Can you make a bash script's option arguments be optional?

Question

I would like either of these inputs to work. That is, the -n option itself is optional – I already know how to do that – but it then may have an optional parameter on top. If no parameter is given, a fallback value will be applied.

command -n 100
command -n

I can only make the former input type work or the latter, but not both.

HAS_NICE_THINGS=0
NICE_THINGS=50       # default value.

while getopts n: option; do
#while getopts n option; do    # NICE_THINGS would always be that default value.
#while getopts nn: option; do    # same.
    case "${option}" in
    n)
        HAS_NICE_THINGS=1
        if [[ ! -z "${OPTARG}" ]] && (( "${OPTARG}" > 0 )) && (( "${OPTARG}" <= 100 )); then
            NICE_THINGS=${OPTARG}
        fi;;
    esac
done

# error message:
# option requires an argument -- n

I'm not entirely sure yet if I would need a boolean for my script, but so far, just in case, I am logging one (HAS_NICE_THINGS).

The end goal I had in mind was to set the JPG quality when eventually saving an image. Though, I can imagine this construct being useful elsewhere as well.

I'm using Ubuntu 18.04.5 and GNU bash, version 4.4.20(1)-release (x86_64-pc-linux-gnu).

Answer 1

Not sensibly with Bash's/POSIX getopts, but you could do it with the "enhanced" getopt (without an s) from util-linux or Busybox. (And those only, in particular many "traditional" getopt implementations are broken wrt. whitespace also)

The man page says of getopts:

optstring contains the option characters to be recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by white space.

there's no mention of optional option-arguments.

Of course you could have another optional option to give the non-default value. E.g. let -n take no argument and just enable nice things, and let -N <arg> take the argument, enable nice things and set the value.

E.g. something like this:

#!/bin/bash
HAS_NICE_THINGS=0
NICE_THINGS_VALUE=50
        
while getopts nN: option; do
    case "${option}" in
    n)
        HAS_NICE_THINGS=1
        shift;;
    N)
        HAS_NICE_THINGS=1
        NICE_THINGS_VALUE=$OPTARG
        shift; shift;;
    esac
done

if [ "$HAS_NICE_THINGS" = 1 ]; then
    echo "nice things enabled with value $NICE_THINGS_VALUE"
fi

would give

$ bash nice-getotps.sh -n
nice things enabled with value 50
$ bash nice-getopts.sh -N42
nice things enabled with value 42

---

The util-linux getopt takes optional option-arguments with the double-colon syntax. It's a bit awkward to use, and you need to mess with eval, but done correctly, it seems to work.

Man page:

-o shortopts [...] Each short option character in shortopts may be followed by one colon to indicate it has a required argument, and by two colons to indicate it has an optional argument.

With a script to just print the raw values so we can check it works properly (getopt-optional.sh):

#!/bin/bash
getopt -T
if [ "$?" -ne 4 ]; then
    echo "wrong version of 'getopt' installed, exiting..." >&2
    exit 1
fi 
params="$(getopt -o an:: -- "$@")"
eval set -- "$params"
while [ "$#" -gt 0 ]; do
    case "$1" in
    -n)
        echo "option -n with arg '$2'"
        shift 2;;
    -a)
        echo "option -a"
        shift;;
    --) 
        shift
        break;;
     *) 
        echo "something else: '$1'"
        shift;;
    esac
done
echo "remaining arguments ($#):"
printf "<%s> " "$@"
echo

we get

$ bash getopt-optional.sh -n -a
option -n with arg ''
option -a
remaining arguments (0):
<> 
$ bash getopt-optional.sh -n'blah blah' -a
 -n 'blah blah' -a --
option -n with arg 'blah blah'
option -a
remaining arguments (0):
<> 

No argument to -n shows up as an empty argument.

Not that you could pass an explicit empty argument anyway, since the option-argument needs to be within the same command line argument as the option itself, and -n is the same as -n"" after the quotes are removed. That makes optional option-arguments awkward to use in that you need to use -nx, as -n x would be taken as the option -n (without an opt-arg), followed by a regular non-option command line argument x. Which is unlike what would happen if -n took a mandatory option-argument.

More about getopt on this Stackoverflow answer to How do I parse command line arguments in Bash?

---

Note again that support for optional option-arguments is limited to those particular implementations of getopt, ones most commonly found on Linux systems. Other implementations of getopt don't even support whitespace in arguments as they rely on word splitting of args=$(getopt...); set -- $args.

The "enhanced" versions of getopt have the -T option to test for support of these enhancements. There's some discussion on the limitations and caveats with getopt here: getopt, getopts or manual parsing - what to use when I want to support both short and long options?

Also, getopt is not a standard tool like getopts is (not sure if that's exactly because of these limitations and differences).

Answer 2

Let's assume that we want to provide an option -a that takes an optional option-argument. Using the option should set the shell variable a_opt in the shell, but it should set it to the value $a_default if no option-argument is given, otherwise it sets it to the value of $OPTARG as usual.

To determine when -a is used with no argument, we have two cases:

1. The option-argument that getopts believes the user is using is actually one of the other options, or
2. The -a option is used at the end of the list command line (getopts would usually generate an error about a missing option in this case).

The first case is handled by looking at $OPTARG to determine whether it looks like one of the other options. If it does, then use the default value, otherwise use $OPTARG as the value. When the default value is used, we need to restart option parsing at the point after the -a, which means shifting off a few arguments (OPTIND-1 of them), resetting OPTIND to 1 and then adding $OPTARG to the front of the list of positional parameters.

This all means that we can't use -a with an argument that looks like one of the options.

The second case is handled by letting the first character of the option string given to getopts be :. This has two effects:

1. getopts no longer emits errors by itself.
2. When an option requiring an option-argument does not get an option-argument, the name of that option is placed in $OPTARG and an : is put into the option variable used with getopts.

This means that we can add a : case to our usual option parsing case statement which tests $OPTARG against a to see whether the user used -a at the very end of the command line.

Code:

#!/usr/local/bin/bash

a_default='A default value'
d_opt=false

while getopts :a:b:c:d opt; do
        case $opt in
                a)
                        case $OPTARG in
                                -[a-d-]*)
                                        shift "$(( OPTIND - 1 ))"
                                        OPTIND=1
                                        set -- "$OPTARG" "$@"
                                        a_opt=$a_default
                                        ;;
                                *)
                                        a_opt=$OPTARG
                        esac
                        ;;

                b)      b_opt=$OPTARG ;;
                c)      c_opt=$OPTARG ;;
                d)      d_opt=true ;;

                :)
                        if [ "$OPTARG" = a ]; then
                                a_opt=$a_default
                        else
                                printf '%s: option requires an argument -- %s\n' \
                                        "$0" "$OPTARG" >&2
                                exit 1
                        fi
                        ;;

                *)      printf '%s: generic option parsing error\n' "$0" >&2
                        exit 1
        esac
done

shift "$(( OPTIND - 1 ))"

printf '%s = "%s"\n' \
        'a_opt' "${a_opt-(Not set)}" \
        'b_opt' "${b_opt-(Not set)}" \
        'c_opt' "${c_opt-(Not set)}" \
        'd_opt' "${d_opt-(Not set)}"

if [ "$#" -gt 0 ]; then
        printf 'Extra argument: "%s"\n' "$@"
fi

Testing:

a_opt gets a default value if -a is used with no option-argument:

$ ./script -a
a_opt = "A default value"
b_opt = "(Not set)"
c_opt = "(Not set)"
d_opt = "false"

a_opt does not get a value if -a isn't used:

$ ./script -b bval
a_opt = "(Not set)"
b_opt = "bval"
c_opt = "(Not set)"
d_opt = "false"

We can give a_opt a value as usual by providing an option-argument with -a:

$ ./script -da aval -c cval
a_opt = "aval"
b_opt = "(Not set)"
c_opt = "cval"
d_opt = "true"

We may still use -- to end option parsing:

$ ./script -d -a -- aval -c cval hello world
a_opt = "A default value"
b_opt = "(Not set)"
c_opt = "(Not set)"
d_opt = "true"
Extra argument: "aval"
Extra argument: "-c"
Extra argument: "cval"
Extra argument: "hello"
Extra argument: "world"