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. Itโs 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 "$LOCKFILE"' EXIT
PRO Tips
Use set -euo pipefail for safer scripts
Always quote variables: โ$variableโ
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
Links#
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