Bash Cheatsheet

Variables

NAME="world"              # Assignment (no spaces around =)
echo "Hello $NAME"        # Double quotes: variable expansion
echo 'Hello $NAME'        # Single quotes: literal string
echo "Path is ${HOME}/bin" # Braces for clarity
readonly PI=3.14          # Read-only variable
unset NAME                # Delete variable
export VAR="value"        # Export to child processes

Special Variables

Variable	Description
$0	Script name
$1 .. $9	Positional arguments
$#	Number of arguments
$@	All arguments (separate words)
$*	All arguments (single string)
$?	Exit status of last command
$$	Current process ID
$!	PID of last background process

Conditionals

if [[ condition ]]; then
  # ...
elif [[ condition ]]; then
  # ...
else
  # ...
fi

Test Operators

Operator	Description
-z "$str"	String is empty
-n "$str"	String is not empty
"$a" == "$b"	String equality
"$a" != "$b"	String inequality
"$a" =~ regex	Regex match
-eq, -ne, -lt, -gt, -le, -ge	Integer comparisons
-f file	File exists and is regular file
-d dir	Directory exists
-r file	File is readable
-w file	File is writable
-x file	File is executable
-s file	File exists and is not empty

Loops

# For loop
for i in 1 2 3; do echo "$i"; done

# C-style for
for ((i=0; i<10; i++)); do echo "$i"; done

# While loop
while [[ condition ]]; do
  # ...
done

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

# Iterate over array
arr=(a b c)
for item in "${arr[@]}"; do echo "$item"; done

String Manipulation

Syntax	Description
${#str}	String length
${str:0:5}	Substring (offset, length)
${str#pattern}	Remove shortest prefix match
${str##pattern}	Remove longest prefix match
${str%pattern}	Remove shortest suffix match
${str%%pattern}	Remove longest suffix match
${str/old/new}	Replace first occurrence
${str//old/new}	Replace all occurrences
${str,,}	Lowercase
${str^^}	Uppercase
${str:-default}	Default if unset/empty
${str:=default}	Assign default if unset/empty

Arrays

arr=(one two three)       # Declare array
arr+=("four")             # Append
echo "${arr[0]}"          # Access element
echo "${arr[@]}"          # All elements
echo "${#arr[@]}"         # Array length
unset arr[1]              # Remove element

declare -A map            # Associative array
map[key]="value"
echo "${!map[@]}"         # All keys

File Operations & Redirection

Syntax	Description
cmd > file	Redirect stdout (overwrite)
cmd >> file	Redirect stdout (append)
cmd 2> file	Redirect stderr
cmd &> file	Redirect stdout and stderr
cmd < file	Read stdin from file
cmd1 | cmd2	Pipe stdout to next command
cmd1 |& cmd2	Pipe stdout and stderr

Functions

greet() {
  local name="$1"       # Local variable
  echo "Hello, $name"
  return 0               # Exit status
}
greet "world"
result=$(greet "world")  # Capture output

Useful Patterns

# Ternary-like
[[ condition ]] && echo "true" || echo "false"

# Default value
val="${1:-default}"

# Check command exists
command -v git &>/dev/null && echo "git found"

# Here document
cat <<EOF
Hello $NAME
EOF

# Trap signals
trap 'echo "Caught SIGINT"; exit' INT

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

Arithmetic

result=$((5 + 3))        # Arithmetic expansion
((count++))              # Increment
((count += 5))           # Add and assign