The Ultimate Bash Scripting Cheatsheet

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

# ── Variables ──
NAME="John"
AGE=30
IS_ADMIN=true
PI=3.14159
EMPTY=""

# Variable interpolation
echo "Hello, $NAME"
echo "Hello, ${NAME}"    # Braces for clarity
echo "Age: $((AGE + 5))" # Arithmetic expansion

# Command substitution
CURRENT_DIR=$(pwd)
FILE_COUNT=$(ls -1 | wc -l)
TODAY=$(date +%Y-%m-%d)
IP_ADDR=$(hostname -I | awk '{print $1}')

# ── Strings ──
GREETING="Hello, World!"
echo ${'#GREETING'}              # Length: 13
echo ${'GREETING:0:5'}           # Substring: "Hello"
echo ${'GREETING:7'}             # From index 7: "World!"
echo ${'GREETING#"World!"'}      # Remove suffix: "Hello, "
echo ${'GREETING#"o"'}           # Remove shortest suffix
echo ${'GREETING##"W"'}          # Remove longest suffix
echo ${'GREETING#*"o"'}          # Remove shortest prefix
echo ${'GREETING##*"o"'}         # Remove longest prefix
echo ${'GREETING^^'}             # Uppercase all
echo ${'GREETING,,'}             # Lowercase all

# String replacement
TEXT="hello world"
echo ${'TEXT/world/World'}        # "hello World"
echo ${'TEXT//l/L'}               # "heLLo WorLd" (all)

# Default values
echo ${'UNDEFINED:-"default"'}   # Use default if unset/null
echo ${'UNDEFINED:="default"'}   # Assign default if unset/null
echo ${'NAME:+"exists"'}          # Use "exists" if NAME is set

# ── Special Variables ──
echo $$     # Current PID
echo $0     # Script name
echo $1-$9 # Positional parameters
echo $#     # Number of parameters
echo $@     # All parameters as separate words
echo $*     # All parameters as single string
echo $?     # Exit status of last command
echo $!     # PID of last background process

#!/usr/bin/env bash

# ── If / Elif / Else ──
AGE=25

if [ "$AGE" -ge 18 ]; then
    echo "Adult"
elif [ "$AGE" -ge 13 ]; then
    echo "Teenager"
else
    echo "Child"
fi

# ── Test Operators ──
FILE="/etc/passwd"

# File tests
[ -f "$FILE" ] && echo "Regular file exists"
[ -d "$FILE" ] && echo "Directory exists"
[ -r "$FILE" ] && echo "Readable"
[ -w "$FILE" ] && echo "Writable"
[ -x "$FILE" ] && echo "Executable"
[ -s "$FILE" ] && echo "Non-empty file"
[ -e "$FILE" ] && echo "Exists"

# String tests
[ "$NAME" = "John" ] && echo "Equals"
[ "$NAME" != "Jane" ] && echo "Not equal"
[ -z "$EMPTY" ] && echo "Empty string"
[ -n "$NAME" ] && echo "Non-empty string"

# Numeric tests
[ 10 -eq 10 ] && echo "Equal"
[ 10 -ne 5 ]  && echo "Not equal"
[ 10 -gt 5 ]  && echo "Greater than"
[ 10 -ge 10 ] && echo "Greater or equal"
[ 5 -lt 10 ]  && echo "Less than"
[ 5 -le 5 ]   && echo "Less or equal"

# ── Case Statement ──
read -p "Choose (start|stop|restart): " ACTION

case "$ACTION" in
    start)
        echo "Starting service..."
        ;;
    stop)
        echo "Stopping service..."
        ;;
    restart|reload)
        echo "Restarting service..."
        ;;
    *)
        echo "Unknown action: $ACTION"
        exit 1
        ;;
esac

# ── For Loops ──
for i in 1 2 3 4 5; do
    echo "Number: $i"
done

for file in *.sh; do
    echo "Processing: $file"
done

for ((i=0; i<10; i++)); do
    echo "Count: $i"
done

# ── While Loop ──
COUNT=0
while [ $COUNT -lt 5 ]; do
    echo "Count: $COUNT"
    COUNT=$((COUNT + 1))
done

# ── Until Loop ──
until [ -f "/tmp/ready" ]; do
    echo "Waiting..."
    sleep 1
done

#!/usr/bin/env bash

# ── Function Definition ──
greet() {
    local name="${'1:-World'}"
    local count="${'2:-1'}"
    echo "Hello, $name! (called $count times)"
}

greet "John"       # Hello, John!
greet               # Hello, World!
greet "Alice" 3     # Hello, Alice! (called 3 times)

# ── Function with return value ──
is_file_exists() {
    local file="$1"
    [ -f "$file" ] && return 0 || return 1
}

is_file_exists "/etc/passwd" && echo "Found" || echo "Not found"

# ── Function that captures output ──
get_file_info() {
    local file="$1"
    if [ ! -f "$file" ]; then
        echo "Error: File not found: $file" >&2
        return 1
    fi
    local size=$(stat -f%z "$file" 2>/dev/null || stat -c%s "$file")
    local lines=$(wc -l < "$file")
    echo "Size: $size bytes, Lines: $lines"
}

INFO=$(get_file_info "script.sh")
echo "$INFO"

# ── Arguments Processing ──
usage() {
    echo "Usage: $0 [-v] [-f FILE] [-n COUNT] NAME"
    echo "  -v          Verbose mode"
    echo "  -f FILE    Input file"
    echo "  -n COUNT   Number of iterations"
    echo "  -h          Show this help"
    exit 1
}

VERBOSE=false
FILE=""
COUNT=10

while getopts "vf:n:h" opt; do
    case $opt in
        v) VERBOSE=true ;;
        f) FILE="$OPTARG" ;;
        n) COUNT="$OPTARG" ;;
        h|*) usage ;;
    esac
done
shift $((OPTIND - 1))
NAME="${'1:-default'}"

# ── Error Handling ──
set -euo pipefail
trap 'echo "Error on line $LINENO"' ERR
trap 'echo "Cleaning up..."; rm -f /tmp/mystatus.$$; exit' EXIT

#!/usr/bin/env bash

# ── awk ──
# Print specific columns
awk '{print $1, $3}' file.txt

# Filter rows
awk '$3 > 100' file.txt

# Custom delimiter
awk -F',' '{print $1, $2}' csv.txt

# Conditional formatting
awk '{printf "%-15s %10d\n", $1, $3}' file.txt

# ── sed ──
# Replace text
sed 's/old/new/g' file.txt          # Global replace
sed 's/old/new/gi' file.txt         # Case-insensitive
sed -i 's/old/new/g' file.txt       # In-place edit
sed '2d' file.txt                   # Delete line 2
sed '1,5d' file.txt                 # Delete lines 1-5
sed -n '/pattern/p' file.txt        # Print matching lines
sed -i 's/^#//' file.txt            # Uncomment lines

# ── grep ──
grep "pattern" file.txt             # Basic search
grep -i "pattern" file.txt          # Case-insensitive
grep -r "pattern" /dir/            # Recursive
grep -n "pattern" file.txt          # With line numbers
grep -v "pattern" file.txt          # Invert match
grep -c "pattern" file.txt          # Count matches
grep -E "pat1|pat2" file.txt        # Extended regex (OR)
grep -A 5 -B 2 "error" log.txt     # Context lines
grep -l "pattern" *.sh              # Files matching

# ── cut ──
cut -d',' -f1,3 file.csv            # Fields 1 and 3
cut -d' ' -f1 file.txt              # First field
cut -c1-10 file.txt                 # Characters 1-10

# ── sort & uniq ──
sort file.txt                       # Sort alphabetically
sort -n file.txt                    # Sort numerically
sort -r file.txt                    # Reverse sort
sort -k2 -n file.txt                # Sort by field 2
sort file.txt | uniq -c             # Count duplicates
sort file.txt | uniq -u             # Remove duplicates