Linux Shell Scripting Cookbook（Third Edition）

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Getting ready

Commands are typed and executed in a terminal session. When a terminal is opened, a prompt is displayed. The prompt can be configured in many ways, but frequently resembles this:

username@hostname$

Alternatively, it can also be configured as root@hostname # or simply as $ or #.

The $ character represents regular users and # represents the administrative user root. Root is the most privileged user in a Linux system.

It is a bad idea to directly use the shell as the root user (administrator) to perform tasks. Typing errors have the potential to do more damage when your shell has more privileges. It is recommended that you log in as a regular user (your shell may denote this as $ in the prompt), and use tools such as sudo to run privileged commands. Running a command as sudo <command> <arguments> will run it as root.

A shell script typically begins with a shebang:

#!/bin/bash

Shebang is a line on which #! is prefixed to the interpreter path. /bin/bash is the interpreter command path for Bash. A line starting with a # symbol is treated by the bash interpreter as a comment. Only the first line of a script can have a shebang to define the interpreter to be used to evaluate the script.

A script can be executed in two ways:

Pass the name of the script as a command-line argument:

        bash myScript.sh

Set the execution permission on a script file to make it executable:

        chmod 755 myScript.sh
        ./myScript.sh.

If a script is run as a command-line argument for bash, the shebang is not required. The shebang facilitates running the script on its own. Executable scripts use the interpreter path that follows the shebang to interpret a script.

Scripts are made executable with the chmod command:

$ chmod a+x sample.sh

This command makes a script executable by all users. The script can be executed as follows:

$ ./sample.sh #./ represents the current directory

Alternatively, the script can be executed like this:

$ /home/path/sample.sh # Full path of the script is used

The kernel will read the first line and see that the shebang is #!/bin/bash. It will identify /bin/bash and execute the script as follows:

$ /bin/bash sample.sh

When an interactive shell starts, it executes a set of commands to initialize settings, such as the prompt text, colors, and so on. These commands are read from a shell script at ~/.bashrc (or ~/.bash_profile for login shells), located in the home directory of the user. The Bash shell maintains a history of commands run by the user in the ~/.bash_history file.

The ~ symbol denotes your home directory, which is usually /home/user, where user is your username or /root for the root user. A login shell is created when you log in to a machine. However, terminal sessions you create while logged in to a graphical environment (such as GNOME, KDE, and so on), are not login shells. Logging in with a display manager such as GDM or KDM may not read a .profile or .bash_profile (most don't), but logging in to a remote system with ssh will read the .profile. The shell delimits each command or command sequence with a semicolon or a new line. Consider this example: $ cmd1 ; cmd2
This is equivalent to these:
$ cmd1
$ cmd2

A comment starts with # and proceeds up to the end of the line. The comment lines are most often used to describe the code, or to disable execution of a line of code during debugging:

# sample.sh - echoes "hello world"
echo "hello world"

Now let's move on to the basic recipes in this chapter.

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