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submitted 2 weeks ago* (last edited 2 weeks ago) by loveknight@programming.dev to c/shell@programming.dev

This is a little tutorial that I found in my search to learn how to use getopt (mind: not getopts, which is a completely different thing). I want to share it here because I find it refreshingly to the point. Just the main code block already tells almost the whole story:

#!/bin/bash
# Set some default values:
ALPHA=unset
BETA=unset
CHARLIE=unset
DELTA=unset

usage()
{
  echo "Usage: alphabet [ -a | --alpha ] [ -b | --beta ]
                        [ -c | --charlie CHARLIE ] 
                        [ -d | --delta   DELTA   ] filename(s)"
  exit 2
}

PARSED_ARGUMENTS=$(getopt -a -n alphabet -o abc:d: --long alpha,bravo,charlie:,delta: -- "$@")
VALID_ARGUMENTS=$?
if [ "$VALID_ARGUMENTS" != "0" ]; then
  usage
fi

echo "PARSED_ARGUMENTS is $PARSED_ARGUMENTS"
eval set -- "$PARSED_ARGUMENTS"
while :
do
  case "$1" in
    -a | --alpha)   ALPHA=1      ; shift   ;;
    -b | --beta)    BETA=1       ; shift   ;;
    -c | --charlie) CHARLIE="$2" ; shift 2 ;;
    -d | --delta)   DELTA="$2"   ; shift 2 ;;
    # -- means the end of the arguments; drop this, and break out of the while loop
    --) shift; break ;;
    # If invalid options were passed, then getopt should have reported an error,
    # which we checked as VALID_ARGUMENTS when getopt was called...
    *) echo "Unexpected option: $1 - this should not happen."
       usage ;;
  esac
done

echo "ALPHA   : $ALPHA"
echo "BETA    : $BETA "
echo "CHARLIE : $CHARLIE"
echo "DELTA   : $DELTA"
echo "Parameters remaining are: $@"

Just be sure to correct the inadvertent mixing of beta and bravo.

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[-] sjohannes@programming.dev 1 points 1 week ago* (last edited 1 week ago)

The article mentions "GNU getopt" but as far as I know GNU doesn't have a getopt utility; the version on most Linux OSes comes from util-linux. (Perhaps the author is confusing it with the C function with the same name.)

Note that other getopt implementations have different features and some are simply broken. For example, BSD getopt doesn't support long options and comes with this known bug:

Arguments containing whitespace or embedded shell metacharacters generally will not survive intact; this looks easy to fix but isn't.

For cross-platform scripts it's probably best to use the getopts shell builtin instead, the downside being it only supports fairly basic (POSIX) syntax even on Bash.

this post was submitted on 22 Jul 2025
20 points (100.0% liked)

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