Stop 05 · Scripting & Programming for DevOpsTopics · 2/8

Bash Scripting Deep Dive

In short

Your first scripts were a list of commands. Real automation scripts take flags, loop over servers, reuse functions and refuse to run with bad input. That's the jump this lesson makes.

Going beyond Phase 2's basics — functions, arrays, argument parsing, and proper error handling — turning ad-hoc one-liners into reliable, reusable automation scripts.

The mascot in a chef hat stands before a row of huge cooking pots, turning a big dial on one of them.

Think of it like this

The wedding caterer's master recipe

A wedding caterer doesn't rewrite the biryani recipe for every function. One master recipe takes 'number of guests' and 'spice level' as inputs, and calls smaller sub-recipes for raita and salan. A good Bash script works the same: arguments in, functions inside.

Watch it happen

Calls inside ./deploy.sh -e prod

push(main)

main

Top of the stack is coloured

1/6The script starts. Top-level code acts like main.

The big ideas

What you need to know

Arrays hold lists

SERVERS=(web1 web2) and "${SERVERS[@]}" loop safely over every item, even ones with spaces.

getopts parses flags

getopts handles -e prod and -n in any order, so users don't have to remember argument positions.

Functions keep it tidy

Wrap repeated steps in functions. Use local inside them so variables don't leak into the rest of the script.

A dry-run flag

A -n flag that prints what would happen lets you test safely before touching real servers.

See it in code

Walk through the code

bash

1#!/usr/bin/env bash2set -euo pipefail3usage() { echo "usage: $0 -e <env> [-n]" >&2; exit 2; }4DRY_RUN=false5while getopts "e:n" opt; do6  case "$opt" in7    e) ENV="$OPTARG" ;;8    n) DRY_RUN=true ;;9    *) usage ;;10  esac11done12[[ -z "${ENV:-}" ]] && usage13SERVERS=(web1 web2 web3)14for s in "${SERVERS[@]}"; do15  if $DRY_RUN; then echo "[dry-run] deploy $s ($ENV)"; else ssh "$s" "./deploy.sh $ENV"; fi16done

1/4Strict mode, plus a usage function that prints help to stderr and exits with code 2.

Run ./deploy.sh -e staging -n first. Check the shellcheck tool too: it catches many Bash bugs before you run anything.

Save yourself the pain

Common mistakes

Oops

Writing for s in $SERVERS, which only loops once over a single joined string.

Do this instead

Use an array and loop with "${SERVERS[@]}", quotes included.

Oops

Building a 500-line Bash script with complex logic and string parsing.

Do this instead

When Bash gets hard to read, move the logic to Python. Keep Bash for gluing commands together.

Your mission

Add flags to a script

~30 min

Quick check

What does "${SERVERS[@]}" (with quotes) do in a for loop?

Go deeper

1Refactor a simple script from Phase 2 to accept command-line arguments properly.
2Practice writing a Bash function and calling it from multiple places in a script.
3Learn to use `set -e` and `set -u` to make scripts fail loudly instead of silently.