1. Notice
1.1 getopts is different from getopt.
1.2 getopts runs on any system running bash in POSIX mode (e.g., set -o posix), getopt is a system tool varies in different systems.
1.3 getopts parses short options, which are a single dash ("-") and a letter or digit.e.g. -2, -d, and -D. It can also parse short options in combination, for instance -2dD. But each option will be treated seperately, i.e. -2dD = -2 -d -D not equal -“2dD”, will need getopt to cope with -“2dD”.
2. Syntax
getopts optstring optname [ arg ]
2.1 Every time you run getopts, it looks for one of the options defined in optstring. If it finds one, it places the option letter in a variable named optname. If the option does not match those defined in optstring, getopts sets variable optname to a question mark ("?"), will need \? in switch ... case.
2.2 If the option is expecting an argument, getopts gets that argument, and places it in $OPTARG. If an expected argument is not found, the variable optname is set to a colon ( “:” ). Getopts then increments the positional index, $OPTIND, that indicates the next option to be processed, don’t need \: in switch ... case.
2.3 However, if you put a colon at the beginning of the optstring, getopts runs in “silent error checking mode.” It will not report any verbose errors about options or arguments, and you need to perform error checking in your script.
2.4 In silent mode, if an option is unexpected, getopts sets optname to “?” and $OPTARG to the unknown option character.
2.5 If the option is OK but an expected argument is not found, optname is set to a colon ( “:” ) and $OPTARG is set to the unknown option character.
2.6 \? needs to appear before : in switch ... case for error checking, e.g.
while getopts ":hi:" option; do
case "$option" in
h)
echo "help function"
;;
i)
ilist=$OPTARG
;;
\?)
echo $option
echo "Invalid option"
;;
:)
echo $option
echo "no argument"
;;
esac
done
2.7 The special option of two dashes ("–") is interpreted by getopts as the end of options.
3. Reference(s)
“Bash Getopts Builtin Command Help and Examples.” Help and Examples, 1 Feb. 2021, https://www.computerhope.com/unix/bash/getopts.htm.