Bash Scripting Cheat Sheet

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

#!/usr/bin/env bash

SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"

set -x  # enable trace
# commands...
set +x  # disable trace

${VAR:-"default_value"}

${VAR:="default_value"}

${DATABASE_URL:?"Error: DATABASE_URL must be configured"

${#VAR}

${VAR#pattern}

${VAR##pattern}

${VAR%pattern}

${VAR%%pattern}

${VAR//search/replace}

${VAR^^}  # UPPERCASE
${VAR,,}  # lowercase

${VAR:offset:length}

if [[ -z "$STR" ]]; then
  echo "String is empty"
fi

if [[ -n "$STR" ]]; then
  echo "String has content"
fi

if [[ "$INPUT" =~ ^[0-9]{4}-[0-9]{2}-[0-9]{2}$ ]]; then
  echo "Valid YYYY-MM-DD date"
fi

[[ -f "$FILE" ]]  # Regular file exists
[[ -d "$DIR" ]]   # Directory exists
[[ -e "$PATH" ]]  # File or dir exists
[[ -s "$FILE" ]]  # File exists and is NOT empty (> 0 bytes)
[[ -x "$FILE" ]]  # File exists and is executable
[[ -L "$LINK" ]]  # Symbolic link

[[ "$A" -eq "$B" ]]  # Equal
[[ "$A" -ne "$B" ]]  # Not equal
[[ "$A" -lt "$B" ]]  # Less than
[[ "$A" -le "$B" ]]  # Less than or equal
[[ "$A" -gt "$B" ]]  # Greater than
[[ "$A" -ge "$B" ]]  # Greater than or equal

case "$1" in
  start|run)
    echo "Starting service..."
    ;;
  stop)
    echo "Stopping service..."
    ;;
  restart)
    echo "Restarting..."
    ;;
  *)
    echo "Usage: $0 {start|stop|restart}" >&2
    exit 1
    ;;
esac

while IFS= read -r line || [[ -n "$line" ]]; do
  echo "Processing: $line"
done < "data.txt"

servers=("web-1" "web-2" "api-1")

for server in "${servers[@]}"; do
  echo "Deploying to $server..."
done

for (( i = 1; i <= 5; i++ )); do
  echo "Attempt $i of 5"
done

# Or syntax with braces:
for i in {1..5}; do
  echo "Step $i"
done

shopt -s nullglob  # avoid literal string if no files match
for file in /var/log/*.log; do
  echo "Archiving $file"
done
shopt -u nullglob

max_attempts=5
attempt=1
delay=2

until curl -sSf "https://api.example.com/health" > /dev/null; do
  if (( attempt >= max_attempts )); then
    echo "Health check failed after $max_attempts attempts" >&2
    exit 1
  fi
  echo "Attempt $attempt failed. Retrying in ${delay}s..."
  sleep "$delay"
  (( attempt++ ))
  (( delay *= 2 ))
done

arr=("alpha" "beta" "gamma")

echo "${arr[0]}"        # First item (alpha)
echo "${arr[-1]}"       # Last item (gamma)
echo "${#arr[@]}"       # Array length (3)
echo "${arr[@]}"        # All items

arr+=("delta")          # Append item
unset 'arr[1]'          # Remove element at index 1

declare -A config

config["host"]="127.0.0.1"
config["port"]="5432"
config["db"]="production"

echo "Connecting to ${config["host"]}:${config["port"]}"

# Iterate over keys:
for key in "${!config[@]}"; do
  echo "$key = ${config[$key]}"
done

CSV="apple,banana,orange,grape"
IFS=',' read -r -a fruits <<< "$CSV"

for fruit in "${fruits[@]}"; do
  echo "Fruit: $fruit"
done

arr=("one" "two" "three")
joined=$(IFS=, ; echo "${arr[*]}")
echo "$joined" # Output: one,two,three

log_message() {
  local level="${1:-INFO}"
  local message="${2:-}"
  local timestamp
  timestamp="$(date -u +"%Y-%m-%dT%H:%M:%SZ")"
  
  echo "[$timestamp] [$level] $message" >&2
}

log_message "ERROR" "Database connection timeout"

$0      # Name of script
$1..$9  # Positional arguments 1 through 9
${10}  # Positional argument 10+ (requires curly braces)
$#      # Total count of positional arguments passed
$@      # All arguments as separate quoted words ("$1" "$2" ...)
$*      # All arguments joined into single string ("$1 $2 ...")
$?      # Exit code of the most recent foreground command
$$      # Process ID (PID) of the current shell script
$!      # Process ID (PID) of the last backgrounded job

while getopts "e:p:vh" opt; do
  case "$opt" in
    e) ENV="$OPTARG" ;;
    p) PORT="$OPTARG" ;;
    v) VERBOSE=1 ;;
    h) echo "Usage: $0 [-e env] [-p port] [-v]"; exit 0 ;;
    *) echo "Invalid option" >&2; exit 1 ;;
  esac
done
shift $((OPTIND - 1))

command > file         # Overwrite stdout to file
command >> file        # Append stdout to file
command 2> file        # Overwrite stderr to file
command 2>> file       # Append stderr to file
command &> file        # Redirect both stdout & stderr to file (Bash)
command > file 2>&1    # Portable POSIX redirect stdout & stderr
command > /dev/null 2>&1 # Discard all output completely

cat << 'EOF' > /tmp/nginx.conf
server {
    listen 80;
    server_name example.com;
    location / {
        proxy_pass http://localhost:3000;
    }
}
EOF

diff -u <(curl -s "https://api.v1.com/data") <(curl -s "https://api.v2.com/data")

make build 2>&1 | tee build.log

tmpdir="$(mktemp -d)"
trap 'rm -rf "$tmpdir"' EXIT

# Script operations...
echo "Working in $tmpdir"
# Cleanup runs automatically whether script succeeds or crashes!

0    # Success (no error)
1    # Catchall for general errors
2    # Misuse of shell built-ins (syntax errors)
126  # Command invoked cannot execute (permissions problem)
127  # Command not found (PATH issue or typo)
128  # Invalid argument to exit
130  # Script terminated by Ctrl+C (SIGINT = 128 + 2)
137  # Script terminated by SIGKILL (128 + 9, e.g. Linux OOM killer)
143  # Script terminated by SIGTERM (128 + 15)

read -r -p "Are you sure you want to deploy to production? [y/N] " response
case "$response" in
  [yY][eE][sS]|[yY])
    echo "Proceeding..."
    ;;
  *)
    echo "Aborted by user."
    exit 1
    ;;
esac