Shell Script Arguments and Functions: Positional Parameters, Return Values, and Calling Conventions

This article covers advanced usage of variables in shell scripts, including argument parsing and variable access.

1. Defining Functions in Shell

Besides its built-in commands, Shell also supports user-defined functions. The benefit is code reuse and easier maintenance.

[function] fun_name[()]
{
    echo $0
    echo $1
    [return x]
}

• A function name must follow the same rules as a variable name.
• The function keyword before the name is optional, and the () after the name is optional, but {} is required.
• The final return x is optional; if omitted, the return value of the function is the exit status of its last command, and a function’s return value can only be a number.
• A function can take any number of arguments.

Inside the function body, access the arguments as $0, $2, … $n. You can also use $@ and $* to represent all of the arguments.

fun_name arg1 arg2 arg3

2. Parsing Passed Arguments

A shell script can receive arguments when it runs, letting different arguments produce different output.

bash xxx.sh arg1 arg2

Variable	Meaning
$n	An argument passed to the script or function, where n is a number counting from zero; the first argument is $1
$0	The filename of the current script — the “zeroth” argument of a shell script
$#	The number of arguments passed to the script or function
$*	All arguments passed to the script or function
$@	All arguments passed to the script or function
$?	The exit status of the last command, or a function’s return value
$$	The process ID of the current shell — for a shell script, the process ID the script is running under

Both $* and $@ represent all arguments passed to a function or script.

• When not enclosed in double quotes (" "), both output all arguments as "$1" "$2" … "$n".
• When enclosed in double quotes (" "):
  • "$*" treats all the arguments as a single whole, outputting them as "$1 $2 … $n".
  • "$@" keeps each argument separate, outputting them as "$1" "$2" … "$n".

Argument parsing has no fixed order and no distinction between positional and optional arguments the way Python’s argparse has — the result is simply a list, and all argument parsing amounts to matching against that list. A variable with a leading - is treated differently from one without.

1. Checking whether the command received any arguments

if [ $# -lt 1 ];
then
echo "no arguments"
else
echo "has $# args"
fi

2. Checking whether the command received a specific argument

j="-x"
for i in $@;
do
if [ $i = $j ];
then
echo "has $j"
fi
done

3. Including Files

A common need is defining a function in one file and calling it from another file. This is where source or . comes in.

# funa in a.sh
funa {
     echo "funa"
}

To call a.sh’s funa from b.sh:

. a.sh
funa

4. Summary

This article covered how functions are defined and used in Shell.