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GETOPT(1) General Commands Manual GETOPT(1)

parse command options

getopt optstring $*

getopt is used to break up options in command lines for easy parsing by shell procedures, and to check for legal options. optstring is a string of recognized option letters (see getopt(3)); if a letter is followed by a colon, the option is expected to have an argument which may or may not be separated from it by whitespace. However, if a letter is followed by two colons, the argument is optional and may not be separated by whitespace - this is an extension not covered by POSIX. The special option ‘--’ is used to delimit the end of the options. getopt will place ‘--’ in the arguments at the end of the options, or recognize it if used explicitly. The shell arguments ($1, $2, ...) are reset so that each option is preceded by a ‘-’ and in its own shell argument; each option argument is also in its own shell argument.

The following code fragment shows how one might process the arguments for a command that can take the options -a and -b, and the option -o, which requires an argument.
args=`getopt abo: $*` 
if [ $? -ne 0 ] 
	echo 'Usage: ...' 
	exit 2 
set -- $args 
while [ $# -ne 0 ] 
	case "$1" 
			flag="$1"; shift;; 
			oarg="$2"; shift; shift;; 
			shift; break;; 
This code will accept any of the following as equivalent:
cmd -aoarg file file 
cmd -a -o arg file file 
cmd -oarg -a file file 
cmd -a -oarg -- file file

getopt prints an error message on the standard error output when it encounters an option letter not included in optstring.

sh(1), getopt(3)

Written by Henry Spencer, working from a Bell Labs manual page. Behavior believed identical to the Bell version.

Note that the construction
set -- `getopt optstring $*`
is not recommended, as the exit value from set will prevent the exit value from getopt from being determined.

Whatever getopt(3) has.
Arguments containing whitespace or embedded shell metacharacters generally will not survive intact; this looks easy to fix but isn't.
The error message for an invalid option is identified as coming from getopt rather than from the shell procedure containing the invocation of getopt; this again is hard to fix.
The precise best way to use the set command to set the arguments without disrupting the value(s) of shell options varies from one shell version to another.
March 16, 2018 OpenBSD-current