Which Linux commands are mostly used?

/ / Published in Cybersecurity, EITC/IS/LSA Linux System Administration, Introduction, Getting started

In the domain of Cybersecurity and Linux System Administration, understanding the fundamental Linux commands is pivotal for both system management and security operations. Linux commands are the building blocks for interacting with the operating system, and their proficiency is essential for effective system administration and cybersecurity tasks. Below, we consider some of the most commonly used Linux commands, providing a detailed explanation of their purposes, functionalities, and examples to illustrate their practical applications.

1. `ls` – List Directory Contents

The `ls` command is used to list the contents of a directory. It is one of the most frequently used commands as it helps users and administrators to see the files and directories within a specified directory.

Usage:

bash
ls [options] [directory]
Common Options:
– `-l`: Long listing format. Provides detailed information about files and directories.
– `-a`: Lists all files, including hidden files (those starting with a dot `.`).
– `-h`: Human-readable format, making file sizes easier to read.

Example:

bash
ls -lah /var/log

This command lists all files in the `/var/log` directory in a long listing format, including hidden files, with human-readable file sizes.

2. `cd` – Change Directory

The `cd` command is used to change the current working directory. It is essential for navigating the file system.

Usage:

bash
cd [directory]
Example:

bash
cd /etc

This command changes the current directory to `/etc`.

3. `pwd` – Print Working Directory

The `pwd` command prints the full path of the current working directory. It is useful for confirming the directory you are currently in.

Usage:

bash
pwd
Example:

bash
pwd

If the current directory is `/home/user`, the command will output `/home/user`.

4. `cp` – Copy Files and Directories

The `cp` command is used to copy files and directories from one location to another.

Usage:

bash
cp [options] source destination
Common Options:
– `-r`: Recursively copy directories and their contents.
– `-i`: Prompt before overwrite.
– `-v`: Verbose mode, showing files being copied.

Example:

bash
cp -r /home/user/docs /backup/docs

This command copies the `docs` directory from `/home/user` to `/backup`.

5. `mv` – Move or Rename Files and Directories

The `mv` command moves files and directories from one location to another. It is also used to rename files and directories.

Usage:

bash
mv [options] source destination
Example:

bash
mv /home/user/file.txt /home/user/docs/file.txt

This command moves `file.txt` from `/home/user` to `/home/user/docs`.

6. `rm` – Remove Files and Directories

The `rm` command is used to remove files and directories.

Usage:

bash
rm [options] file
Common Options:
– `-r`: Recursively remove directories and their contents.
– `-i`: Prompt before each removal.
– `-f`: Force removal without prompt.

Example:

bash
rm -rf /home/user/temp

This command forcefully removes the `temp` directory and its contents.

7. `chmod` – Change File Modes or Access Permissions

The `chmod` command is used to change the access permissions of files and directories.

Usage:

bash
chmod [options] mode file
Example:

bash
chmod 755 /home/user/script.sh

This command sets the permissions of `script.sh` to `755` (rwxr-xr-x).

8. `chown` – Change File Owner and Group

The `chown` command changes the ownership of a file or directory.

Usage:

bash
chown [options] owner[:group] file
Example:

bash
chown root:root /var/www/html

This command changes the owner and group of `/var/www/html` to `root`.

9. `ps` – Report a Snapshot of Current Processes

The `ps` command displays information about the currently running processes.

Usage:

bash
ps [options]
Common Options:
– `-e`: Display all processes.
– `-f`: Full-format listing.

Example:

bash
ps -ef

This command displays a full-format listing of all running processes.

10. `top` – Display Linux Tasks

The `top` command provides a dynamic, real-time view of the system's running processes.

Usage:

bash
top
Example:

bash
top

Running this command will show a real-time display of system processes, CPU usage, memory usage, and more.

11. `kill` – Terminate a Process

The `kill` command is used to send a signal to a process, usually to terminate it.

Usage:

bash
kill [options] pid
Example:

bash
kill -9 1234

This command forcefully terminates the process with PID 1234.

12. `df` – Report File System Disk Space Usage

The `df` command displays the amount of disk space used and available on file systems.

Usage:

bash
df [options]
Common Options:
– `-h`: Human-readable format.
– `-T`: Display file system type.

Example:

bash
df -h

This command shows disk space usage in a human-readable format.

13. `du` – Estimate File Space Usage

The `du` command estimates file space usage.

Usage:

bash
du [options] [directory]
Common Options:
– `-h`: Human-readable format.
– `-s`: Summarize total size.

Example:

bash
du -sh /home/user

This command shows the total size of the `/home/user` directory in a human-readable format.

14. `grep` – Print Lines Matching a Pattern

The `grep` command searches for patterns within files and outputs the matching lines.

Usage:

bash
grep [options] pattern [file]
Common Options:
– `-i`: Ignore case.
– `-r`: Recursively search directories.
– `-v`: Invert match, showing lines that do not match the pattern.

Example:

bash
grep -r "error" /var/log

This command searches for the term "error" in all files within `/var/log`.

15. `find` – Search for Files in a Directory Hierarchy

The `find` command searches for files and directories within a directory hierarchy based on specified criteria.

Usage:

bash
find [path] [expression]
Common Expressions:
– `-name`: Search by name.
– `-type`: Search by type (e.g., `f` for files, `d` for directories).
– `-mtime`: Search by modification time.

Example:

bash
find /home/user -name "*.txt"

This command finds all `.txt` files within `/home/user`.

16. `tar` – Archive Files

The `tar` command is used to create, extract, and manipulate tar archives.

Usage:

bash
tar [options] [archive-file] [file or directory]
Common Options:
– `-c`: Create a new archive.
– `-x`: Extract files from an archive.
– `-v`: Verbose mode.
– `-f`: Specify the archive file.

Example:

bash
tar -cvf backup.tar /home/user/docs

This command creates a tar archive named `backup.tar` containing the `docs` directory.

17. `ssh` – Secure Shell

The `ssh` command is used to securely connect to remote systems.

Usage:

bash
ssh [options] user@hostname
Common Options:
– `-p`: Specify port.
– `-i`: Specify identity file for key-based authentication.

Example:

bash
ssh -i ~/.ssh/id_rsa [email protected]

This command connects to the remote system at `192.168.1.10` using the specified identity file for authentication.

18. `scp` – Secure Copy

The `scp` command is used to securely copy files between hosts.

Usage:

bash
scp [options] source destination
Common Options:
– `-r`: Recursively copy directories.
– `-P`: Specify port.

Example:

bash
scp -P 2222 /home/user/file.txt user@remote:/home/user

This command copies `file.txt` to the remote system on port `2222`.

19. `rsync` – Remote Sync

The `rsync` command is used for fast and versatile file copying and synchronization.

Usage:

bash
rsync [options] source destination
Common Options:
– `-a`: Archive mode, which preserves permissions, timestamps, and other attributes.
– `-v`: Verbose mode.
– `-z`: Compress file data during the transfer.
– `-P`: Show progress during transfer.

Example:

bash
rsync -avz /home/user/docs user@remote:/backup/docs

This command synchronizes the `docs` directory to the remote system, preserving attributes and compressing data during the transfer.

20. `crontab` – Schedule Tasks

The `crontab` command is used to schedule tasks to run at specified intervals.

Usage:

bash
crontab [options] [file]
Common Options:
– `-e`: Edit the crontab file.
– `-l`: List the crontab file.
– `-r`: Remove the crontab file.

Example:

bash
crontab -e

This command opens the crontab file for editing, allowing you to schedule tasks.

21. `sudo` – Execute a Command as Another User

The `sudo` command allows a permitted user to execute a command as the superuser or another user.

Usage:

bash
sudo [options] command
Example:

bash
sudo apt-get update

This command runs `apt-get update` with superuser privileges, updating the package list.

22. `apt-get` – APT Package Handling Utility

The `apt-get` command is used to handle packages in Debian-based distributions.

Usage:

bash
apt-get [options] command
Common Commands:
– `update`: Update the package list.
– `upgrade`: Upgrade all packages.
– `install`: Install a package.
– `remove`: Remove a package.

Example:

bash
sudo apt-get install vim

This command installs the `vim` text editor.

23. `yum` – Yellowdog Updater Modified

The `yum` command is used to manage packages in RPM-based distributions.

Usage:

bash
yum [options] command
Common Commands:
– `update`: Update packages.
– `install`: Install a package.
– `remove`: Remove a package.

Example:

bash
sudo yum install httpd

This command installs the Apache HTTP server.

24. `systemctl` – Control the Systemd System and Service Manager

The `systemctl` command is used to examine and control the systemd system and service manager.

Usage:

bash
systemctl [command] [unit]
Common Commands:
– `start`: Start a service.
– `stop`: Stop a service.
– `restart`: Restart a service.
– `status`: Show the status of a service.

Example:

bash
sudo systemctl status sshd

This command shows the status of the SSH daemon.

25. `journalctl` – Query the Systemd Journal

The `journalctl` command is used to query and display messages from the systemd journal.

Usage:

bash
journalctl [options]
Common Options:
– `-b`: Show messages from the current boot.
– `-u`: Show messages for a specific unit.

Example:

bash
journalctl -u sshd

This command displays journal entries related to the SSH daemon.

26. `netstat` – Network Statistics

The `netstat` command displays network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.

Usage:

bash
netstat [options]
Common Options:
– `-a`: Show all sockets.
– `-t`: Show TCP connections.
– `-u`: Show UDP connections.
– `-n`: Show numerical addresses.

Example:

bash
netstat -tuln

This command shows all listening TCP and UDP ports with numerical addresses.

27. `ifconfig` – Configure a Network Interface

The `ifconfig` command is used to configure network interfaces.

Usage:

bash
ifconfig [interface] [options]
Example:

bash
ifconfig eth0

This command displays the configuration of the `eth0` network interface.

28. `ping` – Send ICMP ECHO_REQUEST to Network Hosts

The `ping` command checks the network connectivity between the host and a target.

Usage:

bash
ping [options] destination
Example:

bash
ping google.com

This command sends ICMP echo requests to `google.com` to check connectivity.

29. `traceroute` – Print the Route Packets Take to the Network Host

The `traceroute` command shows the path packets take to reach a network host.

Usage:

bash
traceroute [options] destination
Example:

bash
traceroute google.com

This command traces the route packets take to `google.com`.

30. `curl` – Transfer Data from or to a Server

The `curl` command is used to transfer data from or to a server using various protocols.

Usage:

bash
curl [options] [URL]
Example:

bash
curl -O http://example.com/file.txt

This command downloads `file.txt` from `example.com`.

31. `wget` – The Non-Interactive Network Downloader

The `wget` command downloads files from the web.

Usage:

bash
wget [options] [URL]
Example:

bash
wget http://example.com/file.txt

This command downloads `file.txt` from `example.com`.

32. `nano` – Simple Text Editor

The `nano` command opens the Nano text editor, which is user-friendly and suitable for beginners.

Usage:

bash
nano [file]
Example:

bash
nano /home/user/file.txt

This command opens `file.txt` in the Nano text editor.

33. `vim` – Vi Improved, a Highly Configurable Text Editor

The `vim` command opens the Vim text editor, which is powerful and highly configurable.

Usage:

bash
vim [file]
Example:

bash
vim /home/user/file.txt

This command opens `file.txt` in the Vim text editor.

34. `awk` – Pattern Scanning and Processing Language

The `awk` command is used for pattern scanning and processing.

Usage:

bash
awk [options] 'pattern {action}' [file]
Example:

bash
awk '{print $1}' /etc/passwd

This command prints the first field of each line in `/etc/passwd`.

35. `sed` – Stream Editor

The `sed` command is used to perform basic text transformations on an input stream.

Usage:

bash
sed [options] 'script' [file]
Example:

bash
sed 's/old/new/g' file.txt

This command replaces all occurrences of `old` with `new` in `file.txt`.

36. `echo` – Display a Line of Text

The `echo` command displays a line of text.

Usage:

bash
echo [options] [string]
Example:

bash
echo "Hello, World!"

This command prints `Hello, World!` to the terminal.

37. `date` – Display or Set the System Date and Time

The `date` command displays or sets the system date and time.

Usage:

bash
date [options] [format]
Example:

bash
date "+%Y-%m-%d %H:%M:%S"

This command displays the current date and time in the specified format.

38. `uptime` – Tell How Long the System Has Been Running

The `uptime` command shows how long the system has been running, along with the current time, number of users, and system load averages.

Usage:

bash
uptime
Example:

bash
uptime

This command displays the system's uptime.

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