Using Variables in Bash

A shell variable can be global or local. Global variables, or environment variables, can be accessed in all shells. In contrast, local variables are visible in the current shell only.

To view all environment variables, use the printenv command. If you need to know the value of a variable, insert the name of your variable as an argument:

printenv PATH

A variable, be it global or local, can be viewed with echo:

echo $PATH

To set a local variable, use a variable name followed by the equal sign, followed by the value:

PROJECT="SLED"

Do not insert spaces around the equal sign, otherwise you get an error. To set a environment variable, use export:

export NAME="tux"

To remove a variable, use unset:

unset NAME

The following table contains some common environment variables which can be used in you shell scripts:

Table 20.5. Useful Environment Variables

`HOME`	the home directory of the current user
`HOST`	the current host name
`LANG`	when a tool is localized, it uses the language from this environment variable. English can also be set to `C`
`PATH`	the search path of the shell, a list of directories separated by colon
`PS1`	specifies the normal prompt printed before each command
`PS2`	specifies the secondary prompt printed when you execute a multi-line command
`PWD`	current working directory
`USER`	the current user

Using Argument Variables

For example, if you have the script foo.sh you can execute it like this:

foo.sh "Tux Penguin" 2000

To access all the arguments which are passed to your script, you need positional parameters. These are $1 for the first argument, $2 for the second, and so on. You can have up to nine parameters. To get the script name, use $0.

The following script foo.sh prints all arguments from 1 to 4:

#!/bin/sh
echo \"$1\" \"$2\" \"$3\" \"$4\"

If you execute this script with the above arguments, you get:

"Tux Penguin" "2000" "" ""

Using Variable Substitution

Variable substitutions apply a pattern to the content of a variable either from the left or right side. The following list contains the possible syntax forms:

${VAR#pattern}

removes the shortest possible match from the left:

file=/home/tux/book/book.tar.bz2
echo ${file#*/}
home/tux/book/book.tar.bz2

${VAR##pattern}

removes the longest possible match from the left:

file=/home/tux/book/book.tar.bz2
echo ${file##*/}
book.tar.bz2

${VAR%pattern}

removes the shortest possible match from the right:

file=/home/tux/book/book.tar.bz2
echo ${file%.*}
/home/tux/book/book.tar

${VAR%%pattern}

removes the longest possible match from the right:

file=/home/tux/book/book.tar.bz2
echo ${file%%.*}
/home/tux/book/book