Bash Cheatsheet

# Assignment (no spaces around =)
NAME="Alice"
AGE=25
readonly PI=3.14
export PATH="$PATH:/usr/local/bin"

# Interpolation & quoting
echo "Hello, $NAME"       # expands
echo 'Hello, $NAME'       # literal
echo "Length: ${#NAME}"    # 5
echo "${NAME,,}"          # lowercase
echo "${NAME^^}"          # uppercase
echo "${NAME:0:3}"        # substring → Ali
echo "${NAME/li/LI}"      # replace → ALIce

# Defaults
echo "${VAR:-default}"     # use default if unset
echo "${VAR:=default}"     # set & use if unset

# Special variables
$0 # script name   $1 # 1st arg   $# # arg count
$@ # all args      $? # exit status
$$ # current PID   $! # last bg PID

# Indexed array
fruits=("apple" "banana" "cherry")
echo "${fruits[0]}"       # apple
echo "${fruits[@]}"       # all
echo "${#fruits[@]}"      # length
fruits+=("date")           # append
unset fruits[1]

# Associative array (bash 4+)
declare -A colors
colors[red]="#ff0000"
echo "${!colors[@]}"      # keys

# Iterate
for f in "${fruits[@]}"; do echo "$f"; done

if [[ "$NAME" == "Alice" ]]; then
  echo "Hi Alice"
elif [[ "$AGE" -gt 18 ]]; then
  echo "Adult"
else
  echo "Minor"
fi

# Operators
-eq -ne -lt -gt -le -ge           # numeric
== != < >                         # string
-z "$a"   # empty   -n "$a"   # not empty
&& || !                           # logical

# Case
case "$1" in
  start) echo "Starting" ;;
  stop)  echo "Stopping" ;;
  *)     echo "Usage: $0 {start|stop}" ;;
esac

for i in 1 2 3; do echo "$i"; done
for i in {1..10}; do echo "$i"; done
for ((i=0; i<10; i++)); do echo "$i"; done

# While / Until
while [[ "$c" -lt 5 ]]; do ((c++)); done
until [[ "$s" == "ready" ]]; do sleep 1; done

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

greet() {
  local name="$1"
  echo "Hello, $name!"
  return 0
}
greet "Alice"

# Capture output
add() { echo $(( $1 + $2 )); }
result=$(add 3 4)

# Error handling
set -euo pipefail
trap "echo Error on line $LINENO" ERR
trap cleanup EXIT

cmd > file.txt        # overwrite
cmd >> file.txt       # append
cmd < file.txt        # stdin from file
cmd 2> err.log        # stderr to file
cmd &> all.log         # stdout+stderr
cmd > /dev/null 2>&1  # discard all

# Here document
cat <<EOF
Hello $NAME
EOF

# Here string
grep "pat" <<< "search this"

# Process substitution
diff <(ls dir1) <(ls dir2)

cat f.txt | grep "err" | wc -l
ps aux | grep "node" | awk '{print $2}'
find . -name "*.tmp" | xargs rm -f

# Command substitution
today=$(date +%Y-%m-%d)

# Background jobs
long_cmd &
jobs   fg %1   wait

# Subshell (doesn't affect parent)
(cd /tmp && ls)

str="Hello World"
echo "${str:6}"        # World
echo "${str:0:5}"      # Hello

file="archive.tar.gz"
echo "${file%.*}"      # archive.tar (shortest from end)
echo "${file%%.*}"     # archive (longest from end)
echo "${file#*.}"      # tar.gz (shortest from start)
echo "${file##*.}"     # gz (longest from start)

echo "${str/World/Bash}" # Hello Bash (first)
echo "${str//l/L}"       # HeLLo WorLd (all)

[[ -f "f" ]]   # regular file
[[ -d "d" ]]   # directory
[[ -e "p" ]]   # exists
[[ -r "f" ]]   # readable
[[ -w "f" ]]   # writable
[[ -x "f" ]]   # executable
[[ -s "f" ]]   # not empty
[[ -L "f" ]]   # symlink
[[ "$a" -nt "$b" ]]  # newer than

# Count lines in *.py
find . -name "*.py" -exec wc -l {} + | tail -1

# Parallel download
cat urls.txt | xargs -P 4 -I {} curl -sO {}

# Quick server
python3 -m http.server 8080

# Largest files
du -ah . | sort -rh | head -20

# Replace in files
find . -name "*.txt" -exec sed -i 's/old/new/g' {} +

# Monitor log
tail -f /var/log/syslog | grep --line-buffered "error"