# Linux Commands Linux is based on File System. Every directory starts from root directory which is called as '/'. The symbol '/' is a parent directory. We need to know these shell command if you want to learn Linux. 1)**mkdir**: Used to create a directory Eg: `ubuntu@ip-172-31-25-118:~$ mkdir test` 2)**ls**: Used to list the files Eg: `ubuntu@ip-172-31-25-118:~$ ls` Output: `test` 3)**pwd**: On which directory you are currently in. It is also known as Present Working Directory. Eg: `ubuntu@ip-172-31-25-118:~$ pwd` Output: /home/ubuntu Whatever command you are executing like echo, cd, ls. These commands have some code or binary files which are stored in /bin directory. Inside / directory there are multiple directories like:- /bin: stores binary files. /home: list all the users. /var: It contains variable information. /log: stores the logs. Eg: `ubuntu@ip-172-31-25-118:~$ cd /` `ubuntu@ip-172-31-25-118:/$ ls` You will see the output on terminal when you perform. 4)**cd**: Used to change the directory. Eg: `ubuntu@ip-172-31-25-118:/$ cd /home/ubuntu/` 5)**lsb\_release -a**: You can identify which OS you are running and on which month is it release Eg: `ubuntu@ip-172-31-25-118:~$ lsb_release -a` Output: `No LSB modules are available.` `Distributor ID: Ubuntu` `Description: Ubuntu 24.04 LTS` `Release: 24.04` `Codename: noble` This indicates that you are using a Ubuntu (flavors of Linux) OS and it is released on April 2024. The term LTS stands for Long Term Support which means you can use such OS in lifelong. 6)**touch** <filename>: You can create a file using **touch** command. Eg: `touch testing` 7)**mv**: Move (like a cut-paste) the files from source to destination Eg: `ubuntu@ip-172-31-25-118:~$ mv testing.txt test/` I am moving **testing.txt** file inside the **test** directory (Moving File to Directory) **Note**:- We need to call folder on Windows whereas in Linux, we are calling folder as directory. If you want to move one directory inside another directory See the example `ubuntu@ip-172-31-25-118:~$ ls` Output:- `A test testing.txt` `ubuntu@ip-172-31-25-118:~$ cd test/` `ubuntu@ip-172-31-25-118:~/test$ ls` Output:- `majedar testing.txt` `ubuntu@ip-172-31-25-118:~/test$ cd ..` `ubuntu@ip-172-31-25-118:~$ mv test A` I have moved the **test** directory inside **A** directory 8)**ls -l**: To distinguish between file and directory we are using ls -l command. `ubuntu@ip-172-31-25-118:~$ ls -l` `total 4` `drwxrwxr-x 2 ubuntu ubuntu 4096 Sep 4 22:00 test` -> See the first word 'd'. Here test is a directory `-rw-rw-r-- 1 ubuntu ubuntu 0 Sep 4 22:03 testing.txt` --> See the first symbol '-'. Here testing.txt is a file Another way to identify the difference between them using color code. Blue color indicates a directory whereas white color considered as a file. You can find color code using **ls** command. **ls -a**: used to see the hidden files. 9)cd .. -> You can step back one directory above. Eg: `ubuntu@ip-172-31-25-118:~$ cd test` `ubuntu@ip-172-31-25-118:~/test$ cd ..` `ubuntu@ip-172-31-25-118:~$` 10)cd ../.. -> You can understand with an example. `ubuntu@ip-172-31-25-118:~$ cd test` `ubuntu@ip-172-31-25-118:~/test$ mkdir majedar` `ubuntu@ip-172-31-25-118:~/test$ cd majedar/` `ubuntu@ip-172-31-25-118:~/test/majedar$ cd ../..` `ubuntu@ip-172-31-25-118:~$` 11)You can create parent directory including it's sub directories Eg: ubuntu@ip-172-31-25-118:~$ **mkdir -p** A/B/C -> I am creating 3 directories. Here '-p' indicates a parent `ubuntu@ip-172-31-25-118:~$ ls` Output:- `A test testing.txt` 12)You can use **clear** command to clear the screen on your terminal. 13)**cp:** used to copy the files and directories (like a copy paste) Eg: I am coping **testing.txt** file inside **test** directory. `ubuntu@ip-172-31-25-118:~$ ls` Output:- `A test testing.txt` `ubuntu@ip-172-31-25-118:~$ cp testing.txt test/` Eg: I am copying **test** directory into **A** directory `ubuntu@ip-172-31-25-118:~$ ls` Output:- `A test testing.txt` `ubuntu@ip-172-31-25-118:~$ cp test A` cp: -r not specified; omitting directory 'test' `ubuntu@ip-172-31-25-118:~$ cp -r test A` Here -r stands for recursive. If we have subdirectory inside parent directory and if we want to copy the parent directory then use -r flag. 14)If you see which commands you have executed in your terminal then you can use **history** command. Eg: `ubuntu@ip-172-31-25-118:~$ history` 15)You can use **rm** command to remove a file Eg: `ubuntu@ip-172-31-25-118:~$ ls` Output:- `A test testing.txt` `ubuntu@ip-172-31-25-118:~$ rm testing.txt` If you want to remove a directory then refer below example `ubuntu@ip-172-31-25-118:~$ ls` Output:- `A test` `ubuntu@ip-172-31-25-118:~$ rm test rm`: cannot remove 'test': Is a directory `ubuntu@ip-172-31-25-118:~$ rm -r test` -> here -r stands for recursive. 16)You can use **vim** editor to edit the file Eg: `ubuntu@ip-172-31-25-118:~$ ls` Output:- `A testing.txt` ubuntu@ip-172-31-25-118:~$ vim testing.txt Then press 'Insert' to insert some content in a file. Once you finish writing a file then press 'Esc' and put wq(write and quit). 17)If you want to see the content of a file use **cat** command. Eg: `ubuntu@ip-172-31-25-118:~$ cat testing.txt` Output:- `Apple` `Mango` `Banana` `Orange` `Pineapple` `Grapes` 18)If you want to see the top 2 lines of a file use **head** command. Eg: `ubuntu@ip-172-31-25-118:~$ head -n 2 testing.txt` Output:- `Apple Mango` To see the last 2 content of a file use **tail** command Eg: `ubuntu@ip-172-31-25-118:~$ tail -n 2 testing.txt` Output:- `Pineapple` `Grapes` 19. If you want to know your current user, use the command **whoami.** Eg: `ubuntu@ip-172-31-25-118:/home$ whoami` ubuntu 20)If you want to add the user in Linux, you need to run below command using 'super user do' which is known as sudo. With sudo you won't be able to create a user in the /home directory. Eg: `ubuntu@ip-172-31-25-118:~$ sudo useradd -m jethalal` If you want to switch to the user jethalal then you need to create a password for the user jethalal. Eg: `ubuntu@ip-172-31-25-118:/home$ sudo passwd jethalal` Output:- `New password:` `Retype new password: passwd: password updated successfully` You need to perform below steps to switch the user. Use **su** (switch user) command. `ubuntu@ip-172-31-25-118:/home$ whoami` Output:- `ubuntu` `ubuntu@ip-172-31-25-118:/home$ su jethalal` Output:- `Password:` <type the password for jethalal user> `$whoami` Output:- `jethalal` To exit from the jethalal user and go back to your current user, use **exit** command. Output:- `$ exit` `ubuntu@ip-172-31-25-118:/home$` 21)If you want to view the user details then run **cat /etc/passwd** command Eg: `ubuntu@ip-172-31-25-118:/home$ cat /etc/passwd` Output:- `ubuntu:x:1000:1000:Ubuntu:/home/ubuntu:/bin/bash jethalal:x:1001:1001::/home/jethalal:/bin/sh` 22)To create a group, run sudo groupadd <group-name> Eg: `ubuntu@ip-172-31-25-118:/home$ sudo groupadd devops` To view a group info run `cat /etc/group` command Eg: `ubuntu@ip-172-31-25-118:/home$ cat /etc/group` Output:- `devops:x:1002:` 23)There are two types of groups:- a)Primary Group:- Whenever you create a user like **ubuntu** bydefault it's group will be created with same name. Eg: `ubuntu@ip-172-31-25-118:/home$ ls` Output: `jethalal ubuntu` `ubuntu@ip-172-31-25-118:/home$ cat /etc/group` Output: `ubuntu:x:1000:` `jethalal:x:1001:` b)Secondary Group:- When you create a group with any name and you have added all the users inside that group, then the group will be called as Secondary Link Eg: `ubuntu@ip-172-31-25-118:/home$ cat /etc/group` `devops:x:1002:` 24)How to add user inside Group a)Adding a single user in a group Syntax: sudo usermod -aG <group-name> <user-name> Here, usermod indicates to modify the user, a stands for add or append, G stands for groupname. Eg: `ubuntu@ip-172-31-25-118:/home$ sudo usermod -aG tester iyer` `ubuntu@ip-172-31-25-118:/home$ cat /etc/group` `tester:x:1005:iyer` b)Adding multiple users in a group Syntax: sudo gpasswd -M <user1>,<user2> <group-name> Eg: `ubuntu@ip-172-31-25-118:/home$ sudo gpasswd -M daya,jethalal devops` `ubuntu@ip-172-31-25-118:/home$ cat /etc/group` `devops:x:1002:daya,jethalal`