if, elif, case and Short-Circuit Operators

Choosing a path

An if statement runs a command list and branches on its status: if cmd; then ...; elif other; then ...; else ...; fi. Any command can be a condition, not just tests: if grep -q ERROR app.log; then is idiomatic. case matches a value against glob patterns and is cleaner than long if/elif chains for things like command-line subcommands or file types: each branch ends with ;;, patterns can be combined with |, and *) is the default. Short-circuit operators chain commands: cmd1 && cmd2 runs cmd2 only if cmd1 succeeds; cmd1 || cmd2 runs cmd2 only if cmd1 fails, which is handy for mkdir -p dir || exit 1 or command -v jq >/dev/null || die "jq required". Avoid the a && b || c idiom as an if/else replacement: if b fails, c runs too. Use a real if when you mean if/else.

A subcommand dispatcher with case

Typical structure of a small CLI tool.

#!/usr/bin/env bash
set -euo pipefail

usage() { printf 'usage: %s {start|stop|status} [service]\n' "$0" >&2; exit 2; }

cmd="${1:-}"
service="${2:-web}"

case "$cmd" in
    start)        echo "starting $service" ;;
    stop|halt)    echo "stopping $service" ;;
    status)
        if systemctl is-active --quiet "$service"; then
            echo "$service is running"
        else
            echo "$service is stopped"
        fi
        ;;
    -h|--help|"") usage ;;
    *)            echo "unknown command: $cmd" >&2; usage ;;
esac

A railway points switch

case is a set of railway points: the train (value) is routed down exactly one track depending on its destination board (pattern), with a siding (*)) for anything unexpected.

Quick check: In `cmd1 || cmd2`, when does cmd2 run?

• Always
• Only if cmd1 succeeds
• Never
• Only if cmd1 fails