A shell script is fundamentally a sequence of commands. In such a script, we very often want to run a command only if a testable condition is true. In earlier chapters, we already saw one way to do this, with the && control operator:

$ grep -q bash /etc/shells && printf 'Bash is a system shell\n'

The preceding command line separates two commands with &&. The second command, which prints a string to standard output, only runs if the first command, grep -q bash /etc/shells, finds the bash string in the /etc/shells file and hence exits with status zero (success).

Using the if keyword, we can make this conditional approach more flexible, and somewhat easier to read:

$ if grep -q bash /etc/shells ; then printf 'Bash is a system shell\n' ; fi

The preceding command line could also be written like this in script form:

if grep -q bash /etc/shells ; then
    printf 'Bash is a system shell\n'
fi

Don't forget the then keyword, with the ; separator before it; both are required. If you prefer, you can put then on a new line after the test command instead.

In shell script, what is expected after the if keyword is not an expression, but a command to run, the outcome of which determines whether the code between the then and fi keywords is run.

The command line tested by if can be any command, builtin or not. For example, you can execute commands only if an attempt to change the working directory with cd was successful:

if cd /usr/local ; then 
    printf >&2 'Successfully changed directory\n' 
fi

We can reverse the test using the ! keyword before the command, separated by spaces:

if ! cd /usr/local/myscript ; then
    printf >&2 'Failed to changed directory\n'
    exit 1
fi

The printf command in this example would only run if the script's attempt to change the working directory to /usr/local/myscript failed. Coupled with an early exit, this can be an effective way to abort a script and prevent the rest of it from running if some essential condition can't be met.

There are also elif and else keywords for using after if to perform successive tests and specify alternatives if the initial command exits with failure:

if cd /usr/local/myscript ; then
    printf >&2 'Changed to primary directory'
elif cd /opt/myscript ; then
    printf >&2 'Changed to secondary directory'
else
    printf >&2 'Couldn'\''t find a directory!'
    exit 1
fi

The script will try to change its working directory to /opt/myscript only if its attempt to change it to /usr/local/myscript fails first. If both attempts fail, it will print an error message, and then exit with 1 (failure).

We can perform a lot of useful logic by testing commands once we know how their exit values work, but very often – perhaps more often – what we actually want is to test some sort of conditional expression, for example:

• Does a variable's value match a string?
• Is a variable empty?
• Is one number greater than another?
• Is a file empty?

There are dedicated commands that are designed to help us do all of this.