Essential Linux Commands: A Practical Reference Guide for Developers

# Print Working Directory
pwd                      # Shows current path

# Change Directory
cd /path/to/directory    # Absolute path
cd subfolder             # Relative path
cd ..                    # Go up one level
cd ../..                 # Go up two levels
cd ~                     # Go to home directory
cd -                     # Go to previous directory

# List Files
ls                       # List files in current dir
ls -l                    # Long listing (permissions, size, date)
ls -a                    # Show hidden files (starting with .)
ls -la                   # Long listing + hidden files
ls -lh                   # Human readable sizes (KB, MB)
ls -lt                   # Sort by modification time
ls -lS                   # Sort by file size
ls -R                    # Recursive listing

# Tree view (if installed)
tree -L 2                # Show 2 levels deep
tree -d                  # Directories only

# Find files
find /var -name "*.log"                    # Find by name
find . -type f -mtime -7                   # Files modified in last 7 days
find . -type f -size +100M                 # Files larger than 100MB
find . -name "*.tmp" -delete               # Find and delete

# Locate (faster, uses database)
locate nginx.conf        # Requires updatedb to be run
sudo updatedb            # Update locate database

# Create files and directories
touch newfile.txt        # Create empty file or update timestamp
mkdir newdir             # Create directory
mkdir -p path/to/nested  # Create nested directories

# Copy files
cp file1.txt file2.txt           # Copy file
cp -r dir1/ dir2/                # Copy directory recursively
cp -i file1.txt file2.txt        # Interactive (ask before overwrite)
cp -p file1.txt file2.txt        # Preserve permissions and timestamps

# Move and rename
mv oldname.txt newname.txt       # Rename file
mv file.txt /new/path/           # Move file
mv -i file.txt dest/             # Ask before overwrite

# Delete files
rm file.txt                      # Delete file
rm -r directory/                 # Delete directory recursively
rm -f file.txt                   # Force delete (no confirmation)
rm -rf directory/                # Force delete directory (use carefully!)
rmdir emptydir                   # Delete empty directory only

# Links
ln -s /path/to/target linkname   # Create symbolic link
ln target hardlink               # Create hard link
readlink -f linkname             # Show where symlink points

# File info
file document.pdf                # Detect file type
stat file.txt                    # Detailed file information
du -sh directory/                # Directory size
du -h --max-depth=1 .            # Size of subdirectories

# Viewing files
cat file.txt             # Output entire file
less file.txt            # Paginated view (q to quit)
head -n 20 file.txt      # First 20 lines
tail -n 20 file.txt      # Last 20 lines
tail -f /var/log/syslog  # Follow file (live updates)

# Searching with grep
grep "error" log.txt              # Find lines containing "error"
grep -i "error" log.txt           # Case insensitive
grep -r "TODO" src/               # Recursive search in directory
grep -n "error" log.txt           # Show line numbers
grep -v "debug" log.txt           # Invert (lines NOT matching)
grep -c "error" log.txt           # Count matches
grep -E "error|warning" log.txt   # Extended regex (OR)
grep -A 3 -B 1 "error" log.txt    # 3 lines after, 1 before match

# Alternative: ripgrep (rg) - faster for codebases
rg "TODO" --type py               # Search only Python files

# Sorting and deduplicating
sort file.txt                     # Sort lines alphabetically
sort -n numbers.txt               # Sort numerically
sort -r file.txt                  # Reverse sort
sort -u file.txt                  # Sort and remove duplicates
uniq file.txt                     # Remove adjacent duplicates
sort file.txt | uniq -c           # Count occurrences

# Cutting and extracting
cut -d',' -f1,3 data.csv          # Extract columns 1 and 3
cut -c1-10 file.txt               # Extract characters 1-10
awk '{print $1, $3}' file.txt     # Print columns 1 and 3
awk -F',' '{print $2}' data.csv   # CSV column 2

# Text transformation
tr 'a-z' 'A-Z' < file.txt         # Convert to uppercase
tr -d ' ' < file.txt              # Delete all spaces
sed 's/old/new/g' file.txt        # Replace text (global)
sed -i 's/old/new/g' file.txt     # Replace in-place
sed -n '10,20p' file.txt          # Print lines 10-20

# Word and line counts
wc -l file.txt                    # Count lines
wc -w file.txt                    # Count words
wc -c file.txt                    # Count bytes

# View permissions
ls -l file.txt
# Output: -rw-r--r-- 1 user group 1234 Jan 25 10:00 file.txt
# Format: [type][owner][group][others]
# r=read(4), w=write(2), x=execute(1)

# Change permissions
chmod 755 script.sh      # rwxr-xr-x (owner all, others read+execute)
chmod 644 file.txt       # rw-r--r-- (owner read+write, others read)
chmod +x script.sh       # Add execute permission
chmod -w file.txt        # Remove write permission
chmod u+x,g+r file.txt   # User add execute, group add read
chmod -R 755 directory/  # Recursive

# Common permission values
# 755 - Directories and executables
# 644 - Regular files
# 600 - Private files (only owner)
# 700 - Private directories

# Change ownership
chown user file.txt              # Change owner
chown user:group file.txt        # Change owner and group
chown -R user:group directory/   # Recursive
chgrp group file.txt             # Change group only

# Special permissions
chmod u+s executable     # SUID (run as owner)
chmod g+s directory/     # SGID (inherit group)
chmod +t /tmp            # Sticky bit (only owner can delete)

# View processes
ps aux                   # All processes (BSD style)
ps -ef                   # All processes (POSIX style)
ps aux | grep nginx      # Find specific process
pgrep nginx              # Get PID by name
top                      # Interactive process viewer
htop                     # Better interactive viewer (if installed)

# Process details
ps aux --sort=-%mem | head   # Top memory consumers
ps aux --sort=-%cpu | head   # Top CPU consumers
lsof -p 1234             # Files opened by PID 1234
lsof -i :8080            # Process using port 8080

# Background and foreground
command &                # Run in background
jobs                     # List background jobs
fg %1                    # Bring job 1 to foreground
bg %1                    # Resume job 1 in background
Ctrl+Z                   # Suspend current process
nohup command &          # Run immune to hangups

# Stop processes
kill 1234                # Send SIGTERM to PID 1234
kill -9 1234             # Send SIGKILL (force kill)
killall nginx            # Kill all by name
pkill -f "python script" # Kill by pattern in command
timeout 10 command       # Kill after 10 seconds

# Common signals
kill -SIGTERM 1234   # or kill -15 (graceful shutdown)
kill -SIGKILL 1234   # or kill -9 (force kill, no cleanup)
kill -SIGHUP 1234    # or kill -1 (reload configuration)
kill -SIGSTOP 1234   # or kill -19 (pause process)
kill -SIGCONT 1234   # or kill -18 (resume process)

# List all signals
kill -l

# Network interfaces
ip addr                  # Show IP addresses
ip link                  # Show network interfaces
ifconfig                 # Legacy (deprecated)
hostname -I              # Show IP addresses only

# DNS and connectivity
ping google.com          # Test connectivity
ping -c 4 google.com     # Ping 4 times then stop
traceroute google.com    # Trace route to host
mtr google.com           # Continuous traceroute
nslookup google.com      # DNS lookup
dig google.com           # Detailed DNS lookup
dig +short google.com    # Just the IP
host google.com          # Simple DNS lookup

# Ports and connections
netstat -tulpn           # Listening ports (legacy)
ss -tulpn                # Listening ports (modern)
ss -s                    # Socket statistics summary
lsof -i :80              # What's using port 80

# HTTP requests
curl https://api.example.com              # GET request
curl -I https://api.example.com           # Headers only
curl -X POST -d '{"key":"val"}' url       # POST JSON
curl -o file.zip https://example.com/file # Download to file
wget https://example.com/file.zip         # Download file

# File transfer
scp file.txt user@host:/path/             # Copy to remote
scp user@host:/path/file.txt ./           # Copy from remote
rsync -avz dir/ user@host:/path/          # Sync directory

# SSH connections
ssh user@hostname                     # Connect to remote host
ssh -p 2222 user@hostname             # Connect on custom port
ssh -i ~/.ssh/key.pem user@hostname   # Use specific key
ssh -L 8080:localhost:80 user@host    # Local port forward
ssh -R 8080:localhost:80 user@host    # Remote port forward
ssh -D 1080 user@host                 # SOCKS proxy

# SSH keys
ssh-keygen -t ed25519 -C "email"      # Generate new key
ssh-copy-id user@hostname             # Copy key to server
ssh-add ~/.ssh/id_ed25519             # Add key to agent

# Disk usage
df -h                    # Filesystem disk usage (human readable)
df -h /                  # Usage for specific mount
du -sh directory/        # Size of directory
du -h --max-depth=1 .    # Sizes of subdirectories
ncdu /var                # Interactive disk usage analyzer

# Find large files
find / -type f -size +500M 2>/dev/null   # Files over 500MB
du -ah . | sort -rh | head -20           # Top 20 largest files/dirs

# Disk information
lsblk                    # List block devices
fdisk -l                 # Partition information (as root)
mount                    # Show mounted filesystems
cat /etc/fstab           # Permanent mount configuration

# Mount and unmount
mount /dev/sdb1 /mnt/usb         # Mount device
umount /mnt/usb                  # Unmount
mount -o remount,rw /            # Remount with different options

# Check and repair
fsck /dev/sdb1           # Check filesystem (unmounted only!)

# Clean up space
# Clear package cache
sudo apt clean                           # Debian/Ubuntu
sudo yum clean all                       # RHEL/CentOS

# Find and remove old logs
sudo journalctl --vacuum-time=7d         # Keep only 7 days of logs
sudo journalctl --vacuum-size=500M       # Limit log size

# Docker cleanup
docker system prune -a                   # Remove unused images, containers

# Clear temp files
sudo rm -rf /tmp/*                       # Clear temp
rm -rf ~/.cache/*                        # Clear user cache

# Operating system info
uname -a                 # Full system info
cat /etc/os-release      # OS version
hostnamectl              # Hostname and OS info
uptime                   # System uptime and load

# Hardware info
lscpu                    # CPU information
free -h                  # Memory usage
cat /proc/meminfo        # Detailed memory info
lsblk                    # Block devices
lspci                    # PCI devices
lsusb                    # USB devices

# System load and resources
top                      # Process and resource monitor
htop                     # Better process monitor
vmstat 1                 # Virtual memory stats (every 1 sec)
iostat 1                 # IO statistics
sar                      # System activity report

# Logs
dmesg                    # Kernel messages
dmesg | tail             # Recent kernel messages
journalctl               # Systemd journal
journalctl -u nginx      # Logs for specific service
journalctl -f            # Follow logs (like tail -f)
tail -f /var/log/syslog  # Traditional syslog

# Service management (systemd)
systemctl status nginx           # Check service status
systemctl start nginx            # Start service
systemctl stop nginx             # Stop service
systemctl restart nginx          # Restart service
systemctl reload nginx           # Reload config without restart
systemctl enable nginx           # Start on boot
systemctl disable nginx          # Don't start on boot
systemctl list-units --type=service --all   # List all services

# Check what's listening
ss -tulpn                        # Listening ports with process info

#!/bin/bash
# Shebang tells system which interpreter to use

# Variables
NAME="John"
echo "Hello, $NAME"
CURRENT_DIR=$(pwd)       # Command substitution
FILES=$(ls -la)

# Conditionals
if [ -f "file.txt" ]; then
    echo "File exists"
elif [ -d "directory" ]; then
    echo "Directory exists"
else
    echo "Not found"
fi

# Common test operators
# -f file    : File exists and is regular file
# -d dir     : Directory exists
# -z string  : String is empty
# -n string  : String is not empty
# -e path    : Path exists (file or directory)
# $a -eq $b  : Numbers are equal
# $a -ne $b  : Numbers are not equal
# $a -gt $b  : a greater than b
# $a -lt $b  : a less than b

# Loops
for file in *.txt; do
    echo "Processing $file"
done

for i in {1..10}; do
    echo "Count: $i"
done

while read line; do
    echo "$line"
done < file.txt

#!/bin/bash
# Example: Backup script

BACKUP_DIR="/backup"
SOURCE_DIR="/var/www"
DATE=$(date +%Y%m%d_%H%M%S)
FILENAME="backup_$DATE.tar.gz"

# Create backup directory if not exists
mkdir -p "$BACKUP_DIR"

# Create compressed backup
tar -czf "$BACKUP_DIR/$FILENAME" "$SOURCE_DIR"

# Check if backup succeeded
if [ $? -eq 0 ]; then
    echo "Backup created: $FILENAME"
    # Keep only last 7 backups
    ls -t "$BACKUP_DIR"/*.tar.gz | tail -n +8 | xargs rm -f
else
    echo "Backup failed!" >&2
    exit 1
fi