https://github.com/meqdaddev/linux-notes
Notes and Resources for Linux OS.
https://github.com/meqdaddev/linux-notes
Last synced: 4 months ago
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Notes and Resources for Linux OS.
- Host: GitHub
- URL: https://github.com/meqdaddev/linux-notes
- Owner: MeqdadDev
- License: mit
- Created: 2024-01-26T20:05:04.000Z (over 2 years ago)
- Default Branch: main
- Last Pushed: 2024-02-27T20:23:35.000Z (over 2 years ago)
- Last Synced: 2025-01-10T09:59:24.823Z (over 1 year ago)
- Size: 14.6 KB
- Stars: 0
- Watchers: 1
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- License: LICENSE
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README
# Linux Notes
Notes and Resources for Linux OS.
```
_ _ _ _ _ _ _ _ __ __
| | |__ __| | \ | | | | | | \ \/ /
| | | | | \| | | | | | \/\/
| |____ _| |_ | |\ | | | | | /\/\
|______| |_ _ _| |_| \_| \___/ /_/\_\
```
## Linux History
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.
## Popular Commands
-------------------------------
### `uptime`
* __`uptime`__ - Tell how long the system has been running.
__Usage Case:__
```bash
uptime
```
_Output:_
```bash
22:23:24 up 9:21, 1 user, load average: 0.85, 1.10, 1.27
```
-------------------------------
### `free`
* __`free`__ - Display amount of free and used memory in the system.
__Usage Case:__
```bash
free
```
_Output:_
```bash
total used free shared buff/cache available
Mem: 7965944 2380788 3041184 378216 2543972 4908056
Swap: 2097148 1104344 992804
```
-------------------------------
### `date`
* __`date`__ - print or set the system date and time.
__Usage Case:__
```bash
date
```
_Output:_
```bash
Fri 26 Jan 2024 22:32:48 IST
```
* __`date +%x`__ - print locale's date representation (e.g., 12/31/99)
```bash
date +%x
```
_Output:_
```bash
26/01/2024
```
-------------------------------
### `head`
* __`head`__ - output the first part of files. (10 lines)
__Usage Case:__
```bash
head test.txt
```
_Output:_
```bash
Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
Line 8
Line 9
Line 10
```
* __`head -n 5`__ - output the first 5 lines of file.
```bash
head test.txt -n 5
```
_Output:_
```bash
Line 1
Line 2
Line 3
Line 4
Line 5
```
-------------------------------
### `tail`
* __`tail`__ - output the last part of files. (10 lines)
__Usage Case:__
```bash
tail test.txt
```
_Output:_
```bash
Line 10
Line 11
Line 12
Line 13
Line 14
Line 15
Line 16
Line 17
Line 18
Line 19
Line 20
```
* __`tail -n 5`__ - output the last 5 lines of file.
```bash
tail test.txt -n 5
```
_Output:_
```bash
Line 1
Line 2
Line 3
Line 4
Line 5
```
-------------------------------
### `history`
* __`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:__
```bash
history
```
_Output:_
```bash
....
....
....
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
....
....
....
```
* __`!`__ - Rerun a specific command from history by using an exclamation mark (!) followed by the line number.
Running `ls` command from history (line 1164):
```bash
!1164
```
_Output:_
```bash
LICENSE README.md hi.txt
```
-------------------------------
### `touch`
* __`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:__
```bash
touch example.txt
```
-------------------------------
### `man`
* __`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.
```bash
man
```
__Usage Case:__
```bash
man ls
```
-------------------------------
### `id`
* __`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.
```bash
id [username]
```
__Usage Case:__
```bash
id meqdad
```
_Output:_
```bash
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:
```bash
id root
```
_Output:_
```bash
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`):
```bash
id
```
_Output:_
```bash
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`
* __`useradd`__ - Create a new user account.
```bash
useradd [options] [User_name]
```
__Usage Case:__
```bash
sudo useradd meqdad
```
_Output:_
```bash
A new user account "meqdad" is created.
```
* __`sudo useradd -d [home_dir_path] [new_user]`__ - To give a home directory path for new users.
```bash
sudo useradd -d /home/test_user test_user
```
_Output:_
```bash
A new user account "test_user" is created and the Home directory is created.
```
-------------------------------
### `userdel`
* __`userdel`__ - Delete a user account.
```bash
userdel [options] [User_name]
```
__Usage Case:__
```bash
sudo userdel meqdad
```
_Output:_
```bash
The user account "meqdad" is deleted.
```
-------------------------------
### `groupadd`
* __`groupadd`__ - Create a new group.
```bash
groupadd [options] [group_name]
```
__Usage Case:__
```bash
sudo groupadd test_group
```
_Output:_
```bash
A new group "test_group" is created.
```
-------------------------------
### `cat /etc/passwd`
* __`cat /etc/passwd`__ - Display user account information.
__Usage Case:__
```bash
cat /etc/passwd
```
OR
```bash
vim /etc/passwd
```
_Output:_
```bash
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]`
* __`passwd [username]`__ - Change or set the password for a user account.
```bash
passwd [username]
```
__Usage Case:__
```bash
passwd meqdad
```
_Output:_
```bash
Changing password for user meqdad.
New password:
Retype new password:
passwd: password updated successfully
```
-------------------------------
### `usermod`
* __`usermod`__ - Modify user account settings.
__Usage Case:__
* Changing the login shell for user "meqdad" to `/bin/zsh`.
```bash
sudo usermod -s /bin/zsh meqdad
```
_Output:_
```bash
The login shell for user "meqdad" is changed to "/bin/zsh".
```
* Adding user "meqdad" to the "admin" group.
```bash
sudo usermod -G admin meqdad
```
_Output:_
```bash
User "meqdad" is added to the "admin" group.
```
* Adding a comment for user "meqdad".
```bash
sudo usermod -c "Meqdad Dev" meqdad
```
_Output:_
```bash
The comment for user "meqdad" is updated to "Meqdad Dev".
```
-------------------------------
### `chage`
* __`chage`__ - Change user password expiry information.
__Usage Case:__
* Changing the maximum password age for user "meqdad" to 90 days.
```bash
sudo chage -M 90 meqdad
```
_Output:_
```bash
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.
```bash
sudo chage -I 10 -W 7 meqdad
```
_Output:_
```bash
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.
```bash
sudo chage -E 2024-12-31 meqdad
```
_Output:_
```bash
The password for user "meqdad" is set to expire on December 31, 2024.
```
-------------------------------
### `find`
* __`find`__ - Search for files and directories in a directory hierarchy.
```bash
find [directory] [options] [expression]
```
__Usage Case:__
```bash
find /home/user -name "*.txt"
```
_Output:_
```bash
/home/user/file1.txt
/home/user/directory/file2.txt
...
```
* Removing the modified files for more than 7 days ago.
```bash
find /var/log -type f -mtime +7 -exec rm {} \;
```
_Output:_
```bash
Removes all files in /var/log that were modified more than 7 days ago.
```
-------------------------------
### `grep`
* __`grep`__ - Search for patterns in text using regular expressions.
```bash
grep [options] pattern [files/directories]
```
__Usage Case:__
```bash
grep "keyword" file.txt
```
_Output:_
```bash
Line containing the keyword in file.txt.
```
* Searching for the specified pattern in all files within the given directory and its subdirectories.
```bash
grep -r "pattern" /path/to/directory
```
_Output:_
```bash
Lines containing the pattern in files within the directory and its subdirectories.
```
* Showing information about Python processes running on the system:
```bash
ps aux | grep python
```
_Output:_
```bash
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`
* __`locate`__ - Find files by name quickly using a pre-built index of file names.
```bash
locate [options] [pattern]
```
__Usage Case:__
```bash
locate test.txt
```
_Output:_
```bash
/home/user/test.txt
/var/www/html/test.txt
...
```
* Searching for files with the ".pdf" extension, ignoring case.
```bash
locate -i "*.pdf"
```
_Output:_
```bash
/home/user/Documents/Document1.pdf
/home/user/Documents/Document2.PDF
...
```
* Find files with ".pdf" extension and filter for those containing "example" in the filename.
```bash
locate -i "*.pdf" | grep "report"
```
_Output:_
```bash
/home/user/Documents/annual_report.pdf
/home/user/Downloads/example_report.pdf
...
```
-------------------------------
### `systemctl`
* __`systemctl`__ - Control the systemd system and service manager.
```bash
systemctl [options] [command] [unit]
```
__Usage Case:__
```bash
systemctl start nginx
```
_Output:_
```bash
Starts the nginx service.
```
* To display the status of the sshd service.
```bash
systemctl status sshd
```
_Output:_
```bash
Displays the status of the sshd service.
```
* To start a service automatically at boot.
```bash
systemctl enable apache2
```
_Output:_
```bash
Enables the apache2 service to start automatically at boot.
```
-------------------------------
### `ps`
* __`ps`__ - Report a snapshot of current processes.
```bash
.....
```
__Usage Case:__
* To display a list of processes running in the current session.
```bash
ps
```
_Output:_
```bash
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.
```bash
ps aux
```
_Output:_
```bash
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.
```bash
ps -ef
```
_Output:_
```bash
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.
```bash
ps -ef | grep python
```
_Output:_
```bash
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
```