An example of how to use getopts in bash

Question

I want to call myscript file in this way:

$ ./myscript -s 45 -p any_string 

or

$ ./myscript -h  #should display help $ ./myscript     #should display help 

My requirements are:

• getopt here to get the input arguments
• check that -s exists, if not return an error
• check that the value after the -s is 45 or 90
• check that the -p exists and there is an input string after
• if the user enters ./myscript -h or just ./myscript then display help

I tried so far this code:

#!/bin/bash while getopts "h:s:" arg; do   case $arg in     h)       echo "usage"        ;;     s)       strength=$OPTARG       echo $strength       ;;   esac done 

But with that code I get errors. How to do it with Bash and getopt?

Answer 1

#!/bin/bash  usage() { echo "Usage: $0 [-s <45|90>] [-p <string>]" 1>&2; exit 1; }  while getopts ":s:p:" o; do     case "${o}" in         s)             s=${OPTARG}             ((s == 45 || s == 90)) || usage             ;;         p)             p=${OPTARG}             ;;         *)             usage             ;;     esac done shift $((OPTIND-1))  if [ -z "${s}" ] || [ -z "${p}" ]; then     usage fi  echo "s = ${s}" echo "p = ${p}" 

Example runs:

$ ./myscript.sh Usage: ./myscript.sh [-s <45|90>] [-p <string>]  $ ./myscript.sh -h Usage: ./myscript.sh [-s <45|90>] [-p <string>]  $ ./myscript.sh -s "" -p "" Usage: ./myscript.sh [-s <45|90>] [-p <string>]  $ ./myscript.sh -s 10 -p foo Usage: ./myscript.sh [-s <45|90>] [-p <string>]  $ ./myscript.sh -s 45 -p foo s = 45 p = foo  $ ./myscript.sh -s 90 -p bar s = 90 p = bar 

Answer 2

The problem with the original code is that:

• h: expects parameter where it shouldn't, so change it into just h (without colon)
• to expect -p any_string, you need to add p: to the argument list

Basically : after the option means it requires the argument.

---

The basic syntax of getopts is (see: man bash):

getopts OPTSTRING VARNAME [ARGS...] 

where:

• OPTSTRING is string with list of expected arguments,

  • h - check for option -h without parameters; gives error on unsupported options;
  • h: - check for option -h with parameter; gives errors on unsupported options;
  • abc - check for options -a, -b, -c; gives errors on unsupported options;

  • :abc - check for options -a, -b, -c; silences errors on unsupported options;

    ^{Notes: In other words, colon in front of options allows you handle the errors in your code. Variable will contain ? in the case of unsupported option, : in the case of missing value.}

• OPTARG - is set to current argument value,

• OPTERR - indicates if Bash should display error messages.

So the code can be:

#!/usr/bin/env bash usage() { echo "$0 usage:" && grep " .)\ #" $0; exit 0; } [ $# -eq 0 ] && usage while getopts ":hs:p:" arg; do   case $arg in     p) # Specify p value.       echo "p is ${OPTARG}"       ;;     s) # Specify strength, either 45 or 90.       strength=${OPTARG}       [ $strength -eq 45 -o $strength -eq 90 ] \         && echo "Strength is $strength." \         || echo "Strength needs to be either 45 or 90, $strength found instead."       ;;     h | *) # Display help.       usage       exit 0       ;;   esac done 

Example usage:

$ ./foo.sh  ./foo.sh usage:     p) # Specify p value.     s) # Specify strength, either 45 or 90.     h | *) # Display help. $ ./foo.sh -s 123 -p any_string Strength needs to be either 45 or 90, 123 found instead. p is any_string $ ./foo.sh -s 90 -p any_string Strength is 90. p is any_string 

See: Small getopts tutorial at Bash Hackers Wiki