Bash Scripting

Automation begins with a script

Every repetitive task you do manually is a bug waiting to happen. Bash scripting turns procedures into repeatable, auditable, version-controlled automation. As a DevOps engineer, bash is the language you reach for first — it is on every Linux system, requires no installation, and integrates directly with every command-line tool.

---

The anatomy of a bash script

deploy.sh

#!/bin/bash
set -euo pipefail

# Variables
APP_NAME="myapp"
DEPLOY_DIR="/opt/${APP_NAME}"
LOG_FILE="/var/log/${APP_NAME}-deploy.log"

# Functions
log() {
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" | tee -a "$LOG_FILE"
}

# Main logic
log "Starting deployment of ${APP_NAME}"
systemctl stop "${APP_NAME}" || true
cp -r ./build/* "${DEPLOY_DIR}/"
systemctl start "${APP_NAME}"
log "Deployment complete"

The first three lines are always the same in a production script:

Line	Purpose
#!/bin/bash	Use bash as interpreter
set -e	Exit on any error
set -u	Exit on unset variable
set -o pipefail	Exit if any pipe command fails

---

Variables

# Assignment, no spaces around =
name="devops"
port=8080
log_dir="/var/log/myapp"

# Reference
echo "$name"
echo "${name}_user"     # braces needed when appending text

# Command substitution
hostname=$(hostname)
date_now=$(date '+%Y-%m-%d')
file_count=$(ls /etc/*.conf | wc -l)

# Readonly
readonly MAX_RETRIES=3

# Arrays
servers=("web-01" "web-02" "db-01")
echo "${servers[0]}"        # web-01
echo "${#servers[@]}"       # 3 (array length)
for s in "${servers[@]}"; do echo "$s"; done

---

Conditionals

# if/elif/else
if [ "$1" = "production" ]; then
  echo "Deploying to production"
elif [ "$1" = "staging" ]; then
  echo "Deploying to staging"
else
  echo "Usage: $0 [production|staging]"
  exit 1
fi

# File tests
if [ ! -f "/etc/nginx/nginx.conf" ]; then
  echo "Config file missing"
  exit 1
fi

if [ -d "/opt/myapp" ]; then
  echo "App directory exists"
fi

# String tests
if [ -z "$API_KEY" ]; then
  echo "API_KEY is not set"
  exit 1
fi

if [ -n "$DEBUG" ]; then
  set -x
fi

# Numeric comparisons
if [ "$count" -gt 10 ]; then
  echo "Too many items"
fi

Test operators

Operator	Meaning
-f file	File exists and is regular file
-d path	Directory exists
-e path	Path exists
-x file	File is executable
-z string	String is empty
-n string	String is not empty
-eq	Equal (numbers)
-ne	Not equal (numbers)
-gt	Greater than
-lt	Less than

---

Loops

# for loop, list
for env in development staging production; do
  echo "Deploying to $env"
done

# for loop, files
for file in /etc/*.conf; do
  echo "Config: $file"
done

# for loop, array
servers=("web-01" "web-02" "web-03")
for server in "${servers[@]}"; do
  ssh "$server" "systemctl status nginx"
done

# while loop
count=0
while [ "$count" -lt 5 ]; do
  echo "Attempt $count"
  count=$((count + 1))
done

# Read file line by line
while IFS= read -r line; do
  echo "Processing: $line"
done < servers.txt

---

Functions

# Define
check_service() {
  local service="$1"     # local keeps variable inside function
  if systemctl is-active --quiet "$service"; then
    echo "$service is running"
    return 0
  else
    echo "$service is not running"
    return 1
  fi
}

deploy() {
  local env="$1"
  local version="$2"
  echo "Deploying version $version to $env"
}

# Call
check_service nginx
deploy production v1.2.3

Always use local for variables inside functions. Without it they are global and can overwrite variables in the main script.

---

Error handling

#!/bin/bash
set -euo pipefail

# Trap, run on exit regardless of how script ends
cleanup() {
  rm -f /tmp/deploy.lock
  echo "Cleanup complete"
}
trap cleanup EXIT

# Trap Ctrl+C
trap 'echo "Interrupted"; exit 1' INT

# Check command exists
if ! command -v docker &>/dev/null; then
  echo "Docker is not installed"
  exit 1
fi

# Check argument provided
if [ $# -eq 0 ]; then
  echo "Usage: $0 <environment>"
  exit 1
fi

---

Input and output

# Redirect stdout
command > output.txt         # overwrite
command >> output.txt        # append

# Redirect stderr
command 2> errors.txt

# Redirect both
command > output.txt 2>&1
command &> output.txt        # shorthand

# Discard output
command > /dev/null 2>&1

# Read user input
read -p "Enter environment: " env
read -sp "Enter password: " pass   # -s hides input

---

Debugging

# Debug mode, prints every command before running it
bash -x script.sh

# Or inside the script
set -x        # enable
set +x        # disable

# Check syntax without running
bash -n script.sh

---

A complete deployment script

scripts/deploy.sh

#!/bin/bash
set -euo pipefail

# ── Configuration ──────────────────────────────────────
APP="devops-chronicles"
DEPLOY_DIR="/opt/${APP}"
SERVICE="${APP}"
LOG="/var/log/${APP}-deploy.log"

# ── Functions ──────────────────────────────────────────
log()  { echo "[$(date '+%H:%M:%S')] $*" | tee -a "$LOG"; }
fail() { echo "ERROR: $*" >&2; exit 1; }

cleanup() { log "Deploy script exited"; }
trap cleanup EXIT

# ── Validation ─────────────────────────────────────────
[ $# -eq 1 ] || fail "Usage: $0 <version>"
VERSION="$1"

command -v systemctl &>/dev/null || fail "systemctl not found"
[ -d "$DEPLOY_DIR" ] || fail "Deploy directory missing: $DEPLOY_DIR"

# ── Deploy ─────────────────────────────────────────────
log "Deploying ${APP} version ${VERSION}"

log "Stopping service"
systemctl stop "$SERVICE"

log "Copying files"
cp -r ./build/* "${DEPLOY_DIR}/"

log "Starting service"
systemctl start "$SERVICE"

# ── Health check ───────────────────────────────────────
log "Running health check"
sleep 3
if curl -sf "http://localhost:3000/health" &>/dev/null; then
  log "Health check passed, deploy complete"
else
  fail "Health check failed, check logs at $LOG"
fi

---

Quick reference

#!/bin/bash
set -euo pipefail           # always start with this

name="value"                # variable
echo "$name"                # reference variable
result=$(command)           # capture output

if [ condition ]; then      # conditional
  commands
fi

for item in list; do        # loop
  commands
done

function_name() {           # function
  local var="$1"
}

trap cleanup EXIT           # run on exit
[ -f file ]                 # file exists
[ -z "$var" ]               # variable is empty
[ $# -eq 0 ]                # no arguments passed
bash -x script.sh           # debug mode
bash -n script.sh           # syntax check only

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Test your knowledge

→ Take the Bash Scripting Quiz