern and hold : #Execution-CycleExecution Cycle #index-Bugs_002c-reporting-214Bugs, reporting : #Reporting-BugsReporting Bugs #index-Case_002dinsensitive-matching-116Case-insensitive matching : #The-_0022s_0022-CommandThe "s" Command #index-Caveat-_002d_002d_002d-_0023n-on-first-line-83Caveat — #n on first line : #Common-CommandsCommon Commands #index-Command-groups-96Command groups : #Common-CommandsCommon Commands #index-Comments_002c-in-scripts-80Comments, in scripts : #Common-CommandsCommon Commands #index-Conditional-branch-202Conditional branch : #Extended-CommandsExtended Commands #index-Conditional-branch-179Conditional branch : #Programming-CommandsProgramming Commands #index-Copy-hold-space-into-pattern-space-162Copy hold space into pattern space : #Other-CommandsOther Commands #index-Copy-pattern-space-into-hold-space-155Copy pattern space into hold space : #Other-CommandsOther Commands #index-Delete-first-line-from-pattern-space-148Delete first line from pattern space : #Other-CommandsOther Commands #index-Disabling-autoprint_002c-from-command-line-9Disabling autoprint, from command line : #Invoking-sedInvoking sed #index-empty-regular-expression-53empty regular expression : #AddressesAddresses #index-Evaluate-Bourne_002dshell-commands-181Evaluate Bourne-shell commands : #Extended-CommandsExtended Commands #index-Evaluate-Bourne_002dshell-commands_002c-after-substitution-111Evaluate Bourne-shell commands, after substitution : #The-_0022s_0022-CommandThe "s" Command #index-Exchange-hold-space-with-pattern-space-169Exchange hold space with pattern space : #Other-CommandsOther Commands #index-Excluding-lines-70Excluding lines : #AddressesAddresses #index-Extended-regular-expressions_002c-choosing-22Extended regular expressions, choosing : #Invoking-sedInvoking sed #index-Extended-regular-expressions_002c-syntax-224Extended regular expressions, syntax : #Extended-regexpsExtended regexps #index-Files-to-be-processed-as-input-34Files to be processed as input : #Invoking-sedInvoking sed #index-Flow-of-control-in-scripts-171Flow of control in scripts : #Programming-CommandsProgramming Commands #index-Global-substitution-102Global substitution : #The-_0022s_0022-CommandThe "s" Command #index-g_t_0040value_007bSSEDEXT_007d_002c-_0040file_007b_002fdev_002fstderr_007d-file-146GNU extensions, /dev/stderr file : #Other-CommandsOther Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-_0040file_007b_002fdev_002fstderr_007d-file-110GNU extensions, /dev/stderr file : #The-_0022s_0022-CommandThe "s" Command #index-g_t_0040value_007bSSEDEXT_007d_002c-_0040file_007b_002fdev_002fstdin_007d-file-198GNU extensions, /dev/stdin file : #Extended-CommandsExtended Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-_0040file_007b_002fdev_002fstdin_007d-file-142GNU extensions, /dev/stdin file : #Other-CommandsOther Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-_0040file_007b_002fdev_002fstdout_007d-file-225GNU extensions, /dev/stdout file : #FootnotesFootnotes #index-g_t_0040value_007bSSEDEXT_007d_002c-_0040file_007b_002fdev_002fstdout_007d-file-145GNU extensions, /dev/stdout file : #Other-CommandsOther Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-_0040file_007b_002fdev_002fstdout_007d-file-109GNU extensions, /dev/stdout file : #The-_0022s_0022-CommandThe "s" Command #index-g_t_0040acronym_007bGNU_007d-extensions_002c-_0040code_007b0_007d-address-66GNU extensions, 0 address : #AddressesAddresses #index-g_t_0040acronym_007bGNU_007d-extensions_002c-0_002c_0040var_007baddr2_007d-addressing-67GNU extensions, 0,addr2 addressing : #AddressesAddresses #index-g_t_0040acronym_007bGNU_007d-extensions_002c-_0040var_007baddr1_007d_002c_002b_0040var_007bN_007d-addressing-68GNU extensions, addr1,+N addressing : #AddressesAddresses #index-g_t_0040acronym_007bGNU_007d-extensions_002c-_0040var_007baddr1_007d_002c_007e_0040var_007bN_007d-addressing-69GNU extensions, addr1,~N addressing : #AddressesAddresses #index-g_t_0040value_007bSSEDEXT_007d_002c-branch-if-_0040code_007bs_002f_002f_002f_007d-failed-200GNU extensions, branch if s/// failed : #Extended-CommandsExtended Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-case-modifiers-in-_0040code_007bs_007d-commands-99GNU extensions, case modifiers in s commands : #The-_0022s_0022-CommandThe "s" Command #index-g_t_0040value_007bSSEDEXT_007d_002c-checking-for-their-presence-204GNU extensions, checking for their presence : #Extended-CommandsExtended Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-disabling-18GNU extensions, disabling : #Invoking-sedInvoking sed #index-g_t_0040value_007bSSEDEXT_007d_002c-evaluating-Bourne_002dshell-commands-183GNU extensions, evaluating Bourne-shell commands : #Extended-CommandsExtended Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-evaluating-Bourne_002dshell-commands-113GNU extensions, evaluating Bourne-shell commands : #The-_0022s_0022-CommandThe "s" Command #index-g_t_0040acronym_007bGNU_007d-extensions_002c-extended-regular-expressions-23GNU extensions, extended regular expressions : #Invoking-sedInvoking sed #index-g_t_0040acronym_007bGNU_007d-extensions_002c-_0040code_007bg_007d-and-_0040var_007bnumber_007d-modifier-interaction-in-_0040code_007bs_007d-command-105GNU extensions, g and number modifier interaction in s command : #The-_0022s_0022-CommandThe "s" Command #index-g_t_0040acronym_007bGNU_007d-extensions_002c-_0040code_007bI_007d-modifier-115GNU extensions, I modifier : #The-_0022s_0022-CommandThe "s" Command #index-g_t_0040acronym_007bGNU_007d-extensions_002c-_0040code_007bI_007d-modifier-56GNU extensions, I modifier : #AddressesAddresses #index-g_t_0040value_007bSSEDEXT_007d_002c-in_002dplace-editing-222GNU extensions, in-place editing : #Reporting-BugsReporting Bugs #index-g_t_0040value_007bSSEDEXT_007d_002c-in_002dplace-editing-13GNU extensions, in-place editing : #Invoking-sedInvoking sed #index-g_t_0040value_007bSSEDEXT_007d_002c-_0040code_007bL_007d-command-189GNU extensions, L command : #Extended-CommandsExtended Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-_0040code_007bM_007d-modifier-117GNU extensions, M modifier : #The-_0022s_0022-CommandThe "s" Command #index-g_t_0040value_007bSSEDEXT_007d_002c-modifiers-and-the-empty-regular-expression-54GNU extensions, modifiers and the empty regular expression : #AddressesAddresses #index-g_t_0040acronym_007bGNU_007d-extensions_002c-_0040samp_007b_0040var_007bn_007d_007e_0040var_007bm_007d_007d-addresses-47GNU extensions, `n~m' addresses : #AddressesAddresses #index-g_t_0040value_007bSSEDEXT_007d_002c-quitting-silently-191GNU extensions, quitting silently : #Extended-CommandsExtended Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-_0040code_007bR_007d-command-197GNU extensions, R command : #Extended-CommandsExtended Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-reading-a-file-a-line-at-a-time-196GNU extensions, reading a file a line at a time : #Extended-CommandsExtended Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-reformatting-paragraphs-188GNU extensions, reformatting paragraphs : #Extended-CommandsExtended Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-returning-an-exit-code-192GNU extensions, returning an exit code : #Extended-CommandsExtended Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-returning-an-exit-code-85GNU extensions, returning an exit code : #Common-CommandsCommon Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-setting-line-length-138GNU extensions, setting line length : #Other-CommandsOther Commands #index-g_t_0040acronym_007bGNU_007d-extensions_002c-special-escapes-220GNU extensions, special escapes : #Reporting-BugsReporting Bugs #index-g_t_0040acronym_007bGNU_007d-extensions_002c-special-escapes-209GNU extensions, special escapes : #EscapesEscapes #index-g_t_0040acronym_007bGNU_007d-extensions_002c-special-two_002daddress-forms-65GNU extensions, special two-address forms : #AddressesAddresses #index-g_t_0040value_007bSSEDEXT_007d_002c-subprocesses-184GNU extensions, subprocesses : #Extended-CommandsExtended Commands #index-g_t_0040value_007bSSEDEXT_007d_002c-subprocesses-114GNU extensions, subprocesses : #The-_0022s_0022-CommandThe "s" Command #index-g_t_0040acronym_007bGNU_007d-extensions_002c-to-basic-regular-expressions-218GNU extensions, to basic regular expressions : #Reporting-BugsReporting Bugs #index-g_t_0040acronym_007bGNU_007d-extensions_002c-to-basic-regular-expressions-72GNU extensions, to basic regular expressions : #Regular-ExpressionsRegular Expressions #index-g_t_0040value_007bSSEDEXT_007d_002c-two-addresses-supported-by-most-commands-120GNU extensions, two addresses supported by most commands : #Other-CommandsOther Commands #index-g_t_0040acronym_007bGNU_007d-extensions_002c-unlimited-line-length-211GNU extensions, unlimited line length : #LimitationsLimitations #index-g_t_0040value_007bSSEDEXT_007d_002c-writing-first-line-to-a-file-208GNU extensions, writing first line to a file : #Extended-CommandsExtended Commands #index-Goto_002c-in-scripts-176Goto, in scripts : #Programming-CommandsProgramming Commands #index-Greedy-regular-expression-matching-78Greedy regular expression matching : #Regular-ExpressionsRegular Expressions #index-Grouping-commands-95Grouping commands : #Common-CommandsCommon Commands #index-Hold-space_002c-appending-from-pattern-space-160Hold space, appending from pattern space : #Other-CommandsOther Commands #index-Hold-space_002c-appending-to-pattern-space-167Hold space, appending to pattern space : #Other-CommandsOther Commands #index-Hold-space_002c-copy-into-pattern-space-164Hold space, copy into pattern space : #Other-CommandsOther Commands #index-Hold-space_002c-copying-pattern-space-into-157Hold space, copying pattern space into : #Other-CommandsOther Commands #index-Hold-space_002c-definition-41Hold space, definition : #Execution-CycleExecution Cycle #index-Hold-space_002c-exchange-with-pattern-space-170Hold space, exchange with pattern space : #Other-CommandsOther Commands #index-In_002dplace-editing-221In-place editing : #Reporting-BugsReporting Bugs #index-In_002dplace-editing_002c-activating-12In-place editing, activating : #Invoking-sedInvoking sed #index-In_002dplace-editing_002c-Perl_002dstyle-backup-file-names-14In-place editing, Perl-style backup file names : #Invoking-sedInvoking sed #index-Inserting-text-before-a-line-126Inserting text before a line : #Other-CommandsOther Commands #index-Labels_002c-in-scripts-173Labels, in scripts : #Programming-CommandsProgramming Commands #index-Last-line_002c-selecting-49Last line, selecting : #AddressesAddresses #index-Line-length_002c-setting-137Line length, setting : #Other-CommandsOther Commands #index-Line-length_002c-setting-17Line length, setting : #Invoking-sedInvoking sed #index-Line-number_002c-printing-133Line number, printing : #Other-CommandsOther Commands #index-Line-selection-43Line selection : #AddressesAddresses #index-Line_002c-selecting-by-number-46Line, selecting by number : #AddressesAddresses #index-Line_002c-selecting-by-regular-expression-match-52Line, selecting by regular expression match : #AddressesAddresses #index-Line_002c-selecting-last-50Line, selecting last : #AddressesAddresses #index-List-pattern-space-135List pattern space : #Other-CommandsOther Commands #index-Mixing-_0040code_007bg_007d-and-_0040var_007bnumber_007d-modifiers-in-the-_0040code_007bs_007d-command-106Mixing g and number modifiers in the s command : #The-_0022s_0022-CommandThe "s" Command #index-Next-input-line_002c-append-to-pattern-space-150Next input line, append to pattern space : #Other-CommandsOther Commands #index-Next-input-line_002c-replace-pattern-space-with-92Next input line, replace pattern space with : #Common-CommandsCommon Commands #index-Non_002dbugs_002c-in_002dplace-editing-223Non-bugs, in-place editing : #Reporting-BugsReporting Bugs #index-Non_002dbugs_002c-_0040code_007bN_007d-command-on-the-last-line-216Non-bugs, N command on the last line : #Reporting-BugsReporting Bugs #index-Non_002dbugs_002c-regex-syntax-clashes-219Non-bugs, regex syntax clashes : #Reporting-BugsReporting Bugs #index-Parenthesized-substrings-98Parenthesized substrings : #The-_0022s_0022-CommandThe "s" Command #index-Pattern-space_002c-definition-40Pattern space, definition : #Execution-CycleExecution Cycle #index-Perl_002dstyle-regular-expressions_002c-multiline-57Perl-style regular expressions, multiline : #AddressesAddresses #index-Portability_002c-comments-81Portability, comments : #Common-CommandsCommon Commands #index-Portability_002c-line-length-limitations-212Portability, line length limitations : #LimitationsLimitations #index-Portability_002c-_0040code_007bN_007d-command-on-the-last-line-215Portability, N command on the last line : #Reporting-BugsReporting Bugs #index-g_t_0040code_007bPOSIXLY_005fCORRECT_007d-behavior_002c-bracket-expressions-76POSIXLY_CORRECT behavior, bracket expressions : #Regular-ExpressionsRegular Expressions #index-g_t_0040code_007bPOSIXLY_005fCORRECT_007d-behavior_002c-enabling-19POSIXLY_CORRECT behavior, enabling : #Invoking-sedInvoking sed #index-g_t_0040code_007bPOSIXLY_005fCORRECT_007d-behavior_002c-escapes-210POSIXLY_CORRECT behavior, escapes : #EscapesEscapes #index-g_t_0040code_007bPOSIXLY_005fCORRECT_007d-behavior_002c-_0040code_007bN_007d-command-217POSIXLY_CORRECT behavior, N command : #Reporting-BugsReporting Bugs #index-Print-first-line-from-pattern-space-153Print first line from pattern space : #Other-CommandsOther Commands #index-Printing-line-number-132Printing line number : #Other-CommandsOther Commands #index-Printing-text-unambiguously-136Printing text unambiguously : #Other-CommandsOther Commands #index-Quitting-193Quitting : #Extended-CommandsExtended Commands #index-Quitting-86Quitting : #Common-CommandsCommon Commands #index-Range-of-lines-58Range of lines : #AddressesAddresses #index-Range-with-start-address-of-zero-61Range with start address of zero : #AddressesAddresses #index-Read-next-input-line-93Read next input line : #Common-CommandsCommon Commands #index-Read-text-from-a-file-195Read text from a file : #Extended-CommandsExtended Commands #index-Read-text-from-a-file-141Read text from a file : #Other-CommandsOther Commands #index-Reformat-pattern-space-186Reformat pattern space : #Extended-CommandsExtended Commands #index-Reformatting-paragraphs-187Reformatting paragraphs : #Extended-CommandsExtended Commands #index-Replace-hold-space-with-copy-of-pattern-space-156Replace hold space with copy of pattern space : #Other-CommandsOther Commands #index-Replace-pattern-space-with-copy-of-hold-space-163Replace pattern space with copy of hold space : #Other-CommandsOther Commands #index-Replacing-all-text-matching-regexp-in-a-line-103Replacing all text matching regexp in a line : #The-_0022s_0022-CommandThe "s" Command #index-Replacing-only-_0040var_007bn_007dth-match-of-regexp-in-a-line-104Replacing only nth match of regexp in a line : #The-_0022s_0022-CommandThe "s" Command #index-Replacing-selected-lines-with-other-text-129Replacing selected lines with other text : #Other-CommandsOther Commands #index-Requiring-_0040value_007bSSED_007d-205Requiring GNU sed : #Extended-CommandsExtended Commands #index-Script-structure-37Script structure : #sed-Programssed Programs #index-Script_002c-from-a-file-33Script, from a file : #Invoking-sedInvoking sed #index-Script_002c-from-command-line-30Script, from command line : #Invoking-sedInvoking sed #index-g_t_0040command_007bsed_007d-program-structure-36sed program structure : #sed-Programssed Programs #index-Selecting-lines-to-process-44Selecting lines to process : #AddressesAddresses #index-Selecting-non_002dmatching-lines-71Selecting non-matching lines : #AddressesAddresses #index-Several-lines_002c-selecting-59Several lines, selecting : #AddressesAddresses #index-Slash-character_002c-in-regular-expressions-55Slash character, in regular expressions : #AddressesAddresses #index-Spaces_002c-pattern-and-hold-39Spaces, pattern and hold : #Execution-CycleExecution Cycle #index-Special-addressing-forms-60Special addressing forms : #AddressesAddresses #index-Standard-input_002c-processing-as-input-35Standard input, processing as input : #Invoking-sedInvoking sed #index-Stream-editor-1Stream editor : #IntroductionIntroduction #index-Subprocesses-182Subprocesses : #Extended-CommandsExtended Commands #index-Subprocesses-112Subprocesses : #The-_0022s_0022-CommandThe "s" Command #index-Substitution-of-text_002c-options-101Substitution of text, options : #The-_0022s_0022-CommandThe "s" Command #index-Text_002c-appending-123Text, appending : #Other-CommandsOther Commands #index-Text_002c-deleting-88Text, deleting : #Common-CommandsCommon Commands #index-Text_002c-insertion-127Text, insertion : #Other-CommandsOther Commands #index-Text_002c-printing-90Text, printing : #Common-CommandsCommon Commands #index-Text_002c-printing-after-substitution-107Text, printing after substitution : #The-_0022s_0022-CommandThe "s" Command #index-Text_002c-writing-to-a-file-after-substitution-108Text, writing to a file after substitution : #The-_0022s_0022-CommandThe "s" Command #index-Transliteration-119Transliteration : #Other-CommandsOther Commands #index-Unbuffered-I_002fO_002c-choosing-27Unbuffered I/O, choosing : #Invoking-sedInvoking sed #index-Usage-summary_002c-printing-5Usage summary, printing : #Invoking-sedInvoking sed #index-Version_002c-printing-3Version, printing : #Invoking-sedInvoking sed #index-Working-on-separate-files-24Working on separate files : #Invoking-sedInvoking sed #index-Write-first-line-to-a-file-207Write first line to a file : #Extended-CommandsExtended Commands #index-Write-to-a-file-144Write to a file : #Other-CommandsOther Commands #index-Zero_002c-as-range-start-address-62Zero, as range start address : #AddressesAddresses    
Previous: 
#Concept-IndexConcept Index ,
Up: 
#TopTop Command and Option Index
This is an alphabetical list of all sed commands and command-line
options.
#index-g_t_0023-_0028comments_0029-79# (comments) : #Common-CommandsCommon Commands #index-g_t_002d_002dexpression-29--expression : #Invoking-sedInvoking sed #index-g_t_002d_002dfile-32--file : #Invoking-sedInvoking sed #index-g_t_002d_002dhelp-4--help : #Invoking-sedInvoking sed #index-g_t_002d_002din_002dplace-11--in-place : #Invoking-sedInvoking sed #index-g_t_002d_002dline_002dlength-16--line-length : #Invoking-sedInvoking sed #index-g_t_002d_002dquiet-7--quiet : #Invoking-sedInvoking sed #index-g_t_002d_002dregexp_002dextended-21--regexp-extended : #Invoking-sedInvoking sed #index-g_t_002d_002dsilent-8--silent : #Invoking-sedInvoking sed #index-g_t_002d_002dunbuffered-26--unbuffered : #Invoking-sedInvoking sed #index-g_t_002d_002dversion-2--version : #Invoking-sedInvoking sed #index-g_t_002de-28-e : #Invoking-sedInvoking sed #index-g_t_002df-31-f : #Invoking-sedInvoking sed #index-g_t_002di-10-i : #Invoking-sedInvoking sed #index-g_t_002dl-15-l : #Invoking-sedInvoking sed #index-g_t_002dn-6-n : #Invoking-sedInvoking sed #index-g_t_002dn_002c-forcing-from-within-a-script-82-n, forcing from within a script : #Common-CommandsCommon Commands #index-g_t_002dr-20-r : #Invoking-sedInvoking sed #index-g_t_002du-25-u : #Invoking-sedInvoking sed #index-g_t_003a-_0028label_0029-command-172: (label) command : #Programming-CommandsProgramming Commands #index-g_t_003d-_0028print-line-number_0029-command-131= (print line number) command : #Other-CommandsOther Commands #index-a-_0028append-text-lines_0029-command-121a (append text lines) command : #Other-CommandsOther Commands #index-b-_0028branch_0029-command-174b (branch) command : #Programming-CommandsProgramming Commands #index-c-_0028change-to-text-lines_0029-command-128c (change to text lines) command : #Other-CommandsOther Commands #index-D-_0028delete-first-line_0029-command-147D (delete first line) command : #Other-CommandsOther Commands #index-d-_0028delete_0029-command-87d (delete) command : #Common-CommandsCommon Commands #index-e-_0028evaluate_0029-command-180e (evaluate) command : #Extended-CommandsExtended Commands #index-G-_0028appending-Get_0029-command-165G (appending Get) command : #Other-CommandsOther Commands #index-g-_0028get_0029-command-161g (get) command : #Other-CommandsOther Commands #index-H-_0028append-Hold_0029-command-158H (append Hold) command : #Other-CommandsOther Commands #index-h-_0028hold_0029-command-154h (hold) command : #Other-CommandsOther Commands #index-i-_0028insert-text-lines_0029-command-125i (insert text lines) command : #Other-CommandsOther Commands #index-L-_0028fLow-paragraphs_0029-command-185L (fLow paragraphs) command : #Extended-CommandsExtended Commands #index-l-_0028list-unambiguously_0029-command-134l (list unambiguously) command : #Other-CommandsOther Commands #index-N-_0028append-Next-line_0029-command-149N (append Next line) command : #Other-CommandsOther Commands #index-n-_0028next_002dline_0029-command-91n (next-line) command : #Common-CommandsCommon Commands #index-P-_0028print-first-line_0029-command-152P (print first line) command : #Other-CommandsOther Commands #index-p-_0028print_0029-command-89p (print) command : #Common-CommandsCommon Commands #index-q-_0028quit_0029-command-84q (quit) command : #Common-CommandsCommon Commands #index-Q-_0028silent-Quit_0029-command-190Q (silent Quit) command : #Extended-CommandsExtended Commands #index-r-_0028read-file_0029-command-140r (read file) command : #Other-CommandsOther Commands #index-R-_0028read-line_0029-command-194R (read line) command : #Extended-CommandsExtended Commands #index-s-command_002c-option-flags-100s command, option flags : #The-_0022s_0022-CommandThe "s" Command #index-T-_0028test-and-branch-if-failed_0029-command-199T (test and branch if failed) command : #Extended-CommandsExtended Commands #index-t-_0028test-and-branch-if-successful_0029-command-177t (test and branch if successful) command : #Programming-CommandsProgramming Commands #index-v-_0028version_0029-command-203v (version) command : #Extended-CommandsExtended Commands #index-w-_0028write-file_0029-command-143w (write file) command : #Other-CommandsOther Commands #index-W-_0028write-first-line_0029-command-206W (write first line) command : #Extended-CommandsExtended Commands #index-x-_0028eXchange_0029-command-168x (eXchange) command : #Other-CommandsOther Commands #index-y-_0028transliterate_0029-command-118y (transliterate) command : #Other-CommandsOther Commands #index-g_t_0040_007b_0040_007d-command-grouping-94{} command grouping : #Common-CommandsCommon Commands    
Table of Contents
#Topsed, a stream editor #Introduction1 Introduction #Invoking-sed2 Invocation #sed-Programs3 sed Programs #Execution-Cycle3.1 How sed Works #Addresses3.2 Selecting lines with sed #Regular-Expressions3.3 Overview of Regular Expression Syntax #Common-Commands3.4 Often-Used Commands #The-_0022s_0022-Command3.5 The s Command #Other-Commands3.6 Less Frequently-Used Commands #Programming-Commands3.7 Commands for sed gurus #Extended-Commands3.8 Commands Specific to GNU sed #Escapes3.9 GNU Extensions for Escapes in Regular Expressions #Examples4 Some Sample Scripts #Centering-lines4.1 Centering Lines #Increment-a-number4.2 Increment a Number #Rename-files-to-lower-case4.3 Rename Files to Lower Case #Print-bash-environment4.4 Print bash Environment #Reverse-chars-of-lines4.5 Reverse Characters of Lines #tac4.6 Reverse Lines of Files #cat-_002dn4.7 Numbering Lines #cat-_002db4.8 Numbering Non-blank Lines #wc-_002dc4.9 Counting Characters #wc-_002dw4.10 Counting Words #wc-_002dl4.11 Counting Lines #head4.12 Printing the First Lines #tail4.13 Printing the Last Lines #uniq4.14 Make Duplicate Lines Unique #uniq-_002dd4.15 Print Duplicated Lines of Input #uniq-_002du4.16 Remove All Duplicated Lines #cat-_002ds4.17 Squeezing Blank Lines #Limitations5 GNU sed's Limitations and Non-limitations #Other-Resources6 Other Resources for Learning About sed #Reporting-Bugs7 Reporting Bugs #Extended-regexpsAppendix A Extended regular expressions #Concept-IndexConcept Index #Command-and-Option-IndexCommand and Option Index    
Footnotes
[ #fnd-11 ] This applies to commands such as =,
a, c, i, l, p.  You can
still write to the standard output by using the 
w or W commands together with the /dev/stdoutspecial file
   
[ #fnd-22 ] Note that GNU sed creates the backup
    file whether or not any output is actually changed.
   
[ #fnd-33 ] Actually,
  if 
sed prints a line without the terminating newline, it will
  nevertheless print the missing newline as soon as more text is sent to
  the same output stream, which gives the “least expected surprise”
  even though it does not make commands like `
sed -n p' exactly
  identical to 
cat.
   
[ #fnd-44 ] This is equivalent to p unless the -ioption is being used.
   
[ #fnd-55 ] This is equivalent to p unless the -ioption is being used.
   
[ #fnd-66 ] All
the escapes introduced here are 
GNUextensions, with the exception of 
\n.  In basic regular
expression mode, setting 
POSIXLY_CORRECT disables them inside
bracket expressions.
   
[ #fnd-77 ] sed guru Greg
Ubben wrote an implementation of the 
dc rpn calculator! 
It is distributed together with sed.
   
[ #fnd-88 ] This requires
another script to pad the output of banner; for example
     #! /bin/sh
     
     banner -w $1 $2 $3 $4 |
       sed -e :a -e '/^.\{0,'$1'\}$/ { s/$/ /; ba; }' |
       ~/sedscripts/reverseline.sed
   
   
[ #fnd-99 ] Some implementations
have a limit of 199 commands per script
   
[ #fnd-1010 ] which is the actual “bug” that prompted the
change in behavior
   
