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.

Steps to parse command line options in Bash with getopts:

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.

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.

Add the operand shift and required-name check below the option loop.
```
shift "$((OPTIND - 1))"
 
if [[ -z $name ]]; then
    printf 'Missing required option: -n\n' >&2
    usage >&2
    exit 2
fi
```
OPTIND points to the first unparsed argument. Shifting by one less than that index removes the recognized options and their values while preserving later file operands.
Append the parsed-value output after the required-name check.
```
printf 'archive=%s\n' "$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.

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

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.
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.
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.