Optional option argument with getopts

while getopts "hd:R:" arg; do
  case $arg in
    h)
      echo "usage" 
      ;;
    d)
      dir=$OPTARG
      ;;
    R)
      if [[ $OPTARG =~ ^[0-9]+$ ]];then
        level=$OPTARG
      else
        level=1
      fi
      ;;
    \?)
      echo "WRONG" >&2
      ;;
  esac
done

• level refers to the parameter of -R, dir refers to parameters of -d

• when I input ./count.sh -R 1 -d test/ it works correctly

• when I input ./count.sh -d test/ -R 1 it works correctly

• but I want to have it work when I input ./count.sh -d test/ -R or ./count.sh -R -d test/

This means that I want -R to have a default value and for the sequence of commands to be more flexible.

Wrong. Actually getopts does support optional arguments! From the bash man page:

If  a  required  argument is not found, and getopts is not silent, 
a question mark (?) is placed in name, OPTARG is unset, and a diagnostic
message is printed.  If getopts is silent, then a colon (:) is placed in name 
and OPTARG is set to the option character found.

When the man page says "silent" it means silent error reporting. To enable it, the first character of optstring needs to be a colon:

while getopts ":hd:R:" arg; do
    # ...rest of iverson's loop should work as posted 
done

Since Bash's getopt does not recognize -- to end the options list, it may not work when -R is the last option, followed by some path argument.

P.S.: Traditionally, getopt.c uses two colons (::) to specify an optional argument. However, the version used by Bash doesn't.

getopts doesn't really support this; but it's not hard to write your own replacement.

while true; do
    case $1 in
      -R) level=1
            shift
            case $1 in
              *[!0-9]* | "") ;;
              *) level=$1; shift ;;
            esac ;;
        # ... Other options ...
        -*) echo "$0: Unrecognized option $1" >&2
            exit 2;;
        *) break ;;
    esac
done

This workaround defines 'R' with no argument (no ':'), tests for any argument after the '-R' (manage last option on the command line) and tests if an existing argument starts with a dash.

# No : after R
while getopts "hd:R" arg; do
  case $arg in
  (...)
  R)
    # Check next positional parameter
    eval nextopt=\${$OPTIND}
    # existing or starting with dash?
    if [[ -n $nextopt && $nextopt != -* ]] ; then
      OPTIND=$((OPTIND + 1))
      level=$nextopt
    else
      level=1
    fi
    ;;
  (...)
  esac
done

I agree with tripleee, getopts does not support optional argument handling.

The compromised solution I have settled on is to use the upper case/lower case combination of the same option flag to differentiate between the option that takes an argument and the other that does not.

Example:

COMMAND_LINE_OPTIONS_HELP='
Command line options:
    -I          Process all the files in the default dir: '`pwd`'/input/
    -i  DIR     Process all the files in the user specified input dir
    -h          Print this help menu

Examples:
    Process all files in the default input dir
        '`basename $0`' -I

    Process all files in the user specified input dir
        '`basename $0`' -i ~/my/input/dir
    
'

VALID_COMMAND_LINE_OPTIONS="i:Ih"
INPUT_DIR=

while getopts $VALID_COMMAND_LINE_OPTIONS options; do
    #echo "option is " $options
    case $options in
        h)
            echo "$COMMAND_LINE_OPTIONS_HELP"
            exit $E_OPTERROR;
        ;;
        I)
            INPUT_DIR=`pwd`/input
            echo ""
            echo "***************************"
            echo "Use DEFAULT input dir : $INPUT_DIR"
            echo "***************************"
        ;;
        i)
            INPUT_DIR=$OPTARG
            echo ""
            echo "***************************"
            echo "Use USER SPECIFIED input dir : $INPUT_DIR"
            echo "***************************"
        ;;
        \?)
            echo "Usage: `basename $0` -h for help";
            echo "$COMMAND_LINE_OPTIONS_HELP"
            exit $E_OPTERROR;
        ;;
    esac
done

This is actually pretty easy. Just drop the trailing colon after the R and use OPTIND

while getopts "hRd:" opt; do
   case $opt in
      h) echo -e $USAGE && exit
      ;;
      d) DIR="$OPTARG"
      ;;
      R)       
        if [[ ${@:$OPTIND} =~ ^[0-9]+$ ]];then
          LEVEL=${@:$OPTIND}
          OPTIND=$((OPTIND+1))
        else
          LEVEL=1
        fi
      ;;
      \?) echo "Invalid option -$OPTARG" >&2
      ;;
   esac
done
echo $LEVEL $DIR

count.sh -d test

> test

count.sh -d test -R

> 1 test

count.sh -R -d test

> 1 test

count.sh -d test -R 2

> 2 test

count.sh -R 2 -d test

> 2 test

Inspired in @calandoa's answer (the only one that actually works!), I've made a simple function that can make it easy to be used multiple times.

getopts_get_optional_argument() {
  eval next_token=\${$OPTIND}
  if [[ -n $next_token && $next_token != -* ]]; then
    OPTIND=$((OPTIND + 1))
    OPTARG=$next_token
  else
    OPTARG=""
  fi
}

An example usage:

while getopts "hdR" option; do
  case $option in
  d)
    getopts_get_optional_argument $@
    dir=${OPTARG}
    ;;
  R)
    getopts_get_optional_argument $@
    level=${OPTARG:-1}
    ;;
  h)
    show_usage && exit 0
    ;;
  \?)
    show_usage && exit 1
    ;;
  esac
done

This gives us a practical way to get "that missing feature" in getopts :)

NOTE that nevertheless command-line options with optional args seems to be discouraged explicitly

> Guideline 7: Option-arguments should not be optional.

but I have no intuitive way to implement my case without this: I have 2 modes that are activated by either using one flag or other, and those have both an argument with a clear default. Introducing a third flag just to disambiguate makes it look a bad CLI style.

I've tested this with many combinations, including all in @aaron-sua's answer and works well.

Try:

while getopts "hd:R:" arg; do
  case $arg in
    h)
      echo "usage" 
    ;;
    d)
      dir=$OPTARG
    ;;
    R)
      if [[ $OPTARG =~ ^[0-9]+$ ]];then
        level=$OPTARG
      elif [[ $OPTARG =~ ^-. ]];then
        level=1
        let OPTIND=$OPTIND-1
      else
        level=1
      fi          
    ;;
    \?)
      echo "WRONG" >&2
    ;;
  esac
done

I think the above code will work for your purposes while still using getopts. I've added the following three lines to your code when getopts encounters -R:

      elif [[ $OPTARG =~ ^-. ]];then
        level=1
        let OPTIND=$OPTIND-1

If -R is encountered and the first argument looks like another getopts parameter, level is set to the default value of 1, and then the $OPTIND variable is reduced by one. The next time getopts goes to grab an argument, it will grab the correct argument instead of skipping it.

---

Here is similar example based on the code from Jan Schampera's comment at this tutorial:

#!/bin/bash
while getopts :abc: opt; do
  case $opt in
    a)
      echo "option a"
    ;;
    b)
      echo "option b"
    ;;
    c)
      echo "option c"

      if [[ $OPTARG = -* ]]; then
        ((OPTIND--))
        continue
      fi

      echo "(c) argument $OPTARG"
    ;;
    \?)
      echo "WTF!"
      exit 1
    ;;
  esac
done

> When you discover that OPTARG von -c is something beginning with a hyphen, then reset OPTIND and re-run getopts (continue the while loop). Oh, of course, this isn't perfect and needs some more robustness. It's just an example.

The following code solves this problem by checking for a leading dash and if found decrements OPTIND to point back to the skipped option for processing. This generally works fine except that you do not know the order the user will place options on the command line - if your optional argument option is last and does not provide an argument getopts will want to error out.

To fix the problem of the final argument missing, the "$@" array simply has an empty string "$@ " appended so that getopts will be satisfied that it has gobbled up yet another option argument. To fix this new empty argument a variable is set that holds the total count of all options to be processed - when the last option is being processed a helper function called trim is called and removes the empty string prior to the value being utilized.

This is not working code, it has only place holders but you can easily modify it and with a little bit of care it can be useful to build a robust system.

#!/usr/bin/env bash 
declare  -r CHECK_FLOAT="%f"  
declare  -r CHECK_INTEGER="%i"  

 ## <arg 1> Number - Number to check
 ## <arg 2> String - Number type to check
 ## <arg 3> String - Error message
function check_number() {
  local NUMBER="${1}"
  local NUMBER_TYPE="${2}"
  local ERROR_MESG="${3}"
  local FILTERED_NUMBER=$(sed 's/[^.e0-9+\^]//g' <<< "${NUMBER}")
  local -i PASS=1
  local -i FAIL=0
    if [[ -z "${NUMBER}" ]]; then 
        echo "Empty number argument passed to check_number()." 1>&2
        echo "${ERROR_MESG}" 1>&2
        echo "${FAIL}"          
  elif [[ -z "${NUMBER_TYPE}" ]]; then 
        echo "Empty number type argument passed to check_number()." 1>&2
        echo "${ERROR_MESG}" 1>&2
        echo "${FAIL}"          
  elif [[ ! "${#NUMBER}" -eq "${#FILTERED_NUMBER}" ]]; then 
        echo "Non numeric characters found in number argument passed to check_number()." 1>&2
        echo "${ERROR_MESG}" 1>&2
        echo "${FAIL}"          
  else  
   case "${NUMBER_TYPE}" in
     "${CHECK_FLOAT}")
         if ((! $(printf "${CHECK_FLOAT}" "${NUMBER}" &>/dev/random;echo $?))); then
            echo "${PASS}"
         else
            echo "${ERROR_MESG}" 1>&2
            echo "${FAIL}"
         fi
         ;;
     "${CHECK_INTEGER}")
         if ((! $(printf "${CHECK_INTEGER}" "${NUMBER}" &>/dev/random;echo $?))); then
            echo "${PASS}"
         else
            echo "${ERROR_MESG}" 1>&2
            echo "${FAIL}"
         fi
         ;;
                      *)
         echo "Invalid number type format: ${NUMBER_TYPE} to check_number()." 1>&2
         echo "${FAIL}"
         ;;
    esac
 fi 
}

 ## Note: Number can be any printf acceptable format and includes leading quotes and quotations, 
 ##       and anything else that corresponds to the POSIX specification. 
 ##       E.g. "'1e+03" is valid POSIX float format, see http://mywiki.wooledge.org/BashFAQ/054
 ## <arg 1> Number - Number to print
 ## <arg 2> String - Number type to print
function print_number() { 
  local NUMBER="${1}" 
  local NUMBER_TYPE="${2}" 
  case "${NUMBER_TYPE}" in 
      "${CHECK_FLOAT}") 
           printf "${CHECK_FLOAT}" "${NUMBER}" || echo "Error printing Float in print_number()." 1>&2
        ;;                 
    "${CHECK_INTEGER}") 
           printf "${CHECK_INTEGER}" "${NUMBER}" || echo "Error printing Integer in print_number()." 1>&2
        ;;                 
                     *) 
        echo "Invalid number type format: ${NUMBER_TYPE} to print_number()." 1>&2
        ;;                 
   esac
} 

 ## <arg 1> String - String to trim single ending whitespace from
function trim_string() { 
 local STRING="${1}" 
 echo -En $(sed 's/ $//' <<< "${STRING}") || echo "Error in trim_string() expected a sensible string, found: ${STRING}" 1>&2
} 
 
 ## This a hack for getopts because getopts does not support optional
 ## arguments very intuitively. E.g. Regardless of whether the values
 ## begin with a dash, getopts presumes that anything following an
 ## option that takes an option argument is the option argument. To fix  
 ## this the index variable OPTIND is decremented so it points back to  
 ## the otherwise skipped value in the array option argument. This works
 ## except for when the missing argument is on the end of the list,
 ## in this case getopts will not have anything to gobble as an
 ## argument to the option and will want to error out. To avoid this an
 ## empty string is appended to the argument array, yet in so doing
 ## care must be taken to manage this added empty string appropriately.
 ## As a result any option that doesn't exit at the time its processed
 ## needs to be made to accept an argument, otherwise you will never
 ## know if the option will be the last option sent thus having an empty
 ## string attached and causing it to land in the default handler.
function process_options() {
local OPTIND OPTERR=0 OPTARG OPTION h d r s M R S D
local ERROR_MSG=""  
local OPTION_VAL=""
local EXIT_VALUE=0
local -i NUM_OPTIONS
let NUM_OPTIONS=${#@}+1
while getopts “:h?d:DM:R:S:s:r:” OPTION "$@";
 do
     case "$OPTION" in
         h)
             help | more
             exit 0
             ;;
         r)
             OPTION_VAL=$(((${NUM_OPTIONS}==${OPTIND})) && trim_string "${OPTARG##*=}" || echo -En "${OPTARG##*=}")
             ERROR_MSG="Invalid input: Integer or floating point number required."
             if [[ -z "${OPTION_VAL}" ]]; then
               ## can set global flags here 
               :;
             elif [[ "${OPTION_VAL}" =~ ^-. ]]; then
               let OPTIND=${OPTIND}-1
               ## can set global flags here 
             elif [ "${OPTION_VAL}" = "0" ]; then
               ## can set global flags here 
               :;               
             elif (($(check_number "${OPTION_VAL}" "${CHECK_FLOAT}" "${ERROR_MSG}"))); then
               :; ## do something really useful here..               
             else
               echo "${ERROR_MSG}" 1>&2 && exit -1
             fi
             ;;
         d)
             OPTION_VAL=$(((${NUM_OPTIONS}==${OPTIND})) && trim_string "${OPTARG##*=}" || echo -En "${OPTARG##*=}")
             [[  ! -z "${OPTION_VAL}" && "${OPTION_VAL}" =~ ^-. ]] && let OPTIND=${OPTIND}-1            
             DEBUGMODE=1
             set -xuo pipefail
             ;;
         s)
             OPTION_VAL=$(((${NUM_OPTIONS}==${OPTIND})) && trim_string "${OPTARG##*=}" || echo -En "${OPTARG##*=}")
             if [[ ! -z "${OPTION_VAL}" && "${OPTION_VAL}" =~ ^-. ]]; then ## if you want a variable value that begins with a dash, escape it
               let OPTIND=${OPTIND}-1
             else
              GLOBAL_SCRIPT_VAR="${OPTION_VAL}"
                :; ## do more important things
             fi
             ;;
         M)  
             OPTION_VAL=$(((${NUM_OPTIONS}==${OPTIND})) && trim_string "${OPTARG##*=}" || echo -En "${OPTARG##*=}")
             ERROR_MSG=$(echo "Error - Invalid input: ${OPTION_VAL}, Integer required"\
                              "retry with an appropriate option argument.")
             if [[ -z "${OPTION_VAL}" ]]; then
               echo "${ERROR_MSG}" 1>&2 && exit -1
             elif [[ "${OPTION_VAL}" =~ ^-. ]]; then
               let OPTIND=${OPTIND}-1
               echo "${ERROR_MSG}" 1>&2 && exit -1
             elif (($(check_number "${OPTION_VAL}" "${CHECK_INTEGER}" "${ERROR_MSG}"))); then
             :; ## do something useful here
             else
               echo "${ERROR_MSG}" 1>&2 && exit -1
             fi
             ;;                      
         R)  
             OPTION_VAL=$(((${NUM_OPTIONS}==${OPTIND})) && trim_string "${OPTARG##*=}" || echo -En "${OPTARG##*=}")
             ERROR_MSG=$(echo "Error - Invalid option argument: ${OPTION_VAL},"\
                              "the value supplied to -R is expected to be a "\
                              "qualified path to a random character device.")            
             if [[ -z "${OPTION_VAL}" ]]; then
               echo "${ERROR_MSG}" 1>&2 && exit -1
             elif [[ "${OPTION_VAL}" =~ ^-. ]]; then
               let OPTIND=${OPTIND}-1
               echo "${ERROR_MSG}" 1>&2 && exit -1
             elif [[ -c "${OPTION_VAL}" ]]; then
               :; ## Instead of erroring do something useful here..  
             else
               echo "${ERROR_MSG}" 1>&2 && exit -1
             fi
             ;;                      
         S)  
             STATEMENT=$(((${NUM_OPTIONS}==${OPTIND})) && trim_string "${OPTARG##*=}" || echo -En "${OPTARG##*=}")
             ERROR_MSG="Error - Default text string to set cannot be empty."
             if [[ -z "${STATEMENT}" ]]; then
               ## Instead of erroring you could set a flag or do something else with your code here..  
             elif [[ "${STATEMENT}" =~ ^-. ]]; then ## if you want a statement that begins with a dash, escape it
               let OPTIND=${OPTIND}-1
               echo "${ERROR_MSG}" 1>&2 && exit -1
               echo "${ERROR_MSG}" 1>&2 && exit -1
             else
                :; ## do something even more useful here you can modify the above as well 
             fi
             ;;                      
         D)  
             ## Do something useful as long as it is an exit, it is okay to not worry about the option arguments 
             exit 0
             ;;          
         *)
             EXIT_VALUE=-1
             ;&                  
         ?)
             usage
             exit ${EXIT_VALUE}
             ;;
     esac
done
}

process_options "$@ " ## extra space, so getopts can find arguments  

You can always decide to differentiate the option with lowercase or uppercase.

However my idea is to call getopts twice and 1st time parse without arguments ignoring them (R) then 2nd time parse only that option with argument support (R:). The only trick is that OPTIND (index) needs to be changed during processing, as it keeps pointer to the current argument.

Here is the code:

#!/usr/bin/env bash
while getopts ":hd:R" arg; do
  case $arg in
    d) # Set directory, e.g. -d /foo
      dir=$OPTARG
      ;;
    R) # Optional level value, e.g. -R 123
      OI=$OPTIND # Backup old value.
      ((OPTIND--)) # Decrease argument index, to parse -R again.
      while getopts ":R:" r; do
        case $r in
          R)
            # Check if value is in numeric format.
            if [[ $OPTARG =~ ^[0-9]+$ ]]; then
              level=$OPTARG
            else
              level=1
            fi
          ;;
          :)
            # Missing -R value.
            level=1
          ;;
        esac
      done
      [ -z "$level" ] && level=1 # If value not found, set to 1.
      OPTIND=$OI # Restore old value.
      ;;
    \? | h | *) # Display help.
      echo "$0 usage:" && grep " .)\ #" $0
      exit 0
      ;;
  esac
done
echo Dir: $dir
echo Level: $level

Here are few tests for scenarios which works:

$ ./getopts.sh -h
./getopts.sh usage:
    d) # Set directory, e.g. -d /foo
    R) # Optional level value, e.g. -R 123
    \? | h | *) # Display help.
$ ./getopts.sh -d /foo
Dir: /foo
Level:
$ ./getopts.sh -d /foo -R
Dir: /foo
Level: 1
$ ./getopts.sh -d /foo -R 123
Dir: /foo
Level: 123
$ ./getopts.sh -d /foo -R wtf
Dir: /foo
Level: 1
$ ./getopts.sh -R -d /foo
Dir: /foo
Level: 1

Scenarios which doesn't work (so the code needs a bit of more tweaks):

$ ./getopts.sh -R 123 -d /foo
Dir:
Level: 123

More information about getopts usage can be found in man bash.

See also: Small getopts tutorial at Bash Hackers Wiki

I just ran into this myself and felt that none of the existing solutions were really clean. After working on it a bit and trying various things, I found that leveraging getopts SILENT mode with :) ... appears to have done the trick along with keeping OPTIND in sync.

---

usage: test.sh [-abst] [-r [DEPTH]] filename
*NOTE: -r (recursive) with no depth given means full recursion

#!/usr/bin/env bash

depth='-d 1'

while getopts ':abr:st' opt; do
    case "${opt}" in
        a) echo a;;
        b) echo b;;
        r) if [[ "${OPTARG}" =~ ^[0-9]+$ ]]; then
               depth="-d ${OPTARG}"
           else
               depth=
               (( OPTIND-- ))
           fi
           ;;
        s) echo s;;
        t) echo t;;
        :) [[ "${OPTARG}" = 'r' ]] && depth=;;
        *) echo >&2 "Invalid option: ${opt}"; exit 1;;
    esac
done
shift $(( OPTIND - 1 ))

filename="$1"
...

I think there are two way.

First is calandoa's answer, Using OPTIND and no silent mode.

Second is Using OPTIND and silent mode.

while getopts ":Rr:" name; do
    case ${name} in
        R)
            eval nextArg=\${$OPTIND}
            # check option followed by nothing or other option.
            if [[ -z ${nextArg} || $nextArg =~ ^-.* ]]; then
                level=1
            elif [[ $nextArg =~ ^[0-9]+$ ]]; then
                level=$nextArg
                OPTIND=$((OPTIND + 1))
            else
                level=1
            fi
            ;;
        r)
            # check option followed by other option.
            if [[ $OPTARG =~ ^-.* ]]; then
                OPTIND=$((OPTIND - 1))
                level2=2
            elif [[ $OPTARG =~ ^[0-9]+$ ]]; then
                level2="$OPTARG"
            else
                level2=2
            fi
            ;;
        :)
            # check no argument
            case $OPTARG in
                r)
                    level2=2
                    ;;
            esac
    esac
done

echo "Level 1 : $level"
echo "Level 2 : $level2"

All solutions presented so far put code in case ... in ... esac, but in my opinion, it would be much more natural to have a modified getopts command, thus I wrote this function:

EDIT:

Now, you can specify the type of an optional arg (see usage info).

Furthermore, instead of testing if $nextArg "looks like" an option(s) arg, the function now checks if $nextArg contains a letter from $optstring. This way, an option letter not contained in $optstring can be used as optional arg, as with getopts' mandatory args.

Latest changes:

• Fixed test if $nextArg is an option arg:
  Test if $nextArg begins with a dash.
  Without this test, optional args that contain a letter from $optstring are not recognised as such.
• Added regexp type specifier (see usage info).
• Fixed: 0 not recognised as optional arg specified to be an int.
• Simplified test if $nextArg is an int.
• Type specifier ::/.../: Use perl to test if $nextArg matches the regexp.
  This way, you benefit from (almost (*)) the full power of Perl regexps.
  (*): See last paragraph of usage info.
• Fixed: Doesn't work with more than one regexp type specifier:
  Use perl instead of grep/sed constructs because non-greedy matching is needed.

Usage:

Invocation: getopts-plus optstring name "$@"

optstring: Like normal getopts, but you may specify options with optional argument by appending :: to the option letter.

However, if your script supports an invocation with an option with optional argument as the only option argument, followed by a non-option argument, the non-option argument will be considered to be the argument for the option.

If you're lucky and the optional argument is expected to be an integer, whereas the non-option argument is a string or vice versa, you may specify the type by appending :::i for an integer or :::s for a string to solve that issue.

If that doesn't apply, you may specify a Perl regexp for the optional arg by appending ::/.../ to the option letter.
See here for an introduction to Perl regexps: https://perldoc.perl.org/perlretut
Please note: ATM, only /.../ will be recognised as a regexp after ::, i. e. neither other delimiters, nor modifiers may be used, so e. g. m#...#a will not be recognised.
If there is a non-option argument after the option with optional argument, it will be considered to be the optional argument only if it matches the regexp.
To be clear: ::/.../ is not meant for argument validation but solely to discriminate between arguments for options with optional argument and non-option arguments.

#!/bin/bash

# Invocation: getopts-plus optstring name "$@"\
# \
# optstring: Like normal getopts, but you may specify options with optional argument
# by appending :: to the option letter.\
# \
# However, if your script supports an invocation with an option with optional
# argument as the only option argument, followed by a non-option argument,
# the non-option argument will be considered to be the argument for the option.\
# \
# If you're lucky and the optional argument is expected to be an integer, whereas
# the non-option argument is a string or vice versa, you may specify the type by
# appending :::i for an integer or :::s for a string to solve that issue.\
# \
# If that doesn't apply, you may specify a Perl regexp for the optional arg by appending
# ::/.../ to the option letter.\
# See here for an introduction to Perl regexps: https://perldoc.perl.org/perlretut
# Please note: ATM, only /.../ will be recognised as a regexp after ::,\
# i. e. neither other delimiters, nor modifiers may be used, so e. g. m#...#a will
# not be recognised.\
# If there is a non-option argument after the option with optional argument, it will
# be considered to be the optional argument only if it matches the regexp.\
# To be clear: ::/.../ is not meant for argument validation but solely to discriminate
# between arguments for options with optional argument and non-option arguments.
function getopts-plus
{
	local optstring=$1
	local -n name=$2

	shift 2

	local optionalArgSuffixRE='::(?::[si]|/.*?/)?'
	local optionalArgTypeCaptureRE=':::([si])|::(/.*?/)'

	# If we pass 'opt' for 'name' (as I always do when using getopts) and there is
	# also a local variable 'opt', the "outer" 'opt' will always be empty.
	# I don't understand why a local variable interferes with caller's variable with
	# same name in this case; however, we can easily circumvent this.
	local opt_

	# Extract options with optional arg

	local -A isOptWithOptionalArg

	while read opt_; do
		# Using an associative array as set
		isOptWithOptionalArg[$opt_]=1
	done <<<$(perlGetCaptures "$optstring" "([a-zA-Z])$optionalArgSuffixRE")

	# Extract all option letters (used to weed out possible optional args that are option args)
	local optLetters=$(perlGetCaptures "$optstring" "([a-zA-Z])(?:$optionalArgSuffixRE|:)?")

	# Save original optstring, then remove our suffix(es)
	local optstringOrg=$optstring
	optstring=$(perl -pe "s#$optionalArgSuffixRE##g" <<<$optstring)

	getopts $optstring name "$@" || return # Return value is getopts' exit value.

	# If current option is an option with optional arg and if an arg has been provided,
	# check if that arg is not an option and if it isn't, check if that arg matches(*)
	# the specified type, if any, and if it does or no type has been specified,
	# assign it to OPTARG and inc OPTIND.
	#
	# (*) We detect an int because it's easy, but we assume a string if it's not an int
	# because detecting a string would be complicated.
	# So it sounds strange to call it a match if we know that the optional arg is specified
	# to be a string, but merely that the provided arg is not an int, but in this context,
	# "not an int" is equivalent to "string". At least I think so, but I might be wrong.

	if ((isOptWithOptionalArg[$name])) && [[ ${!OPTIND} ]]; then
		local nextArg=${!OPTIND} foundOpt=0

		# Test if $nextArg is an option arg
		if [[ $nextArg == -* ]]; then
			# Check if $nextArg contains a letter from $optLetters.
			# This way, an option not contained in $optstring can be
			# used as optional arg, as with getopts' mandatory args.

			local i

			# Start at char 1 to skip the leading dash
			for ((i = 1; i < ${#nextArg}; i++)); do
				while read opt_; do
					[[ ${nextArg:i:1} == $opt_ ]] && foundOpt=1 && break 2
				done <<<$optLetters
			done

			((foundOpt)) && return
		fi

		# Extract type of optional arg if specified
		local optArgType=$(perlGetCaptures "$optstringOrg" "$name(?:$optionalArgTypeCaptureRE)" '$1$2')

		local nextArgIsOptArg=0

		case $optArgType in
			/*/) # Check if $nextArg matches regexp
				perlMatch "$nextArg" "$optArgType" && nextArgIsOptArg=1
				;;
			[si]) # Check if $nextArg is an int
				local nextArgIsInt=0

				[[ $nextArg =~ ^[0-9]+$ ]] && nextArgIsInt=1

				# Test if specified type and arg type match (see (*) above).
				# N.B.: We need command groups since && and || between commands have same precedence.
				{ [[ $optArgType == i ]] && ((nextArgIsInt)) || { [[ $optArgType == s ]] && ((! nextArgIsInt)); }; } && nextArgIsOptArg=1
				;;
			'') # No type or regexp specified => Assume $nextArg is optional arg.
				nextArgIsOptArg=1
				;;
		esac

		if ((nextArgIsOptArg)); then
			OPTARG=$nextArg && ((OPTIND++))
		fi
	fi
}

# Uses perl to match \<string\> against \<regexp\>.\
# Returns with code 0 on a match and 1 otherwise.
function perlMatch # Args: <string> <regexp>
{
	perl -e 'q('"$1"') =~ '"$2"' and exit 0; exit 1;'
}

# Uses perl to match \<string\> against \<regexp\>
# and prints each capture on a separate line.\
# If \<regexp\> contains more than one capture group,
# you must specify the \<line format\> which is an
# arbitrary Perl string containing your desired backrefs.\
# By default, merely $1 will be printed.
function perlGetCaptures # Args: <string> <regexp> [<line format>]
{
	local lineFmt=${3:-\$1}

	# Matching repeatedly with g option gives one set of captures at a time.
	perl -e 'while (q('"$1"') =~ m#'"$2"'#g) { print(qq('"$lineFmt"') . "\n"); }'
}

The same script without comments inside function bodies in case you don't need them:

#!/bin/bash

# Invocation: getopts-plus optstring name "$@"\
# \
# optstring: Like normal getopts, but you may specify options with optional argument
# by appending :: to the option letter.\
# \
# However, if your script supports an invocation with an option with optional
# argument as the only option argument, followed by a non-option argument,
# the non-option argument will be considered to be the argument for the option.\
# \
# If you're lucky and the optional argument is expected to be an integer, whereas
# the non-option argument is a string or vice versa, you may specify the type by
# appending :::i for an integer or :::s for a string to solve that issue.\
# \
# If that doesn't apply, you may specify a Perl regexp for the optional arg by appending
# ::/.../ to the option letter.\
# See here for an introduction to Perl regexps: https://perldoc.perl.org/perlretut
# Please note: ATM, only /.../ will be recognised as a regexp after ::,\
# i. e. neither other delimiters, nor modifiers may be used, so e. g. m#...#a will
# not be recognised.\
# If there is a non-option argument after the option with optional argument, it will
# be considered to be the optional argument only if it matches the regexp.\
# To be clear: ::/.../ is not meant for argument validation but solely to discriminate
# between arguments for options with optional argument and non-option arguments.
function getopts-plus
{
	local optstring=$1
	local -n name=$2

	shift 2

	local optionalArgSuffixRE='::(?::[si]|/.*?/)?'
	local optionalArgTypeCaptureRE=':::([si])|::(/.*?/)'

	local opt_

	local -A isOptWithOptionalArg

	while read opt_; do
		isOptWithOptionalArg[$opt_]=1
	done <<<$(perlGetCaptures "$optstring" "([a-zA-Z])$optionalArgSuffixRE")

	local optLetters=$(perlGetCaptures "$optstring" "([a-zA-Z])(?:$optionalArgSuffixRE|:)?")

	local optstringOrg=$optstring
	optstring=$(perl -pe "s#$optionalArgSuffixRE##g" <<<$optstring)

	getopts $optstring name "$@" || return

	if ((isOptWithOptionalArg[$name])) && [[ ${!OPTIND} ]]; then
		local nextArg=${!OPTIND} foundOpt=0

		if [[ $nextArg == -* ]]; then
			local i

			for ((i = 1; i < ${#nextArg}; i++)); do
				while read opt_; do
					[[ ${nextArg:i:1} == $opt_ ]] && foundOpt=1 && break 2
				done <<<$optLetters
			done

			((foundOpt)) && return
		fi

		local optArgType=$(perlGetCaptures "$optstringOrg" "$name(?:$optionalArgTypeCaptureRE)" '$1$2')

		local nextArgIsOptArg=0

		case $optArgType in
			/*/)
				perlMatch "$nextArg" "$optArgType" && nextArgIsOptArg=1
				;;
			[si])
				local nextArgIsInt=0

				[[ $nextArg =~ ^[0-9]+$ ]] && nextArgIsInt=1

				{ [[ $optArgType == i ]] && ((nextArgIsInt)) || { [[ $optArgType == s ]] && ((! nextArgIsInt)); }; } && nextArgIsOptArg=1
				;;
			'')
				nextArgIsOptArg=1
				;;
		esac

		if ((nextArgIsOptArg)); then
			OPTARG=$nextArg && ((OPTIND++))
		fi
	fi
}

# Uses perl to match \<string\> against \<regexp\>.\
# Returns with code 0 on a match and 1 otherwise.
function perlMatch # Args: <string> <regexp>
{
	perl -e 'q('"$1"') =~ '"$2"' and exit 0; exit 1;'
}

# Uses perl to match \<string\> against \<regexp\>
# and prints each capture on a separate line.\
# If \<regexp\> contains more than one capture group,
# you must specify the \<line format\> which is an
# arbitrary Perl string containing your desired backrefs.\
# By default, merely $1 will be printed.
function perlGetCaptures # Args: <string> <regexp> [<line format>]
{
	local lineFmt=${3:-\$1}

	perl -e 'while (q('"$1"') =~ m#'"$2"'#g) { print(qq('"$lineFmt"') . "\n"); }'
}

Some tests using the latest version:

Optional arg type of -g specified as integer, no int passed but followed by a non-option string arg.

$ . ./getopts-plus.sh
$ while getopts-plus 'b:c::de::f::g:::ia' opt -ab 99 -c 11 -def 55 -g "hello you"; do e opt OPTARG; echo; printf "%.0s-" $(seq 1 25); echo -e "\n"; done

opt == 'a'

OPTARG == ''

-------------------------

opt == 'b'

OPTARG == '99'

-------------------------

opt == 'c'

OPTARG == '11'

-------------------------

opt == 'd'

OPTARG == ''

-------------------------

opt == 'e'

OPTARG == ''

-------------------------

opt == 'f'

OPTARG == '55'

-------------------------

opt == 'g'

OPTARG == '' <-- Empty because "hello you" is not an int

Like above, but with int arg.

$ OPTIND=1
$ while getopts-plus 'b:c::de::f::g:::ia' opt -ab 99 -c 11 -def 55 -g 7 "hello you"; do e opt OPTARG; echo; printf "%.0s-" $(seq 1 25); echo -e "\n"; done

opt == 'a'

OPTARG == ''

-------------------------

opt == 'b'

OPTARG == '99'

-------------------------

opt == 'c'

OPTARG == '11'

-------------------------

opt == 'd'

OPTARG == ''

-------------------------

opt == 'e'

OPTARG == ''

-------------------------

opt == 'f'

OPTARG == '55'

-------------------------

opt == 'g'

OPTARG == '7' <-- The passed int

Added optional option -h with regexp /^(a|b|ab|ba)$/, no arg passed.

$ OPTIND=1
$ while getopts-plus 'b:c::de::f::g:::ih::/^(a|b|ab|ba)$/a' opt -ab 99 -c 11 -def 55 -gh "hello you"; do e opt OPTARG; echo; printf "%.0s-" $(seq 1 25); echo -e "\n"; done

opt == 'a'

OPTARG == ''

-------------------------

opt == 'b'

OPTARG == '99'

-------------------------

opt == 'c'

OPTARG == '11'

-------------------------

opt == 'd'

OPTARG == ''

-------------------------

opt == 'e'

OPTARG == ''

-------------------------

opt == 'f'

OPTARG == '55'

-------------------------

opt == 'g'

OPTARG == ''

-------------------------

opt == 'h'

OPTARG == '' <-- Empty because "hello you" does not match the regexp

Like above, but with an arg matching the regexp.

$ OPTIND=1
$ while getopts-plus 'b:c::de::f::g:::ih::/^(a|b|ab|ba)$/a' opt -ab 99 -c 11 -def 55 -gh ab "hello you"; do e opt OPTARG; echo; printf "%.0s-" $(seq 1 25); echo -e "\n"; done

opt == 'a'

OPTARG == ''

-------------------------

opt == 'b'

OPTARG == '99'

-------------------------

opt == 'c'

OPTARG == '11'

-------------------------

opt == 'd'

OPTARG == ''

-------------------------

opt == 'e'

OPTARG == ''

-------------------------

opt == 'f'

OPTARG == '55'

-------------------------

opt == 'g'

OPTARG == ''

-------------------------

opt == 'h'

OPTARG == 'ab' <-- The arg that matches the regexp

Added another regexp-typed optional option -i with regexp /^\w+$/ (using the Perl token \w which means alphanumeric or underscore), no arg passed.

$ OPTIND=1
$ while getopts-plus 'b:c::de::f::g:::ih::/^(a|b|ab|ba)$/ai::/^\w+$/' opt -ab 99 -c 11 -def 55 -gh ab -i "hello you"; do e opt OPTARG; echo; printf "%.0s-" $(seq 1 25); echo -e "\n"; done
[23:10:49]

opt == 'a'

OPTARG == ''

-------------------------

opt == 'b'

OPTARG == '99'

-------------------------

opt == 'c'

OPTARG == '11'

-------------------------

opt == 'd'

OPTARG == ''

-------------------------

opt == 'e'

OPTARG == ''

-------------------------

opt == 'f'

OPTARG == '55'

-------------------------

opt == 'g'

OPTARG == ''

-------------------------

opt == 'h'

OPTARG == 'ab'

-------------------------

opt == 'i'

OPTARG == '' <-- Empty because "hello you" contains a space.

Like above, but with an arg matching the regexp.

$ OPTIND=1
$ while getopts-plus 'b:c::de::f::g:::ih::/^(a|b|ab|ba)$/ai::/^\w+$/' opt -ab 99 -c 11 -def 55 -gh ab -i foo_Bar_1 "hello you"; do e opt OPTARG; echo; printf "%.0s-" $(seq 1 25); echo -e "\n"; done
[23:15:23]

opt == 'a'

OPTARG == ''

-------------------------

opt == 'b'

OPTARG == '99'

-------------------------

opt == 'c'

OPTARG == '11'

-------------------------

opt == 'd'

OPTARG == ''

-------------------------

opt == 'e'

OPTARG == ''

-------------------------

opt == 'f'

OPTARG == '55'

-------------------------

opt == 'g'

OPTARG == ''

-------------------------

opt == 'h'

OPTARG == 'ab'

-------------------------

opt == 'i'

OPTARG == 'foo_Bar_1' <-- Matched because it contains only alphanumeric chars and underscores.

Old thread, but figured I'd share what I did anyway (which is also mostly older than this thread). I got fed up with trying to make getopt do what I wanted, and in a fit of frustration did this so that both short and long opts with optional arguments are supported. It's the long long way, somebody is sure to laugh, but it works exactly how I want it to - overly commented examples of all three cases are below:

#!/usr/bin/bash
# Begin testme.sh

shopt -s extglob;
VERSION="1.0"

function get_args(){
  while test -n "${1}" ; do
    case "${1}" in
      -a | --all)
         # dumb single argument example
         PROCESS_ALL="yes"
         shift 1
      ;;
      -b | --buildnum)
         # requires a second argument so use check_arg() below
         check_arg $1 $2
         BUILD_NUM=${2}
         shift 2
      ;;
      -c | --cache)
        # Example where argument is not required, don't use check_arg()
        if [ echo ${2} | grep -q "^-" ]; then
          # no argument given, use default cache value
          CACHEDIR=~/mycachedir
          # Note: this could have been set upon entering the script
          #       and used the negative above as well
          shift 1
        else
          cache=${2}
          shift 2
        fi
      ;;
      -h | --help)
        showhelp
        exit 0
      ;;
      -v | --version)
        echo -e "$(basename ${0}) ${VERSION}\n"
        exit 0
      ;;
      # Handle getopt style short args (reason for shopts above)
      -+([a-z,A-Z]))
        # split up the arguments and call recursively with trailing break
        arg="${1}"
        newargs=$( echo ${1} | sed 's@-@@' | \
                               sed 's/.\{1\}/& /g' | \
                               sed 's/[^ ]* */-&/g')
        newargs="${newargs} $(echo ${@} | sed "s@${arg}@@")"
        get_args ${newargs}
        break;
      ;;
      *)
        echo -e "Invalid argument ${1}!\n\n"
        showhelp
        exit 1
      ;;
    esac
  done
}

# Super lazy, but I didn't want an if/then/else for every required arg
function check_arg(){
  if [ echo "${2}" | grep -q "^-" ]; then
    echo "Error:  $1 requires a valid argument."
    exit 1
  fi
}

function showhelp(){
  echo ""
  echo "`basename ${0}` is a utility to..."
}

# Process command line arguments
get_args $@
...
# End testme.sh

I've never run into it, but I suppose there might be a case where I'd need the second argument to begin with a '-' character, in which case, I'd strip it out before calling get_args(). I have used with with fixed position arguments as well, and in that case they are at the end, but same solution. Also, I suppose a portable version could just handle the combined short args in *), but I figure if bash is too heavy a requirement, the you are on your own.