Bash scripting basics: variables, conditions, loops and exit codes

Why engineers still write Bash

Bash (the Bourne Again SHell) is the command interpreter on almost every Linux server, jump host, firewall appliance shell and network operating system that exposes a Linux prompt. If you type the same five commands every morning, a Bash script can run them for you. It needs no extra installation, starts instantly and glues together tools that already exist, such as ping, ssh, grep and curl.

Bash is not the right tool for everything. Once a job needs data structures, API calls with JSON or careful error handling, Python is usually easier to read and test. A sensible rule is: if the script grows past about 100 lines, consider rewriting it in Python.

Your first script

Save the following as check_hosts.sh, make it executable with chmod +x check_hosts.sh and run it with ./check_hosts.sh.

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

SITE="branch-01"
HOSTS=("192.168.10.1" "192.168.10.2" "192.168.10.10")

for h in "${HOSTS[@]}"; do
  if ping -c 1 -W 2 "$h" > /dev/null 2>&1; then
    echo "$SITE $h up"
  else
    echo "$SITE $h DOWN"
  fi
done

• Shebang: the first line tells the system which interpreter to use. /usr/bin/env bash finds Bash wherever it is installed.
• set -euo pipefail: stop on any failing command (-e), treat unset variables as errors (-u) and make a pipeline fail if any part fails (pipefail).
• Variables: assign with no spaces around =; read with "$NAME". Arrays use "${ARRAY[@]}".
• Redirection: > /dev/null 2>&1 throws away normal output and errors, because you only care about the exit code.

Conditions, exit codes and functions

Every command returns an exit code: 0 means success, anything else means failure. if simply runs a command and checks that code. For comparisons you use the test brackets [[ ... ]].

backup_dir="/var/backups/net"

log() { echo "$(date '+%F %T') $*"; }

if [[ ! -d "$backup_dir" ]]; then
  log "creating $backup_dir"
  mkdir -p "$backup_dir"
fi

case "${1:-status}" in
  status) log "status requested" ;;
  clean)  find "$backup_dir" -name '*.cfg' -mtime +30 -print ;;
  *)      echo "usage: $0 [status|clean]"; exit 2 ;;
esac

$1 is the first argument, and ${1:-status} supplies a default if none was given. Ending with a meaningful exit code lets schedulers and other scripts know whether your job worked.

Quoting and common mistakes

Tip: run shellcheck script.sh before you share a script. This free linter catches quoting bugs and risky patterns that are easy to miss by eye.