Bash

#!/usr/bin/env bash
set -euo pipefail  # Exit on error, unset vars, pipe failures

# Variables
NAME="Alice"
echo "Hello, $NAME"
echo "Hello, ${NAME}!"  # Explicit boundary

# Read user input
read -p "Enter name: " USER_NAME

# Command substitution
CURRENT_DIR=$(pwd)
DATE=$(date +%Y-%m-%d)

# Arithmetic
COUNT=$((5 + 3))
echo $((10 % 3))   # 1

# if/elif/else
if [[ "$ENV" == "production" ]]; then
  echo "prod"
elif [[ "$ENV" == "staging" ]]; then
  echo "staging"
else
  echo "dev"
fi

# File tests
if [[ -f "$FILE" ]]; then echo "is a file"; fi
if [[ -d "$DIR" ]]; then echo "is a dir"; fi
if [[ -z "$VAR" ]]; then echo "empty string"; fi
if [[ -n "$VAR" ]]; then echo "non-empty"; fi

# Number comparison
if [[ $COUNT -gt 10 ]]; then echo "more than 10"; fi

# for loop
for i in {1..5}; do
  echo "Item $i"
done

# Loop over files
for file in *.log; do
  echo "Processing $file"
done

# Loop over array
FRUITS=("apple" "banana" "cherry")
for fruit in "${FRUITS[@]}"; do
  echo "$fruit"
done

# while loop
COUNT=0
while [[ $COUNT -lt 5 ]]; do
  echo $COUNT
  ((COUNT++))
done

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

# Define function
log() {
  local level="$1"
  local message="$2"
  echo "[$(date +%H:%M:%S)] [$level] $message" >&2
}

log "INFO" "Starting deployment"
log "ERROR" "Something went wrong"

# Return values (exit codes)
file_exists() {
  [[ -f "$1" ]]   # Returns 0 (true) or 1 (false)
}

if file_exists "config.yml"; then
  echo "Config found"
fi

# Capture output
result=$(my_function arg1 arg2)

# Redirect stdout
command > output.txt      # Overwrite
command >> output.txt     # Append

# Redirect stderr
command 2> error.log
command 2>&1              # stderr to stdout
command > all.log 2>&1   # Both to file

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

# Pipe stdout to next command
ps aux | grep nginx
cat access.log | grep ' 500 ' | wc -l

# tee — write to file AND pass through to stdout
command | tee output.log | grep error

# Here string
grep 'pattern' <<< "$variable"

# xargs: build command args from stdin
find . -name '*.log' | xargs rm
find . -name '*.txt' | xargs -I{} cp {} /backup/{}

# Parallel execution
cat urls.txt | xargs -P 4 curl -O

# Process substitution — treat command output as a file
diff <(sort file1.txt) <(sort file2.txt)

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

#!/usr/bin/env bash
set -euo pipefail  # Exit on error, unset var, pipe failure

# trap: run cleanup on exit or signal
TMPDIR=$(mktemp -d)
trap 'rm -rf "$TMPDIR"' EXIT

# Trap Ctrl+C and TERM
trap 'echo "Interrupted"; exit 130' INT TERM

# Explicit error check (doesn't require set -e)
if ! command_that_may_fail; then
  echo "Failed" >&2
  exit 1
fi

# Default value if command fails
VALUE=$(get_value) || VALUE="fallback"

# Exit code conventions: 0=success, 1=general error, 127=not found