How to parse command line options in Bash with getopts

Named options make a shell script easier to automate because callers do not have to remember a fragile positional order. The Bash getopts builtin handles short options one at a time while leaving later file operands available to the script.

Each successful getopts call stores the option letter in a variable, places a required option value in OPTARG, and advances OPTIND to the next argument. A leading colon in the option string enables silent error handling, so the script can print a specific message for an unknown option or a missing value.

The parser accepts -n NAME for a required archive label, -d for dry-run mode, and -h for help. A -- marker ends option parsing explicitly, which allows a remaining file operand such as -old.log to begin with a hyphen.

1. Create archive-plan.sh with its interpreter line, usage function, and default values.

   archive-plan.sh

   #!/usr/bin/env bash
   set -u
    
   usage() {
       printf 'Usage: %s -n NAME [-d] [FILE...]\n' "${0##*/}"
   }
    
   name=
   dry_run=0

   The name variable starts empty because -n is required, while dry_run starts at 0 because -d is optional.

2. Add the getopts loop after the default values.

   while getopts ':n:dh' option; do
       case $option in
           n) name=$OPTARG ;;
           d) dry_run=1 ;;
           h) usage; exit 0 ;;
           :) printf 'Missing value for -%s\n' "$OPTARG" >&2; usage >&2; exit 2 ;;
           \?) printf 'Unknown option: -%s\n' "$OPTARG" >&2; usage >&2; exit 2 ;;
       esac
   done

   In :n:dh, the first colon enables silent error handling and the colon after n marks -n as requiring a value. The d and h options do not take values.

3. Add the operand shift and required-name check below the option loop.

   shift "$((OPTIND - 1))"
    
   if [[ -z $name ]]; then
       printf 'Missing required option: -n\n' >&2
       usage >&2
       exit 2
   fi

   OPTIND points to the first unparsed argument. Shifting by one less than that index removes the recognized options and their values while preserving later file operands.

4. Append the parsed-value output after the required-name check.

   printf 'archive=%s\n' "$name"
   printf 'dry_run=%s\n' "$dry_run"
    
   for file in "$@"; do
       printf 'input=%s\n' "$file"
   done

   Quoted "$@" expansion passes each remaining operand as one value, including names that contain spaces.

5. Compare archive-plan.sh with the consolidated file.

   archive-plan.sh

   #!/usr/bin/env bash
   set -u
    
   usage() {
       printf 'Usage: %s -n NAME [-d] [FILE...]\n' "${0##*/}"
   }
    
   name=
   dry_run=0
    
   while getopts ':n:dh' option; do
       case $option in
           n) name=$OPTARG ;;
           d) dry_run=1 ;;
           h) usage; exit 0 ;;
           :) printf 'Missing value for -%s\n' "$OPTARG" >&2; usage >&2; exit 2 ;;
           \?) printf 'Unknown option: -%s\n' "$OPTARG" >&2; usage >&2; exit 2 ;;
       esac
   done
    
   shift "$((OPTIND - 1))"
    
   if [[ -z $name ]]; then
       printf 'Missing required option: -n\n' >&2
       usage >&2
       exit 2
   fi
    
   printf 'archive=%s\n' "$name"
   printf 'dry_run=%s\n' "$dry_run"
    
   for file in "$@"; do
       printf 'input=%s\n' "$file"
   done

6. Check archive-plan.sh for Bash syntax errors.

   $ bash -n archive-plan.sh

   No output means Bash parsed the complete file without finding a syntax error.

7. Confirm that -n without a value reaches the missing-value branch.

   $ bash archive-plan.sh -n
   Missing value for -n
   Usage: archive-plan.sh -n NAME [-d] [FILE...]

   The script exits with status 2, allowing a caller to distinguish invalid usage from a successful parse.

8. Run the parser with both supported options and two file operands.

   $ bash archive-plan.sh -d -n nightly -- app.log -old.log
   archive=nightly
   dry_run=1
   input=app.log
   input=-old.log

   The -- marker prevents -old.log from being read as another option. Both input= lines confirm that the shift retained the operands after parsing.