getopts for Parsing Command-Line Flags

#!/bin/bash
set -- -v -f myfile.txt

while getopts "vf:" opt; do
    case $opt in
        v) echo "verbose mode on" ;;
        f) echo "file argument: $OPTARG" ;;
    esac
done

#!/bin/bash
set -- -n "Ada" -c 3

while getopts "n:c:" opt; do
    case $opt in
        n) name=$OPTARG ;;
        c) count=$OPTARG ;;
    esac
done

echo "name=$name count=$count"

#!/bin/bash
set -- -x

while getopts ":vf:" opt; do
    case $opt in
        v) echo "verbose" ;;
        f) echo "file: $OPTARG" ;;
        \?) echo "unknown option: -$OPTARG" ;;
        :) echo "option -$OPTARG requires an argument" ;;
    esac
done

#!/bin/bash
set -- -v file1.txt file2.txt

while getopts "v" opt; do
    case $opt in
        v) verbose=1 ;;
    esac
done
shift $((OPTIND - 1))

echo "remaining arguments: $*"

1. Forgetting the leading colon in the option string (e.g. :vf:) disables getopts's default error messages in favor of manual handling via the ? and : cases.
2. Not resetting OPTIND to 1 if getopts is used more than once in the same shell session (e.g. across multiple function calls), causing later calls to skip arguments.
3. Forgetting a colon after an option letter that takes an argument (like f:), which makes getopts think -f takes no argument.

• getopts "optstring" name parses flags one at a time from the positional parameters, in a loop.
• A colon after a letter in the optstring (e.g. f:) means that option requires an argument, stored in $OPTARG.
• A leading colon in the optstring enables silent error handling: ? for an unknown option, : for a missing required argument.
• After the loop, shift $((OPTIND - 1)) removes the parsed options, leaving only the remaining positional arguments.