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Linux Fundamental Commands

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Linux Fundamental Commands

This repository contains a comprehensive list of fundamental Linux commands that are commonly used in Bash, SH, and other UNIX shells.

Table of Contents

Navigation

  • cd: Change directory. 
    cd path/to/directory
  • ls: List directory contents.
ls
ls -l
ls -a
  • pwd: Print working directory.
pwd

File Management

  • cp: Copy files and directories.
cp source_file destination_file
  • mv: Move or rename files and directories.
mv old_file new_file
mv file /path/to/directory
  • rm: Remove files and directories.
rm file
rm -r directory
  • mkdir: Create directories.
mkdir directory_name
  • rmdir: Remove empty directories.
rmdir directory_name

Text Processing

  • cat: Concatenate and display files.
cat filename
  • grep: Search for patterns in files.
grep pattern filename
  • sed: Stream editor for text transformation.
sed 's/old/new/' filename
  • awk: Pattern scanning and processing language.
awk '/pattern/ { print $1 }' filename

System Information

  • uname: Print system information.
uname -a

df: Display disk space usage.

df -h

free: Display amount of free and used memory.

free -h

User Management

  • whoami: Print the current user.
whoami
  • who: Display who is logged in.
who
  • passwd: Change user password.
passwd

Network

  • ping: Check the network connection to a server.
ping server_name
  • ifconfig / ip: Display and configure network interfaces.
ifconfig
ip addr show
  • wget: Non-interactive network downloader.
wget URL

Package Management

  • apt-get: Command-line package handling utility (Debian-based).
sudo apt-get install package_name
  • yum: Package manager for RPM-based distributions.
sudo yum install package_name
  • the command to download a file from internet or over a network:
wget URL
curl URL -o outputfile

Permissions

  • chmod: Change file permissions.
chmod permissions filename
  • chown: Change file owner and group.
chown user:group filename
  • sudo -i or sudo su - are equivalent commands. These commands start a new shell as the superuser (root) with root's environment variables and settings.
sudo -i: This command starts an interactive shell session as the root user, loading root's environment variables.
sudo su -: This is an alternative syntax to achieve the same result—switching to the root user with a login shell.
  • sudo su allows you to switch to another user account. It invokes the su command with superuser privileges granted by sudo, enabling you to log in as another user.
It's generally followed by - <username> to switch to a specific user account,
for example, sudo su - john to switch to user john's environment.
  • sudo -u executes a command as a specified user.
sudo -u <username> command: This executes command as the user <username>.

Processes

  • ps: Report a snapshot of current processes.
ps aux
  • kill: Send a signal to a process.
kill PID
  • top / htop: Display Linux tasks.
top
htop

Miscellaneous

  • date: Print or set the system date and time.
date
  • history: Command history.
history
  • tar: Archive files.
tar -cvf archive_name.tar directory_to_compress

Sample Shell Scripts

1. Backup Script

This script creates a backup of specified directories and compresses them into a tarball with a timestamp.

#!/bin/bash

# Directories to backup
backup_dirs="/var/www /etc /home/user"

# Backup destination directory
backup_dest="/backup"

# Create backup directory if it doesn't exist
mkdir -p "$backup_dest"

# Backup filename with timestamp
backup_file="backup-$(date +%Y%m%d%H%M%S).tar.gz"

# Perform backup
tar -czvf "$backup_dest/$backup_file" $backup_dirs

# Optionally, clean up old backups (e.g., keep last 7 days)
find "$backup_dest" -name "backup-*" -mtime +7 -exec rm {} \;

2. Log Rotation Script

This script rotates logs and archives older log files to save disk space.

#!/bin/bash

# Log directory
log_dir="/var/log/app"

# Maximum number of log files to keep
max_logs=5

# Rotate and compress logs
for logfile in $(ls -t $log_dir/*.log | tail -n +$max_logs); do
    gzip -9 "$logfile"
done

# Remove compressed logs older than 7 days
find $log_dir/*.log.gz -mtime +7 -exec rm {} \;

3. System Maintenance Script

This script performs routine maintenance tasks such as updating packages and cleaning up temporary files.

#!/bin/bash

# Update package lists and upgrade packages
apt-get update
apt-get upgrade -y

# Clean up unused packages and old kernels
apt-get autoremove -y
apt-get autoclean

# Clear temporary files
rm -rf /tmp/*

# Optionally, reboot the system if required
# reboot

4. Service Monitoring Script

This script checks the status of critical services and restarts them if they are not running.

#!/bin/bash
# List of services to monitor
services=("apache2" "mysql")

# Check status and restart if necessary
for service in "${services[@]}"; do
    systemctl is-active --quiet $service
    if [ $? -ne 0 ]; then
        systemctl start $service
        echo "Service $service restarted at $(date)" >> /var/log/service-monitor.log
    fi
done

Tips for Using Shell Scripts in Production:

  • Permissions: Ensure scripts have appropriate permissions (chmod +x script.sh) and are run with appropriate privileges.
  • Testing: Test scripts in a non-production environment before deploying them to production.
  • Logging: Implement logging to track script executions and outputs.
  • Error Handling: Include error checking and handling to manage unexpected conditions gracefully.
  • Security: Avoid hardcoding sensitive information (e.g., passwords) directly into scripts; use environment variables or secure vaults.

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Linux Fundamental Commands