Diff for "BashFAQ/101"

How do I process options in a bash script?

For example, how do I code my bash script to accept a bunch of options like

foobar -a --include something

First up, there are some GNU and POSIX standards for how to do this.

do-it-yourself

while "$1"; do
    case "$1" in
        -a|--all) ALL=yes ;shift ;;
        -i|--include) INCLUDE="$2"; shift; shift ;;
        -*) echo "$PROG: Bad option '$1', exiting." >&2; exit 1;;
        *) break;;
    esac
done
echo args="$@"

This is all well and good and probably portable everywhere - but it's crufty and it doesn't honour the standards. For example, how do you handle concatenation of single-letter options? What if the user writes -isomething without a space or --include=something? What about '--' or inter-mixing options and arguments? It's all a lot harder than it looks!!

getopt(1)

This example refers to GNU getopt(1), part of the util-linux-ng package. As for other systems - please add info here!!

TEMP=`getopt -o ai: --long all,include: -n 'example.bash' -- "$@"`

if [ $? != 0 ] ; then echo "Terminating..." >&2 ; exit 1 ; fi

eval set -- "$TEMP"

while true ; do
    case "$1" in
        -a|--all) echo "Option a" ; shift ;;
        -i|--include) echo "Option i = $2"; shift; shift ;;
        --) break;;
    esac
done

echo args="$@"

This is better as it obeys the standards and gives the user a fairly predictable user-interface. There is still the disadvantage that options are coded in at least 2, probably 3 places - in the call to getopt(1), in the case statement that processes them and presumably in the help message that you are going to get around to writing one of these days. This is a classic opportunity for errors to creep in as the code is written and maintained - often not discovered till much, much later. You also need to remember to "double shift" options that accept values (eg --include above). Finally, getopt(1) provides no way to print help for the calling script.

process-getopt(1)

This depends on bash-2.04 or later and GNU getopt(1) as above.

process-getopt is a GPL script that you can download and install somewhere - in this case, it's in /usr/bin:

PROG=$(basename $0)
VERSION='1.2'
USAGE="A tiny example using process-getopt(1)"

# call process-getopt functions to define some options:
source /usr/bin/process-getopt

SLOT=""
SLOT_func()   { [ "${1:-""}" ] && SLOT="yes"; }      # callback for SLOT option
add_opt SLOT "boolean option" s "" slot

TOKEN=""
TOKEN_func()  { [ "${1:-""}" ] && TOKEN="$2"; }      # callback for TOKEN option
add_opt TOKEN "this option takes a value" t n token number

add_std_opts     # define the standard options --help etc:

TEMP=$(call_getopt "$@") || exit 1
eval set -- "$TEMP" # just as with getopt(1)

# remove the options from the command line
process_opts "$@" || shift "$?"

echo "SLOT=$SLOT"
echo "TOKEN=$TOKEN"
echo "args=$@"

Here, all information about each option is defined in one place making for much easier authoring and maintenance. A lot of the dirty work is handled automatically and standards are obeyed as in getopt(1) - because it calls getopt for you. Options and any option arguments are popped off the command line so that you end up with a "$@" with just the non-option arguments - no need to remember to shift them off. '--' is handled automatically. As an added bonus you get a nicely formatted help page (for  -h, --help ) and a starter for a man page (using an easter-egg option  --print-man-page ).

process-getopt at sourceforge

author's website

Linux Gazette article on process-getopt

Full disclosure: bhepple at freeshell dot org wrote this entry and is the author of process-getopt(1). Constructive criticism to that email address is most welcome!!

CategoryShell