Linux Commands and System Administration Essentials

Posted on Oct 31, 2024 in Computers

1. Viewing Directories

Use the following command to view directories:

  • find -type d (finds all directories)
  • ls -l (lists files and directories in long format)

2. Folders Storing System Files

The following folders store system files:

  • / (root directory)
  • /dev (device files)

3. Controlling Processes: Foreground (FG) and Background (BG)

In Linux, you can run multiple tasks simultaneously. To run a program in the background, add an ampersand (&) to the end of the command line.

Example:

# cp -r /usr/src/linux /tmp &

When the program finishes, the system will display a message:

[Done] cp -r /usr/src/linux /tmp

To move a background process to the foreground, use the fg command.

3.1. Running Commands in Background and Using jobs, fg, and bg

To run a command in the background, use the & operator at the end of the command.

Example:

[cfm@salt]$ firefox &
[1] 23710
[cfm@salt]$

To view running background processes, use the jobs command.

Example:

[cfm@salt]$ jobs
[1]+ Running firefox &
[cfm@salt]$

To bring a background process to the foreground, use the fg command with the job number.

Example:

[cfm@salt]$ fg 1
firefox

To stop a foreground process and move it to the background, press Ctrl+Z and then use the bg command.

Example:

[1]+ Stopped firefox
[cfm@salt]$ bg 1
[1]+ firefox &
[cfm@salt]$

3.2. Important Note about Background Processes

If a background process requires user interaction through the console, it will remain stalled.

4. Deleting a Folder and its Contents

Use the rm -r command to delete a folder and its contents.

5. Background (BG) and Foreground (FG) Process Control

Linux allows running multiple tasks concurrently. To run a program in the background, append an ampersand (&) to the command.

6. nice and renice Commands

These commands allow you to launch a process with a specific priority (nice) and change the priority of an existing process (renice).

6.1. nice Command

  • Starts a program with an altered priority.
  • Nice range is from +19 (very nice/low priority) to -20 (not very nice/high priority).
  • The default nice value is 10.

Examples:

$ nice -10 long-running-command &
$ nice -n 10 long-running-command &
$ nice -15 important-command &
$ nice -n -15 important-command &

6.2. renice Command

  • Changes the nice level of an existing process.
  • Users cannot increase the nice level (lower the priority) of a process without appropriate permissions.

Examples:

$ renice 20 2984 
$ renice -15 3598
$ renice 15 -u miguel

7. Deleting a Directory

Use the rm command to delete a directory.

Examples:

$ rm filename
$ rm directory_name
$ rm -r directory_name

8. Linux File Systems

Common Linux file systems include:

  • ext3
  • ext2
  • swap

9. Creating a Hard Link

Use the ln command to create a hard link.

Example:

$ ln original_file hard_link

10. Files Containing User and Password Information

  • /etc/passwd (stores user information)
  • /etc/shadow (stores password information)

11. Changing Process Priority

Use the nice and renice commands to change process priority.

11.1. renice Command Details

The renice command alters the priority of running processes.

Syntax:

renice priority [[-p] pid ...] [[-g] pgrp ...] [[-u] user ...]

Options:

  • -g: Interprets parameters as process group IDs.
  • -u: Interprets parameters as usernames.
  • -p: Interprets parameters as process IDs (default).

Examples:

renice +15 785
renice +20 -u fred

12. UMASK

UMASK is used to determine the default file permissions. For example, if you want default permissions of 644, the umask would be 022 (666 – 022 = 644).

13. Viewing Processes

Use the ps aux command to view all running processes, including their memory and CPU usage.

14. Generating SSH Keys

To generate SSH keys, use the following command:

ssh-keygen -t rsa

This command creates a public/private RSA key pair. You will be prompted to enter a filename for the private key (default is /home/user/.ssh/id_rsa) and optionally a passphrase.

15. Switching Between Consoles

Use Ctrl+Alt+F1 through Ctrl+Alt+F6 to switch between virtual consoles.

16. Privileged Ports

Ports below 1024 are considered privileged ports. Some common privileged ports include:

  • SSH: 22
  • Telnet: 23
  • HTTPS: 443
  • NetBIOS: 137
  • CUPS (printing): 631

There are a total of 65535 ports available.