Must Know Linux/Unix Commands

File Management becomes easy if you know the right commands.

Sometimes, commands are also referred as "programs" since whenever you run a command, actually, it's the corresponding program code, written for the command, which is being executed.

Let's learn the must know Linux commands.

 

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Listing files (ls)

 

If you want to see the list of files on your UNIX or Linux system, use the 'ls' command.

It shows the files /directories in your current directory. 

 

Note:

  • Directories are denoted in blue color.
  • Files are denoted in white.
  • You will find similar color schemes in different flavors of Linux.

Suppose, your "Music" folder has following sub-directories and files.

You can use 'ls-R' to shows all the files not only in directories but also subdirectories 

 

NOTE: The command is case-sensitive. If you enter, "ls - r" you will get an error

 'ls -al' gives detailed information of the files. The command provides information in a columnar format. The columns provide the following information:

 

1st Column

File type and access permissions

2nd Column

# of HardLinks to the File

3rd Column

Owner and the creator of the file

4th Column

Group of the owner

5th Column

File size in Bytes

6th Column

Date and Time

7th Column

Directory or File name

 

Let's see an example - 

Listing Hidden Files

Hidden items in UNIX/Linux begin with -at the start, of the file or directory.

Any Directory/file starting with a '.' will not be seen unless you request for it.  To view hidden files, use the command

ls  - a

Creating & Viewing Files

The 'cat' command is used to display text files. It can also be used for copying, combining and creating new text files.  Let's see how it works

To create a new file, use the command

  1.  cat > filename
  2.  Add content
  3.  Press 'ctrl + d' to return to command prompt.

To view a file, use the command -

cat

Let's see the file we just created -  

Let's see another file sample2 

The syntax to combine 2 files is -

cat file1 file2 > newfilename

Let's combine sample 1 and sample 2.

As soon as you insert this command and hit enter, the files are concatenated, but you do not see a result. This is because Bash Shell (Terminal) is silent type.  It will never give you a confirmation message like "OK" or "Command Successfully Executed". It will only show a message when something goes wrong or when an error has occurred.

In order to view the new combo file "sample" use the command

cat sample

Note: Only text files can be displayed and combined using this command.

Deleting Files

The 'rm' command removes files from the system without confirmation.  To delete a file use syntax -

rm  

 

Moving and Re-naming files

In order to move a file, use the command

mv

Suppose we want to move the file "sample2" to location /home/guru99/Documents. Executing the command

mv sample2  /home/guru99/Documents

 

mv command needs super user permission. Currently, we are executing the command as a standard user. Hence we get the above error. To overcome the error use command

sudo

Sudo program allows regular users to run programs with the security privileges of the superuser or root.

Sudo command will ask for password authentication. Though, you do not need to know the root password. You can supply your own password. After authentication, the system will invoke the requested command.

Sudo maintains a log of each command run. System administrators can trackback the person responsible for undesirable changes in the system

For renaming file:

mv filename newfilename

NOTE: By default, the password you entered for sudo is retained for 15 minutes per terminal. This eliminates the need of entering the password time and again.

You only need root/sudo privileges, only if the command involves files or directories not owned by the user or group running the comman

Directory Manipulations

Enough with File manipulations!  Let's learn some directory commands

Creating Directories

Directories can be created on a Linux operating system using the following command

mkdir

This command will create a subdirectory in your present working directory, which is usually your "Home Directory".

For example,

mkdir mydirectory

If you want to create a directory in a different location other than 'Home directory', you could use the following command -

mkdir

For example:

mkdir /tmp/MUSIC

will create a directory 'Music' under '/tmp' directory

You can also create more than one directory at a time. 

Removing Directories

In order to remove a directory, use the command -

rmdir

Example

rmdir mydirectory

will delete the directory mydirectory

Tip: Ensure that there is no file / sub-directory under the directory that you want to delete. Delete the files/sub-directory first before deleting the parent directory. 

Renaming Directory

The 'mv' (move) command (covered earlier) can also be used for renaming directories. Use the below given format:

mv directoryname newdirectoryname

Let us try it:

Other Important Commands

The 'Man' command

Man stands for manual which is a reference book of a Linux operating system. It is similar to HELP file found in popular softwares.

To get help on any command that you do not understand, you can type

man

The terminal would open the manual page for that command.

For an example, if we type man man and hit enter; terminal would give us information on man command

The History Command

History command shows all the commands that you have used in the past for the current terminal session. This can help you refer to  the old commands you have entered and re-use them in your operations again.

The clear command

This command clears all the clutter on the terminal and gives you a clean window to work on, just like when you launch the terminal.

Pasting commands into the terminal

Many a times you would have to type in long commands on the Terminal. Well, it can be annoying at times, and if you want to avoid such a situation then copy, pasting the commands can come to rescue.

For copying, the text from a source, you would use Ctrl + c, but for pasting it on the Terminal you need to use Ctrl + Shift + v. You can also try Shift + Insert or select Edit>Paste on the menu

Summary

 

Below is a summary of commands we have learned in this tutorial

Command

Description

ls

Lists all files and directories in the present working directory

ls - R

Lists files in sub-directories as well

ls - a

Lists hidden files as well

ls - al

Lists files and directories with detailed information like permissions , size , owner etc.

cat > filename

Creates a new file

cat filename

Displays the file content

cat file file2 > file3

Joins two files (file1, file2) and stores the output in a new file (file3)

mv  file "new file path"

Moves the files to the new location

mv filename new_file_name

Renames the file to a new filename

sudo

Allows regular users to run programs with the security privileges of the superuser or root

rm

Deletes a file

man

Gives help information on a command

history

Gives a  list of all past commands typed in the current terminal session

clear

Clears the terminal

mkdir

Creates a new directory in the present working directory

mkdir

Create a new directory at the specified path

rmdir 

Deletes a directory

mv

Renames a directory

 

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