Notes and Resources for Linux OS.

 _        _ _ _   _   _   _   _  __  __
| |      |__ __| | \ | | | | | | \ \/ /
| |        | |   |  \| | | | | |  \/\/
| |____   _| |_  | |\  | | | | |  /\/\
|______| |_ _ _| |_| \_|  \___/  /_/\_\

Linux was created by Linus Torvalds in 1991 as an open-source operating system kernel. Initially a hobby project, Linux rapidly evolved with contributions from developers worldwide, incorporating features from Unix-like systems and gaining stability.

Throughout the mid-1990s and early 2000s, Linux gained traction in the server market due to its robustness, scalability, and cost-effectiveness. Distributions like Red Hat, Debian, and Slackware emerged, catering to various user needs.

In the late 2000s and early 2010s, Linux expanded into desktop computing and mobile devices. Desktop-oriented distributions such as Ubuntu and Linux Mint gained popularity, while Android, based on the Linux kernel, became the dominant OS for mobile devices globally.

Linux solidified its position in cloud computing, powering infrastructure for major cloud providers like AWS, GCP, and Azure. Its flexibility, performance, and compatibility with virtualization technologies have made it a cornerstone of modern cloud-native architectures.

Today, Linux enjoys widespread popularity across servers, desktops, embedded systems, and cloud computing. Its vibrant open-source community actively contributes to its development and improvement, emphasizing freedom, security, and innovation in the digital infrastructure landscape.

• uptime - Tell how long the system has been running.

Usage Case:

uptime

Output:

22:23:24 up  9:21,  1 user,  load average: 0.85, 1.10, 1.27

• free - Display amount of free and used memory in the system.

Usage Case:

free

Output:

               total        used        free      shared  buff/cache   available
Mem:         7965944     2380788     3041184      378216     2543972     4908056
Swap:        2097148     1104344      992804

• date - print or set the system date and time.

Usage Case:

date

Output:

Fri 26 Jan 2024 22:32:48 IST

• date +%x - print locale's date representation (e.g., 12/31/99)

date +%x

Output:

26/01/2024

• head - output the first part of files. (10 lines)

Usage Case:

head test.txt

Output:

Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
Line 8
Line 9
Line 10

• head <file_name> -n 5 - output the first 5 lines of file.

head test.txt -n 5

Output:

Line 1
Line 2
Line 3
Line 4
Line 5

• tail - output the last part of files. (10 lines)

Usage Case:

tail test.txt

Output:

Line 10
Line 11
Line 12
Line 13
Line 14
Line 15
Line 16
Line 17
Line 18
Line 19
Line 20

• tail <file_name> -n 5 - output the last 5 lines of file.

tail test.txt -n 5

Output:

Line 1
Line 2
Line 3
Line 4
Line 5

• history - display a list of previously executed commands in a terminal session. It shows a numbered list of commands along with their execution sequence. This command is particularly useful for reviewing and reusing commands from your command-line history.

Usage Case:

history

Output:

....
....
....
 1152  sudo apt-get update
 1153  sudo apt-get upgrade
 1154  dmesg
 1155  sudo dmesg
 1156  cd Desktop/
 1157  cd Tmp/
 1158  ls
 1159  mkdir Meqdad
 1160  ls
 1161  cd Meqdad/
 1162  ls
 1163  sudo code .
 1164  ls
 1165  touch hi.txt
 1166  python3
 1167  python --version
....
....
....

• !<command_id> - Rerun a specific command from history by using an exclamation mark (!) followed by the line number.

Running ls command from history (line 1164):

!1164

Output:

LICENSE  README.md  hi.txt

• touch - Create new empty files or update the timestamp of existing files. When used to create a new file, it's followed by the name of the file you want to create.

Usage Case:

touch example.txt

• man - Stands for "manual". It's used to display the manual pages for other commands installed on your system. These manual pages provide detailed information about commands, their options, syntax, and often include examples and explanations.

man <command_name>

Usage Case:

man ls

• id - To display the user and group identifiers associated with the current user or a specified username. It provides information about the user's UID (User Identifier), GID (Group Identifier), and supplementary group IDs.

id [username]

Usage Case:

id meqdad

Output:

uid=1000(meqdad) gid=1000(meqdad) groups=1000(meqdad),4(adm),20(dialout),24(cdrom),27(sudo),30(dip),46(plugdev),122(lpadmin),135(lxd),136(sambashare)

Running id command for the root user:

id root

Output:

uid=0(root) gid=0(root) groups=0(root)

• id - When you run the id command without specifying a username, it displays information about the current user.

Running id command the current user (meqdad):

id

Output:

uid=1000(meqdad) gid=1000(meqdad) groups=1000(meqdad),4(adm),20(dialout),24(cdrom),27(sudo),30(dip),46(plugdev),122(lpadmin),135(lxd),136(sambashare)

• useradd - Create a new user account.

useradd [options] [User_name]

Usage Case:

sudo useradd meqdad

Output:

A new user account "meqdad" is created.

• sudo useradd -d [home_dir_path] [new_user] - To give a home directory path for new users.

sudo useradd -d /home/test_user test_user

Output:

A new user account "test_user" is created and the Home directory is created.

• userdel - Delete a user account.

userdel [options] [User_name]

Usage Case:

sudo userdel meqdad

Output:

The user account "meqdad" is deleted.

• groupadd - Create a new group.

groupadd [options] [group_name]

Usage Case:

sudo groupadd test_group

Output:

A new group "test_group" is created.

• cat /etc/passwd - Display user account information.

Usage Case:

cat /etc/passwd

OR

vim /etc/passwd

Output:

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
...
...
...

Explanation of Output:

• Username (Login Name): daemon - This is the username or login name of the user account.

• Password Placeholder: x - In modern Unix-like operating systems, the actual encrypted password is stored in the /etc/shadow file for security reasons. This placeholder (x) indicates that the password is stored in /etc/shadow.

• User ID (UID): 1 - This is the user's unique numerical identifier. User IDs 0-99 are typically reserved for system users and services.

• Group ID (GID): 1 - This is the primary group ID of the user. In this case, both the user ID and the group ID are 1, which is often the case for system-related accounts.

• User Information: daemon - This field typically contains additional information about the user, such as the full name or description. In this example, it's the same as the username.

• Home Directory: /usr/sbin - This is the path to the user's home directory. For system accounts like daemon, this might be set to a directory related to the system's operation.

• Login Shell: /usr/sbin/nologin - This is the path to the user's login shell. If the user is not intended to log in interactively, a shell like /usr/sbin/nologin is used to prevent login.

• passwd [username] - Change or set the password for a user account.

passwd [username]

Usage Case:

passwd meqdad

Output:

Changing password for user meqdad.
New password:
Retype new password:
passwd: password updated successfully

• usermod - Modify user account settings.

Usage Case:

• Changing the login shell for user "meqdad" to /bin/zsh.

sudo usermod -s /bin/zsh meqdad

Output:

The login shell for user "meqdad" is changed to "/bin/zsh".

• Adding user "meqdad" to the "admin" group.

sudo usermod -G admin meqdad

Output:

User "meqdad" is added to the "admin" group.

• Adding a comment for user "meqdad".

sudo usermod -c "Meqdad Dev" meqdad

Output:

The comment for user "meqdad" is updated to "Meqdad Dev".

• chage - Change user password expiry information.

Usage Case:

• Changing the maximum password age for user "meqdad" to 90 days.

sudo chage -M 90 meqdad

Output:

The maximum password age for user "meqdad" is set to 90 days.

• Changing the maximum password age for user "meqdad" to 10 days with warning message on the 7th day.

sudo chage -I 10 -W 7 meqdad

Output:

The password for user "meqdad" will expire in 10 days, and a warning will be issued 7 days prior.

• Changing the maximum password age for user "meqdad" to a specific date.

sudo chage -E 2024-12-31 meqdad

Output:

The password for user "meqdad" is set to expire on December 31, 2024.

• find - Search for files and directories in a directory hierarchy.

find [directory] [options] [expression]

Usage Case:

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

Output:

/home/user/file1.txt
/home/user/directory/file2.txt
...

• Removing the modified files for more than 7 days ago.

find /var/log -type f -mtime +7 -exec rm {} \;

Output:

Removes all files in /var/log that were modified more than 7 days ago.

• grep - Search for patterns in text using regular expressions.

grep [options] pattern [files/directories]

Usage Case:

grep "keyword" file.txt

Output:

Line containing the keyword in file.txt.

• Searching for the specified pattern in all files within the given directory and its subdirectories.

grep -r "pattern" /path/to/directory

Output:

Lines containing the pattern in files within the directory and its subdirectories.

• Showing information about Python processes running on the system:

ps aux | grep python

Output:

user     12345  0.0  0.0  1234  5678 ?        S    Jan01   0:00 python script.py
user     23456  0.0  0.0  1234  5678 ?        S    Jan01   0:00 /usr/bin/python3 another_script.py

• locate - Find files by name quickly using a pre-built index of file names.

locate [options] [pattern]

Usage Case:

locate test.txt

Output:

/home/user/test.txt
/var/www/html/test.txt
...

• Searching for files with the ".pdf" extension, ignoring case.

locate -i "*.pdf"

Output:

/home/user/Documents/Document1.pdf
/home/user/Documents/Document2.PDF
...

• Find files with ".pdf" extension and filter for those containing "example" in the filename.

locate -i "*.pdf" | grep "report"

Output:

/home/user/Documents/annual_report.pdf
/home/user/Downloads/example_report.pdf
...

• systemctl - Control the systemd system and service manager.

systemctl [options] [command] [unit]

Usage Case:

systemctl start nginx

Output:

Starts the nginx service.

• To display the status of the sshd service.

systemctl status sshd

Output:

Displays the status of the sshd service.

• To start a service automatically at boot.

systemctl enable apache2

Output:

Enables the apache2 service to start automatically at boot.

• ps - Report a snapshot of current processes.

.....

Usage Case:

• To display a list of processes running in the current session.

ps

Output:

    PID TTY          TIME CMD
   5277 pts/1    00:00:00 bash
   5284 pts/1    00:00:00 ps

• To display a detailed list of all processes, including those owned by other users.

ps aux

Output:

USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root           1  0.1  0.1 167164 11740 ?        Ss   21:50   0:01 /sbin/init splash
root           2  0.0  0.0      0     0 ?        S    21:50   0:00 [kthreadd]
root           3  0.0  0.0      0     0 ?        I<   21:50   0:00 [rcu_gp]
root           4  0.0  0.0      0     0 ?        I<   21:50   0:00 [rcu_par_gp]
root           5  0.0  0.0      0     0 ?        I<   21:50   0:00 [slub_flushwq]
root           6  0.0  0.0      0     0 ?        I<   21:50   0:00 [netns]
...
...
root        1115  0.0  0.2 118256 23296 ?        Ssl  21:50   0:00 /usr/bin/python3 /usr/share/unattended-upgrades/
mosquit+    1122  0.0  0.1  15512  8320 ?        Ss   21:50   0:00 /usr/sbin/mosquitto -c /etc/mosquitto/mosquitto.
colord      1168  0.0  0.1 245572 13012 ?        Ssl  21:50   0:00 /usr/libexec/colord
postgres    1214  0.0  0.3 221024 29696 ?        Ss   21:50   0:00 /usr/lib/postgresql/14/bin/
postgres    1245  0.0  0.1 221136  9232 ?        Ss   21:50   0:00 postgres: 14/main: checkpointer

• To display a full-format listing of all processes with additional information.

ps -ef

Output:

UID          PID    PPID  C STIME TTY          TIME CMD
root           1       0  0 21:50 ?        00:00:01 /sbin/init splash
root           2       0  0 21:50 ?        00:00:00 [kthreadd]
root           3       2  0 21:50 ?        00:00:00 [rcu_gp]
root           4       2  0 21:50 ?        00:00:00 [rcu_par_gp]
root           5       2  0 21:50 ?        00:00:00 [slub_flushwq]
root           6       2  0 21:50 ?        00:00:00 [netns]
...
...
lp          1221    1103  0 21:50 ?        00:00:00 /usr/lib/cups/notifier/dbus dbus://
rtkit       1241       1  0 21:50 ?        00:00:00 /usr/libexec/rtkit-daemon
postgres    1245    1214  0 21:50 ?        00:00:00 postgres: 14/main: checkpointer 
postgres    1246    1214  0 21:50 ?        00:00:00 postgres: 14/main: background writer 
postgres    1247    1214  0 21:50 ?        00:00:00 postgres: 14/main: walwriter 

• To display a full-format listing of all processes with additional information and filters for Python processes.

ps -ef | grep python

Output:

root        1115       1  0 21:50 ?        00:00:00 /usr/bin/python3 /usr/share/unattended-upgrades/unattended-upgrade-shutdown --wait-for-signal
meqdad      5663    5277  0 22:22 pts/1    00:00:00 grep --color=auto python