Shell Scripting

After this lesson, you will be able to: Write bash scripts with variables, conditionals, loops, and functions to automate real tasks like backups, deploys, and log rotation.

Shell scripting is how ops engineers turn ad-hoc commands into repeatable automation. This lesson covers the syntax + the safety habits that prevent disasters.

Prerequisites:Networking from the CLI

Shebang + safety options (always)

Every production bash script starts with these. Memorise them.

tsx

#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n\t'

# Why each:
# -e   exit on any error
# -u   exit on undefined variable
# -o pipefail   exit if any part of a pipe fails
# IFS  prevent word-splitting bugs on filenames with spaces

# Without these, scripts silently swallow failures.

Variables, conditionals, loops, functions

Cover sheet of bash syntax.

tsx

name='alex'                   # no spaces around =
echo "Hello, $name"
echo "Date: $(date +%Y-%m-%d)"

# Conditional
if [[ -f /etc/passwd ]]; then
  echo 'exists'
elif [[ -d /etc ]]; then
  echo 'dir'
else
  echo 'no'
fi

# String comparison: ==  ; numeric: -eq -lt -gt
if [[ "$1" == 'deploy' ]]; then ...
if (( count > 5 )); then ...

# Loops
for f in *.log; do echo "$f"; done
for i in {1..5}; do echo "$i"; done
while read -r line; do echo "$line"; done < input.txt

# Functions
backup() {
  local src="$1"
  local dest="$2"
  tar -czf "$dest" "$src"
}
backup /var/www /backups/www-$(date +%F).tar.gz

A real deploy script

Drop this in /usr/local/bin/deploy.sh, chmod 755, run via cron or systemd timer.

bash

#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n\t'

APP_DIR=/srv/myapp
REPO=https://github.com/me/myapp
LOG=/var/log/myapp/deploy.log

log() {
  echo "[$(date '+%F %T')] $*" | tee -a "$LOG"
}

log 'Starting deploy'

cd "$APP_DIR"
log 'Pulling latest'
git fetch --all --quiet
git reset --hard origin/main

log 'Installing dependencies'
npm ci --omit=dev

log 'Building'
npm run build

log 'Restarting service'
sudo systemctl restart myapp

log 'Done'

Test and debug scripts

`bash -n script.sh`, syntax check without running. `bash -x script.sh`, print each command as it runs (debugger). `shellcheck script.sh`, static analysis catches 80% of bash bugs. INSTALL IT. Run scripts in a test directory before pointing at production. `set -x` inside the script turns on tracing; `set +x` turns it off.

Common mistakes only experienced shell-scripters avoid

Skipping `set -euo pipefail`. Without it, errors silently propagate. Unquoted variables. `rm $FILE` with $FILE empty is `rm` (lists files); with a space, splits incorrectly. Always quote: `rm "$FILE"`. Trusting `cd` to succeed. `cd /nonexistent && rm -rf *` is a horror story without `set -e`. Using `eval`. Almost never necessary; almost always a security hole. Skipping shellcheck. It catches gotchas you wouldn't.

Quick Check

What's wrong with `cd /var/www && rm -rf *` if /var/www doesn't exist?

Pick the failure mode.

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