Cheatsheet

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Cheatsheet#

Bash (Bourne Again Shell) is the default command-line interpreter for most Linux distributions and macOS. Created by Brian Fox in 1989 as a free replacement for the Bourne Shell, Bash has become the de facto standard for shell scripting. It’s both a command interpreter and a programming language, allowing users to automate tasks and create powerful scripts.

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Advanced File Operations

# Find and process files
find . -type f -name "*.log" -mtime +30 -exec rm {} \;

# Mass rename files
for f in *.jpeg; do
    mv "$f" "${f%.jpeg}.jpg"
done

# Create directory structure with parents
mkdir -p project/{src,docs,tests}/{main,utils}

# Secure file deletion
shred -u -z -n 3 sensitive_file.txt

Process Management

# Run command in background and save output
nohup long_running_script.sh > output.log 2>&1 &

# Get process ID of last background command
echo $!

# Kill all processes matching pattern
pkill -f "python.*myapp"

# Monitor process resource usage
top -p $(pgrep -d',' python)

String Manipulation

string="Hello World"

# Substring extraction
echo ${string:6:5}  # "World"

# String replacement
echo ${string//o/0}  # "Hell0 W0rld"

# Convert to uppercase/lowercase
echo ${string^^}  # "HELLO WORLD"
echo ${string,,}  # "hello world"

# Strip prefix/suffix
filename="script.sh.bak"
echo ${filename%.bak}  # "script.sh"

Array Operations

# Declare array
declare -a fruits=("apple" "banana" "orange")

# Add element
fruits+=("mango")

# Get length
echo ${#fruits[@]}

# Slice array
echo ${fruits[@]:1:2}

# Associative array (hash/dictionary)
declare -A config
config[host]="localhost"
config[port]="8080"

Error Handling and Debugging

# Enable debugging
set -x  # Print commands
set -e  # Exit on error
set -u  # Error on undefined variables

# Error handling function
handle_error() {
    echo "Error on line $1"
    exit 1
}
trap 'handle_error $LINENO' ERR

# Conditional error checking
command || { echo "Command failed"; exit 1; }

Advanced Input/Output

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

# Here document
cat << EOF > config.yml
server:
port: 8080
host: localhost
EOF

# Process substitution
diff <(sort file1.txt) <(sort file2.txt)

Network Operations

# Check if port is open
nc -zv localhost 80

# Simple HTTP server
while true; do
    echo -e "HTTP/1.1 200 OK\n\nHello" | nc -l 8080
done

# Download with progress
wget --progress=bar:force -O output.file url

# Monitor network connections
watch -n 1 'netstat -tuln'

Date and Time Operations

# CONTENT_HERE

SECTION

# Format current date
now=$(date +"%Y-%m-%d_%H-%M-%S")

# Date arithmetic
tomorrow=$(date -d "tomorrow" +%Y-%m-%d)
last_week=$(date -d "7 days ago" +%Y-%m-%d)

# Create timestamp for files
touch -t 202312311200.00 file.txt

# Measure execution time
SECONDS=0
sleep 5
echo "Operation took $SECONDS seconds"

Security and Permissions

# Set secure permissions
chmod 750 script.sh
umask 027

# Generate random password
openssl rand -base64 32

# Calculate file hash
sha256sum file.txt

# Find SUID files
find / -perm /4000 -type f 2>/dev/null

Advanced Script Features

# Getopts for argument parsing
while getopts ":h:p:v" opt; do
    case $opt in
        h) host="$OPTARG";;
        p) port="$OPTARG";;
        v) verbose=1;;
        \?) echo "Invalid option: -$OPTARG";;
    esac
done

# Function with return value
is_number() {
    [[ $1 =~ ^[0-9]+$ ]]
    return $?
}

# Lock file to prevent multiple instances
LOCKFILE="/tmp/script.lock"
if ! mkdir "$LOCKFILE" 2>/dev/null; then
    echo "Script is already running"
    exit 1
fi
trap 'rm -rf "$LOCKFILE"' EXIT

PRO Tips

  • Use set -euo pipefail for safer scripts

  • Always quote variables: “$variable”

  • Use shellcheck for script validation

  • Use $(command) instead of backticks

  • Learn to use man pages effectively

  • Use ctrl+r for history search

  • Create aliases for common commands

  • Use screen or tmux for persistent sessions

  • Master regex patterns for text processing

  • Keep scripts modular and documented

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