A couple of notes on special variables available in the Bash shell.
$0: Name of the script
The special variable $0 contains the name of the script. It is useful when
returning usage instructions to the user that run the script. For example:
{% highlight bash %} echo "Usage: $0 --option-1 --option-2"; {% endhighlight %}
$#: Number of arguments
The special variable $# contains the number of arguments passed to the
script. If no arguments are passed, it is equal to 0. It is useful to examine
the value of this variable before taking further actions, if a script must
be run with a minimum number of arguments.
Here is an example of $# used together with $0:
{% highlight bash %} if [ $# -eq 0 ]; then echo "Insufficient number of arguments!"; echo "Usage: $0 /path/to/file"; fi {% endhighlight %}
$?: Most recent exit code
The special variable $? contains the exit code of the most recently
executed command. It is useful to examine this to determine whether or
not the previous command completed succesfully or returned an error.
Typically, commands that complete succesfully return an exit code equal to 0, with other values indicating different errors, but check each command's documentation to verify this.
In the following example, the existense of the word "test" in a file is checked:
{% highlight bash %} grep test /path/to/file > /dev/null; if [ $? -eq 0 ]; then echo "/path/to/file contains the word 'test'"; else echo "/path/to/file does not contain the word 'test'"; fi {% endhighlight %}