Linux Command Reference

This reference provides an overview of essential Linux commands for file management, process control, networking, system information, and more. It serves as a quick guide for both beginners and experienced users.

File and Directory Commands

Command	Description
`ls`	List files and directories.
`cd <directory>`	Change the current directory.
`pwd`	Print the current working directory.
`mkdir <directory>`	Create a new directory.
`rmdir <directory>`	Remove an empty directory.
`rm <file>`	Delete a file.
`rm -r <directory>`	Recursively delete a directory and its contents.
`cp <source> <destination>`	Copy files or directories.
`mv <source> <destination>`	Move or rename files or directories.
`touch <file>`	Create an empty file or update the timestamp.
`cat <file>`	Display the contents of a file.
`less <file>`	View the contents of a file one page at a time.
`head <file>`	Display the first lines of a file.
`tail <file>`	Display the last lines of a file.
`find <path> -name <pattern>`	Search for files and directories by name.
`grep <pattern> <file>`	Search for patterns within files.

File Permissions and Ownership

Command	Description
`chmod <permissions> <file>`	Change the permissions of a file or directory.
`chown <owner> <file>`	Change the owner of a file or directory.
`chgrp <group> <file>`	Change the group ownership.
`umask <mask>`	Set default file creation permissions.

Process Management

Command	Description
`ps`	Display information about running processes.
`top`	Display real-time system statistics.
`htop`	An enhanced version of `top` (may require installation).
`kill <PID>`	Terminate a process by its PID.
`killall <process_name>`	Terminate all processes with the given name.
`bg`	Resume a suspended job in the background.
`fg`	Bring a background job to the foreground.
`jobs`	List current jobs.
`nice -n <priority> <command>`	Start a process with a given priority.
`renice <priority> -p <PID>`	Change the priority of an existing process.

Networking Commands

Command	Description
`ifconfig` or `ip addr`	Display network interface information.
`ping <host>`	Send ICMP ECHO_REQUEST packets to network hosts.
`traceroute <host>`	Trace the route packets take to a network host.
`netstat -tuln`	List open ports and listening services.
`nslookup <domain>`	Query Internet name servers interactively.
`dig <domain>`	DNS lookup utility.
`ssh user@host`	Connect to a remote host securely.
`scp <source> <destination>`	Securely copy files between hosts.
`curl <url>`	Transfer data from or to a server.
`wget <url>`	Retrieve files from the web.

System Information

Command	Description
`uname -a`	Display system information.
`df -h`	Show disk space usage.
`du -sh <directory>`	Estimate file space usage.
`free -h`	Display memory usage.
`uptime`	Show how long the system has been running.
`who`	Show who is logged on.
`last`	Show last logged in users.
`history`	Show command history.
`man <command>`	Display the manual page for a command.
`info <command>`	Display info documentation.

Package Management

Debian/Ubuntu-based Systems (using apt):

Command	Description
`sudo apt update`	Update package index.
`sudo apt upgrade`	Upgrade installed packages.
`sudo apt install <package>`	Install a package.
`sudo apt remove <package>`	Remove a package.
`sudo apt search <package>`	Search for a package.

Red Hat/CentOS-based Systems (using yum or dnf):

Command	Description
`sudo yum update`	Update packages.
`sudo yum install <package>`	Install a package.
`sudo yum remove <package>`	Remove a package.
`sudo yum search <package>`	Search for a package.

Compression and Archiving

Command	Description
`tar -cvf archive.tar <files>`	Create a tar archive.
`tar -xvf archive.tar`	Extract a tar archive.
`tar -zcvf archive.tar.gz <files>`	Create a compressed tar archive with gzip.
`tar -zxvf archive.tar.gz`	Extract a gzip compressed tar archive.
`zip archive.zip <files>`	Create a zip archive.
`unzip archive.zip`	Extract a zip archive.
`gzip <file>`	Compress a file with gzip.
`gunzip <file.gz>`	Decompress a gzip file.

User and Group Management

Command	Description
`sudo adduser <username>`	Add a new user.
`sudo passwd <username>`	Change a user's password.
`sudo deluser <username>`	Delete a user.
`sudo addgroup <groupname>`	Add a new group.
`sudo delgroup <groupname>`	Delete a group.
`sudo usermod -aG <group> <username>`	Add a user to a group.
`id <username>`	Display user identity.

Disk Management

Command	Description
`fdisk -l`	List disk partitions.
`mount <device> <mount_point>`	Mount a filesystem.
`umount <mount_point>`	Unmount a filesystem.
`df -h`	Report filesystem disk space usage.
`du -sh <directory>`	Estimate file space usage.
`fsck <device>`	Check and repair a filesystem.

System Control and Shutdown

Command	Description
`sudo shutdown -h now`	Shutdown the system immediately.
`sudo shutdown -r now`	Reboot the system immediately.
`sudo reboot`	Reboot the system.
`sudo systemctl start <service>`	Start a system service.
`sudo systemctl stop <service>`	Stop a system service.
`sudo systemctl restart <service>`	Restart a system service.
`sudo systemctl enable <service>`	Enable a service to start on boot.
`sudo systemctl disable <service>`	Disable a service from starting on boot.
`sudo systemctl status <service>`	Check the status of a service.

Environment Variables and Shell

Command	Description
`echo $VARIABLE`	Display the value of an environment variable.
`export VARIABLE=value`	Set or modify an environment variable.
`alias <name>='<command>'`	Create a shell alias.
`unalias <name>`	Remove a shell alias.
`env`	Display all environment variables.

Text Processing

Command	Description
`grep <pattern> <file>`	Search for a pattern in files.
`sed 's/old/new/g' <file>`	Replace text in a file using stream editor.
`awk '/pattern/ {action}' <file>`	Pattern scanning and processing language.
`sort <file>`	Sort lines of text files.
`uniq <file>`	Report or filter out repeated lines.
`diff <file1> <file2>`	Compare files line by line.
`wc <file>`	Print newline, word, and byte counts.
`tee <file>`	Read from standard input and write to standard output and files.

Redirection and Pipes

Use redirection and pipes to control input and output:

> - Redirect output to a file (overwrite).
>> - Redirect output to a file (append).
< - Redirect input from a file.
| - Pipe the output of one command as input to another.
2>&1 - Redirect standard error to standard output.

Examples

# Redirect output to a file
ls -l > filelist.txt

# Append output to a file
echo "New line" >> file.txt

# Redirect error messages to a file
command 2> error.log

# Pipe output to another command
ps aux | grep apache

# Suppress output
command > /dev/null 2>&1

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