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Processing Shell Script Options with the getopts Command

The getopts command simplifies the task of validating and parsing command line options and arguments for your shell scripts.  Without this built-in command, writing shell script code to perform these common tasks could quickly become a complex and lengthy undertaking.

The syntax for the getopts command is as follows:

getopts option-list variable

option-list contains valid options for the script, and variable is set to the option if it is found in option-list.  If the option is not in option-list, variable is set to the "?" character.

The following pet of code illustrates a basic implementation of the command:

#!/bin/ksh

PROG_NAME=$(basename $0)
A_FLAG=FALSE
B_FLAG=FALSE

while getopts abl: OPTION
do
    case ${OPTION} in
        a) A_FLAG=TRUE;;
        b) B_FLAG=TRUE;;
        l) LOGFILE=${OPTARG};;
      \?) print -u2 "Usage: ${PROG_NAME} [ -a -b -l logfile_name ]"
           exit 2;;
    esac
done

print ${A_FLAG}
print ${B_FLAG}
print ${LOGFILE}

There are a few key points you should understand about getopts and this example:

- getopts is designed to be executed inside of a loop, processing one option each iteration.  Once getopts processes the last option, it returns a non-zero value and the loop terminates.

- options requiring an argument ("l" in this example) need to be followed by a ":" (colon) on the getopts command line.

- option arguments are stored in the special variable OPTARG for script processing.

- if an invalid option is used, correct usage will be displayed and the script will exit with a return code of 2.

Here is a sample of the output for the above code (the script is named getopts_sample):

# ./getopts_sample -a -b -l testlog.txt
TRUE
TRUE
testlog.txt

You may want to run the above code on your system to see what happens when an invalid option is used.

The previous has been a basic introduction to the getopts command.  A more detailed understanding can be obtained by reviewing your shell's man page.