Essential Linux/Unix Commands

1. who : The ‘$ who’ command displays all the users who have logged into the system currently.

2. pwd : The ‘$pwd’ command stands for ‘present working directory’ and as the name says,it displays the directory in which we are currently (directory is same as folder for Windows OS users).

3. mkdir : The ‘$ mkdir’ stands for ‘make directory’ and it creates a new directory.We have used ‘$ cd’ (which is discussed below) to get into the newly created directory and again on giving ‘$ pwd’ command,we are displayed with the new ‘newfolder’ directory.

4. rmdir : The ‘$ rmdir’ command deletes any directory we want to delete and you can remember it by its names ‘rmdir’ which stands for ‘remove directory’.

5. cd : The ‘$ cd’ command stands for ‘change directory’ and it changes your current directory to the ‘newfolder’ directory.You can understand this a double clicking a folder and then you do some stuff in that folder.

6. ls : The ‘$ ls’ command simply displays the contents of a directory.

7. touch : The ‘$ touch’ command creates a file(not directory) and you can simple add an extension such as .txt after it to make it a Text File.

8. cp : This ‘$ cp ‘ command stands for ‘copy’ and it simply copy/paste the file wherever you want to.In the above example we are copying a file ‘file.txt’ from the directory harssh to a new directory new.

9. mv : The ‘$ mv’ command stands for ‘move’ and it simply move a file from a directory to anothe directory.In the above example a file named ‘file.txt’ is being moved into a new directory ‘new’

10. rm : The ‘$ rm ‘ command for remove and the ‘-r’ simply forcibly deletes a file without prompting the user.Try ‘$ rm filename.txt’ at your terminal 

11. chmod : The ‘$ chmod’ command stands for change mode command.As there are many modes in Unix that can be used to manipulate files in the Unix environment.Basically there are 3 modes that we can use with the ‘chmod’ command
1. +w (stands for write and it changes file permissions to write)
2. +r (stands for read and it changes file permissions to read)
3. +x (generally it is used to make a file executable)

12. cal : The ‘$ cal’ means calendar and it simply display a calendar on to your screen.

13. file : The ‘$ file’ command displays the type of file.As i mentioned earlier linux treats everything as file so on
executing the command file on a directory(Downloads) it displays directory as the output
14. sort : As the name suggests the ‘$ sort’ sorts the contents of the file according to the ASCII rules.

15. grep : grep is an acronym for ‘globally search a regular expression and print it’.The ‘$ grep’ command searches the specified input fully(globally) for a match with the supplied pattern and displays it.
In the example, this would search for the word ‘picture’ in the file newsfile and if found,the lines containing it would be displayed on the screen.
$ grep picture newsfile

grep -l error cluster-2010*.log  // if you are grep log failure, it will only print filename if not true.

grep -n error cluster-2010*.log  // it will return line number which not true.

16. man : The ‘$ man’ command stands for ‘manual’ and it can display the in-built manual for most of the commands that we ever need.In the above example we can read about the ‘$ pwd’ command.
$ man pwd
17. lpr : The ‘$ lpr’ command send a file to the printer for printing.
$ lpr new.txt
18. passwd : The ‘$ passwd’ command simply changes the password of the user.In above case ‘harssh’ is the user.
$ passwd
Output: Changing password for harssh.
(current) UNIX password:
19. clear : The ‘$ clear’ command is used to clean up the terminal so that you can type with more accuracy 
$ clear