Bash - Scripting Quick Reference

Practical Bash scripting reference — variables, conditionals, loops, functions, arrays, and error handling for real automation scripts.

Bash — Scripting Quick Reference

Variables and Quoting

The basics that trip people up most often.

BASH

NAME="orders-api"
readonly MAX_RETRIES=3                  ## cannot be reassigned later
echo "Service: $NAME"
echo "Service: ${NAME}_prod"            ## braces needed when concatenating
unset NAME

## Quoting rules
echo "$NAME has spaces preserved"       ## double quotes expand variables
echo '$NAME stays literal'              ## single quotes do not expand

Parameter Breakdown:

${NAME}: Braces are required whenever the variable name runs into other text
Double quotes: Expand variables and command substitution, preserve word splitting
Single quotes: Completely literal — nothing inside is expanded

Conditionals

Branch logic based on tests and exit codes.

BASH

if [ "$ENV" = "production" ]; then
  echo "Deploying to production"
elif [ "$ENV" = "staging" ]; then
  echo "Deploying to staging"
else
  echo "Unknown environment: $ENV"
  exit 1
fi

## File and string test operators
[ -f "$CONFIG_FILE" ] && echo "config exists"
[ -d "/opt/orders-api" ] && echo "dir exists"
[ -z "$NAME" ] && echo "NAME is empty"
[ -n "$NAME" ] && echo "NAME is set"

Command Parameter Table:

Operator	Description
`-f`	True if path exists and is a regular file
`-d`	True if path exists and is a directory
`-z` / `-n`	True if string is empty / non-empty

Loops

Iterate over lists, files, and ranges.

BASH

for service in orders-api payments-api auth-api; do
  echo "Restarting $service"
  systemctl restart "$service"
done

for i in {1..5}; do
  echo "Attempt $i"
done

while read -r line; do
  echo "Processing: $line"
done < input.txt

until ping -c1 db.internal.razorpay.net &>/dev/null; do
  echo "Waiting for database..."
  sleep 2
done

Parameter Breakdown:

read -r: Prevents backslashes in input from being interpreted as escapes
{1..5}: Bash brace expansion — generates a numeric range inline
until: Loops until the condition becomes true, opposite of while

Functions

Reusable blocks with arguments and return values.

BASH

deploy_service() {
  local service_name="$1"
  local version="$2"

  echo "Deploying $service_name version $version"
  if [ -z "$version" ]; then
    echo "Error: version required" >&2
    return 1
  fi
  ## ... deployment logic here
  return 0
}

deploy_service "orders-api" "v2.3.1"
if [ $? -eq 0 ]; then
  echo "Deploy succeeded"
fi

Parameter Breakdown:

local: Scopes a variable to the function only — always use it to avoid leaking globals
$1, $2: Positional arguments passed into the function
$?: Exit code of the last command — 0 means success by convention

Arrays

Store and loop over lists of values.

BASH

SERVICES=("orders-api" "payments-api" "auth-api")
echo "${SERVICES[0]}"                   ## first element
echo "${SERVICES[@]}"                   ## all elements
echo "${#SERVICES[@]}"                  ## array length
SERVICES+=("notifications-api")         ## append

for service in "${SERVICES[@]}"; do
  echo "$service"
done

## Associative arrays (Bash 4+)
declare -A ENV_PORTS=( ["staging"]=8080 ["prod"]=9090 )
echo "${ENV_PORTS[prod]}"

Parameter Breakdown:

"${SERVICES[@]}": Quoted, expands each element as a separate word — always quote this
${#SERVICES[@]}: The # prefix returns the count, not a value
declare -A: Required to create an associative (key-value) array

String Manipulation

Common patterns for parsing and formatting text.

BASH

FILE="backup-2026-06-19.tar.gz"
echo "${FILE%.tar.gz}"                  ## strip suffix -> backup-2026-06-19
echo "${FILE#backup-}"                  ## strip prefix -> 2026-06-19.tar.gz
echo "${FILE/2026/2027}"                ## replace first match
echo "${#FILE}"                         ## string length

NAME="orders-api"
echo "${NAME^^}"                        ## uppercase -> ORDERS-API
echo "${NAME,,}"                        ## lowercase -> orders-api

Command Parameter Table:

Syntax	Description
`${VAR%pattern}`	Strip shortest match from the end
`${VAR#pattern}`	Strip shortest match from the start
`${VAR/old/new}`	Replace first occurrence of `old` with `new`

Input/Output Redirection

Control where output and errors go.

BASH

./deploy.sh > deploy.log                ## stdout to file, overwrite
./deploy.sh >> deploy.log               ## stdout to file, append
./deploy.sh 2> errors.log               ## stderr only
./deploy.sh > all.log 2>&1              ## both stdout and stderr to one file
./deploy.sh &> all.log                  ## shorthand for the line above
./deploy.sh < input.txt                 ## stdin from a file
command1 | command2                     ## pipe stdout of one into another

Parameter Breakdown:

2>&1: Redirects file descriptor 2 (stderr) to wherever 1 (stdout) currently points
Order matters: > file 2>&1 works, 2>&1 > file does not merge correctly
&>: Bash shorthand that does both redirections safely in one token

Exit Codes and Error Handling

Make scripts fail loudly instead of silently.

BASH

#!/bin/bash
set -e                                  ## exit immediately on any error
set -u                                  ## error on undefined variables
set -o pipefail                         ## fail if any command in a pipe fails
set -euo pipefail                       ## all three combined, common at the top of scripts

trap 'echo "Error on line $LINENO"' ERR
trap 'echo "Cleaning up..."; rm -f /tmp/lock' EXIT

cleanup() {
  rm -f /tmp/deploy.lock
}
trap cleanup EXIT

Parameter Breakdown:

set -e: Stops the script on the first command that fails — essential for deploy scripts
set -o pipefail: Without this, false | true reports success because the last command succeeded
trap ... EXIT: Runs cleanup code no matter how the script exits, including on error

Arguments and Option Parsing

Handle script inputs properly.

BASH

echo "Script name: $0"
echo "First arg: $1"
echo "All args: $@"
echo "Arg count: $#"

while getopts "e:v:h" opt; do
  case $opt in
    e) ENVIRONMENT="$OPTARG" ;;
    v) VERSION="$OPTARG" ;;
    h) echo "Usage: $0 -e <env> -v <version>"; exit 0 ;;
    *) echo "Invalid option"; exit 1 ;;
  esac
done

## Usage: ./deploy.sh -e production -v 2.3.1

Parameter Breakdown:

getopts: Standard way to parse -flag value style arguments
$@ vs $*: $@ preserves each argument as a separate word when quoted, $* does not
OPTARG: Holds the value passed after a flag that expects one

Useful One-Liners

Patterns worth keeping in a snippets file.

BASH

## Retry a command up to 3 times with a delay
for i in 1 2 3; do ./flaky-script.sh && break || sleep 5; done

## Check if a command exists before using it
command -v jq >/dev/null 2>&1 || { echo "jq is required"; exit 1; }

## Run a command with a timeout
timeout 30s ./long-running-task.sh

## Get the directory the running script lives in
SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"