Command-Line Options

We have already seen many examples of the positional parameters (variables called 1, 2, 3, etc.) that the shell uses to store the command-line arguments to a shell script or function when it runs. We have also seen related variables like * (for the string of all arguments) and # (for the number of arguments).

Indeed, these variables hold all of the information on the user's command-line. But consider what happens when options are involved. Typical UNIX commands have the form command [-options]args, meaning that there can be 0 or more options. If a shell script processes the command teatime alice hatter, then $1 is "alice" and $2 is "hatter". But if the command is teatime -o alice hatter, then $1 is -o, $2 is "alice", and $3 is "hatter".

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You might think you could write code like this to handle it:

if [ $1 = -o ]; then
    code that processes the -o option
    1=$2
    2=$3
fi
    
normal processing of $1 and $2...

But this code has several problems. First, assignments like 1=$2 are illegal because positional parameters are read-only. Even if they were legal, another problem is that this kind of code imposes limitations on how many arguments the script can handle—which is very unwise. Furthermore, if this command had several possible options, the code to handle all of them would get very messy very quickly.