A command consists of several elements. The first element is always the actual command, followed by parameters or options. Commands are executed when you press Enter. Before doing so, easily edit the command line, add options, or correct typing errors. One of the most frequently used commands is ls, which can be used with or without arguments. Entering the plain ls command shows the contents of the current directory.

Options are prefixed with a hyphen. The command ls -l, for example, shows the contents of the same directory in full detail (long listing format). Next to each filename, see the date when the file was created, the file size in bytes, and further details, which are covered later. One important option that exists for many commands is --help. By entering ls --help, display all the options for the ls command.

It is important to get the “quoting” right. If a filename contains a space, either escape the space using a back slash (\) or enclose the filename in single or double quotes. Otherwise Bash interprets a filename like My Documents as the names of two files or directories. The difference between single and double quotes is that variable expansion takes place within double quotes. Single quotes ensure that the shell sees the quoted string literally.

To use the shell efficiently, it is really useful to have some knowledge of the file and directory structures of a Linux system. You can think of directories as electronic folders in which files, programs, and subdirectories are stored. The top level directory in the hierarchy is the root directory, referred to as /. This is the place from which all other directories can be accessed.

The /home directory contains the directories in which the individual users can store their personal files. Figure 27.1, “Excerpt from a Standard Directory Tree” shows the standard directory tree in Linux, with the home directories of the example users xyz, linux, and tux. The directory tree of a Linux system has a functional structure that follows the Filesystem Hierarchy Standard (FHS). The following list provides a brief description of the standard directories in Linux.

Figure 27.1. Excerpt from a Standard Directory Tree

There are two important features of the shell that can make your work a lot easier:

When working with files or directories, it is important specify the correct path. However, you do not need to enter the entire (absolute) path from the root directory to the respective file. You can start from the current directory. Address your home directory directly with ~. This means that there are two ways to list the file Testfile in the directory test: by entering the relative path with ls test or by specifying the absolute path with ls ~/test.

To list the contents of home directories of other users, enter ls ~username. In the example directory tree, one of the sample users is tux. In this case, ls ~tux would list the contents of the home directory of tux.

Refer to the current directory with a dot (.). The next higher level in the tree is represented by two dots (..). By entering ls .., see the contents of the parent directory of the current directory. The command ls ../.. shows the contents of the directory two levels higher in the hierarchy.

Another convenience offered by the shell is wild cards for pathname expansion. There are three different types of these in Bash:

Assuming that your test directory contains the files Testfile, Testfile1, Testfile2, and datafile, the command ls Testfile? lists the files Testfile1 and Testfile2. With ls Test*, the list also includes Testfile. ls *fil* shows all the sample files. Finally, you can use the set wild card to address all sample files whose last character is a number: ls Testfile[1-9] or, using classes, ls Testfile[[:digit:]].

Of the four types of wild cards, the most inclusive one is the asterisk. It could be used to copy all files contained in one directory to another one or to delete all files with one command. The command rm *fil*, for instance, would delete all files in the current directory whose name includes the string fil.

Linux includes two small programs for viewing text files directly in the shell. Rather than starting an editor to read a file like Readme.txt, simply enter less Readme.txt to display the text in the console window. Use Space to scroll down one page. Use Page Up and Page Down to move forward or backward in the text. To exit less, press Q.

Instead of less, you can also use the older program more. However, it is less convenient because it does not allow you to scroll backwards.

The program less got its name from the the precept that less is more and can also be used to view the output of commands in a convenient way. To see how this works, read Section 27.1.7, “Pipes and Redirection”.

Normally, the standard output in the shell is your screen or the console window and the standard input is the keyboard. To forward the output of a command to an application like less, use a pipeline.

To view the files in the test directory, enter the command ls test | less. The contents of the test directory are then displayed with less. This only makes sense if the normal output with ls would be too lengthy. For instance, if you view the contents of the dev directory with ls /dev, you only see a small portion in the window. View the entire list with ls /dev | less.

It is also possible to save the output of commands to a file. For example, echo "test one" > Content generates a new file called Content that contains the words test one. View the file with less Content.

You can also use a file as the input for a command. For example, with tr replace characters from standard input that redirected from the file Content and write the result to standard output: replace t with x by calling tr t x < Content. The output of tr is sent to the screen.

If you need a new file containing the output, pipe the output of tr to a file. To test this, change into test and enter the command tr t x < ../Content > new. Finally, view new with less new.

Just like the standard output, the standard error output is sent to the console. However, to redirect the standard error output to a file named errors, append 2> errors to the corresponding command. Both standard output and standard error are saved to one file named alloutput if you append >& alloutput. Finally, to append the output of a command to an already existing file, the command must be followed by >> instead of >.

Now that you have already created a number of files and directories, consider the subject of archives and data compression. Suppose you want to have the entire test directory packed in one file that you can save on a USB stick as a backup copy or send by e-mail. To do so, use the command tar (for tape archiver). With tar --help, view all the options for the tar command. The most important of these options are explained here:

To pack the test directory with all its files and subdirectories into an archive named testarchive.tar, use the options -c and -f. For testing purposes, also add -v to follow the progress of the archiving, although this option is not mandatory. After using cd to change to your home directory where the test directory is located, enter tar -cvf testarchive.tar test. After that, view the contents of the archive file with tar -tf testarchive.tar. The test directory with all its files and directories has remained unchanged on your hard disk. To unpack the archive, enter tar -xvf testarchive.tar, but do not try this yet.

For file compression, the obvious choice is gzip or, for a even better compression ratio, bzip2. Just enter gzip testarchive.tar (or bzip2 testarchive.tar, but gzip is used in this example). With ls, now see that the file testarchive.tar is no longer there and that the file testarchive.tar.gz has been created instead. This file is much smaller and therefore much better suited for transfer via e-mail or storage on a USB stick.

Now, unpack this file in the test2 directory created earlier. To do so, enter cp testarchive.tar.gz test2 to copy the file to that directory. Change to the directory with cd test2. A compressed archive with the .tar.gz extension can be unzipped with the gunzip command. Enter gunzip testarchive.tar.gz, which results in the file testarchive.tar, which then needs to be extracted or untarred with tar -xvf testarchive.tar. You can also unzip and extract a compressed archive in one step with tar -xvf testarchive.tar.gz (adding the -z option is no longer required). With ls, you can see that a new test directory has been created with the same contents as your test directory in your home directory.

mtools are a set of commands for working with MS-DOS file systems. The commands included in mtools allow you to address the first floppy drive as a:, just like under MS-DOS, and the commands are like MS-DOS commands except they are prefixed with an m.

After this crash course, you should be familiar with the basics of the Linux shell or command line. You may want to clean up your home directory by deleting the various test files and directories using the rm and rmdir commands. In Section 27.3, “Important Linux Commands”, find a list of the most important commands and a brief description of their functions.