Positional Parameters and Special Bash Parameters

1. Positional parameters are the argument vector

Arguments supplied after the script name become $1, $2, and so on. Inside a function, the same notation refers to that function call.

cat > show-args.sh <<'EOF'
#!/usr/bin/env bash
printf 'arg1=<%s>\n' "${1-}"
printf 'arg2=<%s>\n' "${2-}"
printf 'count=%d\n' "$#"
EOF

bash show-args.sh api "staging blue"

2. $0 and $# describe the current invocation

if (( $# < 2 )); then
  printf 'usage: %s SERVICE ENVIRONMENT\n' "$0" >&2
  exit 64
fi

3. Quoted "$@" preserves every original argument

For forwarding or iteration, "$@" is the crucial form. It expands to one quoted shell word per original positional argument.

show() {
  printf 'received=%d\n' "$#"
  for arg in "$@"; do
    printf 'arg=<%s>\n' "$arg"
  done
}

show "api gateway" staging "*.yaml"

4. "$*" joins arguments into one string

Quoted "$*" joins all positional parameters using the first character of IFS. That is useful occasionally for display, but it is not a substitute for "$@" when argument boundaries matter.

set -- "api gateway" staging "*.yaml"
joined="$*"
printf 'joined=<%s>\n' "$joined"

5. $? is the most recent foreground pipeline status

grep -q '^root:' /etc/passwd
status=$?
printf 'grep status=%d\n' "$status"

Capture the value immediately if it matters; any later foreground command can replace it.

6. $$ and $! expose process identifiers

sleep 1 &
pid=$!
printf 'shell=%s background=%s\n' "$$" "$pid"
wait "$pid"
printf 'background status=%d\n' "$?"

7. Other special parameters are contextual tools

$- exposes active shell option letters, while $_ has context-dependent behavior related to previous commands. These can be useful diagnostically, but application logic is usually clearer with explicit variables.

8. Validate required operands before doing work

service=${1:-}
environment=${2:-}

if [[ -z $service || -z $environment ]]; then
  printf 'usage: %s SERVICE ENVIRONMENT\n' "$0" >&2
  exit 64
fi

case $environment in
  dev|staging|prod) ;;
  *) printf 'unsupported environment: %s\n' "$environment" >&2; exit 65 ;;
esac

Argument-count validation checks shape; semantic validation checks whether values are actually acceptable.

9. Positional defaults work best for small interfaces

service=${1:-api}
environment=${2:-staging}
replicas=${3:-2}

printf 'service=%s env=%s replicas=%s\n' \
  "$service" "$environment" "$replicas"

As optional parameters multiply, named options are usually easier to remember and less error-prone than positional defaults.

10. Preserve the vector when wrapping another command

run_wrapped() {
  printf 'running %d argument(s)\n' "$#" >&2
  "$@"
}

run_wrapped printf 'service=%s env=%s\n' "api gateway" staging

11. Hands-on lab: positional deployment planner

mkdir -p "$HOME/devops-academy/bash/chapter06/lesson01"
cd "$HOME/devops-academy/bash/chapter06/lesson01"

cat > plan.sh <<'EOF'
#!/usr/bin/env bash

if (( $# < 2 || $# > 3 )); then
  printf 'usage: %s SERVICE ENVIRONMENT [REPLICAS]\n' "$0" >&2
  exit 64
fi

service=$1
environment=$2
replicas=${3:-2}

case $environment in
  dev|staging|prod) ;;
  *) printf 'invalid environment: %s\n' "$environment" >&2; exit 65 ;;
esac

if [[ ! $replicas =~ ^[0-9]+$ ]] || (( 10#$replicas < 1 )); then
  printf 'replicas must be a positive integer\n' >&2
  exit 66
fi

printf 'PLAN service=%s env=%s replicas=%d\n' \
  "$service" "$environment" "$((10#$replicas))"
EOF

bash plan.sh "api gateway" staging 3
bash plan.sh worker prod
bash plan.sh api qa || true

12. Knowledge check

13. Summary

Positional parameters are Bash's raw CLI interface. Use $# for arity, $0 for invocation naming, and quoted "$@" to preserve argument boundaries. Validate both shape and meaning before operational work.

14. Further reading

• GNU Bash Reference Manual — Special Parameters.
• GNU Bash Reference Manual — Shell Parameters.
• POSIX Shell Command Language — special parameters.
• ShellCheck documentation — argument forwarding and quoting.