Some basic Linux commands everyone must know

November 7 20097 Commented

Categorized Under: General

This tutorial is based on the following software environment.

Ubuntu 8.04.2, PuTTY

I assume you know how to login and logout from Linux system and one Linux system is available in your network. Ask your Linux administrator for your credentials like user name/password information. For connecting to Linux you need any Telnet/SSH client like PuTTY. If you don’t have one yet, download PuTTY from the following site.

http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html

OK, open your client, give host name/IP address and press Open button. First Linux asks for user name and then password. Give the information and you are now in command prompt.

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Now let us get into commands. Please note that everything in Linux is case-sensitive, so all commands should be in appropriate case.

  • pwd  – the command is called “print working directory” which is used to print the current directory. When you are logged in, you are taken into your home directory. When Linux administrator creates your user account, he specifies your home directory where you generally keep files, folders etc. Type the command pwd in command prompt and hit [ Enter ].

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  •  who – this give you information regarding currently logged in users. Suppose you want to know how many users are connected to Linux system along with you. This command gives details like user id, logged in time, terminals to which users are connected etc. This has another variation who am i which gives information about your session. Type the command who in your command prompt and hit [ Enter ].

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First column is user id, second column is the terminal and third one is the logged in time. Now type who am i, hit [ Enter ] and verify the result.

  • cd - short name of  “change directory” which is used to switch to another directory from your current working directory. This command is used with the argument directory name to which we should change, cd <directory name>. If you type cd without any argument, you are changed to your home directory. Type cd /usr in your command prompt and hit enter. Then type pwd command and hit [ Enter ].

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 In the above diagram you could see that /usr is printed on executing pwd command. This means that we changed our working directory to /usr using cd command. Now just type cd and hit [ Enter ]. Validate the result using pwd command.

  • ls - List command used to show files and directories. If you just type ls, it lists files and directories in the current working directory. Optionally you may give the directory name as argument to list content in that particular directory. You may add combination of additional arguments to get more features in the result set. Some arguments are given below.
    •  -l – this argument gives detailed listing of files and directories like owner of the file, size of the file, whether the file is a directory, last modified time etc. This is a common command everybody uses.
    • -R – The argument -R is used for recursive purpose. This means it list all files in sub directories also. It searches all inner directories recursively and give results.
    • -a – The argument -a lists all hidden files.
    • -t – sort by modification time in descending order
    • directory name – If you give directory name as argument, it lists content in that directory rather than listing current working directory.

Ok, now let us put everything into an example. Go to your command prompt, type ls -lt /usr and hit [ Enter ].

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The above command gives detailed listing of files in the directory /usr in descending order of modified time. The first column is about file permissions. The first character shows whether it is a directory. For eg, in the above screenshot we could see that bin is a directory because the first character of file permission column is d. output.log is a file because the first character is -. Second column gives the number of links to the file. Third column shows the owner of the file and fourth column is about owner group. Fifth column shows file size in bytes and sixth column about last modified time. Final column shows the name of the file. Please note that in Linux everything is file. So directory is also a file in Linux system.

  • mkdir - Used to create directories. You should give directory name as argument. Let us create a directory in your home directory. Go to your home directory using cd command. Type the following command and hit [ Enter ].

mkdir testdir

The above command creates testdir in your home directory. Verify the same using ls command.

  •  touch – This command is used to change the timestamp of the file. But one of the most import function of the command is to create an empty file if it do not exist. Go to your system directory, type the following command and hit [ Enter ].

touch sample.txt

It creates sample.txt file. Verify the same using ls command.

  • catcat command is very useful for many purposes like creating files, adding and appending content to files etc. It also diplays the content of the file to standard output or another file. You may concatenate multiple files and display the content. Let us try some examples. Type the following command and hit [ Enter ].

cat > sample.txt

It waits for the input which should be added to the file in the next line. Type the text “Hello how are you?” and hit [ Enter ]. Then press [ Ctrl+d ]. This add the text to sample.txt file. Now let us display the content of the file.

cat sample.txt

You get the message in screen. Ok, now rather than writing the content to standard output, let us write it into another file.

cat sample.txt > samplecopy.txt

The above command creates samplecopy.txt file if it do not exists already and add the content of the file sample.txt. Verify the result using cat command.

Let us append the content. Type the following command and hit [ Enter ].

cat >> sample.txt

In the next line just add the text “I am fine”, hit [ Enter ] and press [ Ctrl+d ]. Verify the content of the file sample.txt using cat command. You may combine multiple files and show the output. For eg,

cat sample.txt samplecopy.txt, displays the content from both files. You may send it to another file also.

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  •  cp - copy command is used to copy files and directories. You have to give source and destination of files and directories. For eg, when you copy file, you may give the destination as either file name or directory name to which the file should be copied. You may copy entire directory also to another location. Some sample examples are given below.

cp sample.txt newsample.txt, this copies the file sample.txt to newsample.txt. You have to give source file name as first argument followed by destination. Verify the result.

Now let us copy the file sample.txt to our testdir directory which we already created. Type the following command and hit [ Enter ].

cp sample.txt testdir, this copies the file to directory. Here we provide directory name as second argument. Verify the result using cd and ls commands. Now let us copy the directory testdir. Go to your home directory and type the following command. Hit [ Enter ].

cp -R testdir newdir, this command copies testdir and its content to another directory newdir. Verify the result. Check whether sample.txt file in testdir is also available in newdir directory. -R argument is used to copy directories recursively.

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  •  mv – move command is used to rename files and directories. It is also used to move files from source to destination. Let us try some examples. Go to your home directory and execute following commands.

mv sample.txt log.txt, this command rename sample.txt to log.txt. source file name is given as first argument.

mv testdir logdir, this rename testdir directory to logdir directory. Please note that the content inside directory are not renamed.

mv log.txt logdir, this command moves log.txt file to logdir directory.

  • rm – remove command is used to delete files and directories. Some examples are given below.

rm newsample.txt, deletes newsample.txt file. You can give multiple file names in single command delimited by space to delete multiple files in a stretch. For eg, rm file1 file2 remove both files.

rm -R logdir, this command deletes logdir directory recursively.

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  • chmod – this command is used to change file permissions. Every file or directory in Linux is associated with file permissions. There are three levels of permissions, read, edit and execute. You can’t modify the file if you don’t have write permission. Similarly if you want to execute a shell script file, you need execute permission on it.

Users are categorized based on the following groups.  

  1. Owner – the owner of the file, basically if the file is created by you, you are the owner of the file.
  2. Owner Group – the group in which the owner is a member. Owner group level permission allow other users who also belong to this group to share the same permission level.
  3. Others – those users who do not belong to the above categories.

The command ls -lt gives details about file permissions in first column. For eg, in the above screen shot let us examine the permission of newdir directory. The first character d specifies that it is a directory. Next three characters is about owner permissions. rwx shows that owner has all rights. Next three characters are related to owner group file permission. Here the members of owner group have no write permission. Similarly last three characters are related to other members permission. Here also they don’t have write permission.

chmod could be used with different argument types, but I always like to use number arguments.    

4 – read access

2 – write access

1 – execute access

You should add these integers according to the given permission. Let us try some examples.

chmod 777 samplecopy.txt, this command gives full rights to all three types of users. First number is for owner, second for owner group and third for others. Suppose you want to protect your file so that others are not allowed to read it. You may give the command as

chmod 700 samplecopy.txt

Similarly if you want to give just read access other than you, you may give command as,

chmod 744 samplecopy.txt

  • passwdthis command is used to change the password of the current user. It asks for current password, once you supply it you may type the new password to change it. 

 

  • grep – this command is used to search for a particular pattern in a file or other input. This is commonly used with other commands like ls, cat etc as input using pipe symbol. One of the common use of the command is to search for a particular file in a directory. Suppose you have an image directory and you would like to know whether a particular image exists in directory. Then you may give command like this,

ls -lt | grep image.gif, this gives output row from detailed listing. For eg, just go through the following screenshot.

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In the above example, we just want to search for a pattern “copy” which is found in samplecopy.txt file listing record.

  • vi - vi is a text editor used to edit files which is a very import command in Linux. Some Linux versions like Ubuntu give an improved version of vi editor called vim. vi has three modes; insert mode, command mode and line mode.   
    • Insert mode – For making any changes in the file, you should be in insert mode.
    • Command mode – When you open vi editor you will be in this mode. This mode is used to do many shortcut tasks like delete words, switch to insert mode, copy and paste operation etc. When you are in insert mode you may press [ Esc ] key to get into command mode.
    • Line mode – this mode is used to issue commands for saving file, quitting editor etc. By pressing [ : ] key, you may get into this mode from command mode.

vi editor has large number of commands, you should be reasonably familiar with it in order to use Linux system. Let us do some examples here. Go to your home directory, type the following command and hit [ Enter ].

vi example

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The editor appears. Now we are in command prompt. If you want to get into insert mode press [ i ] key. Once you press [ i ] key, you may enter text into the editor. Just type “Hello how are you?” in the editor. Once you made modifications in the editor, you should go to command prompt by pressing [ Esc ] key. Now you are in command prompt. Now let us save the file. Save command is executed at line mode. Press [ : ] key. For saving the file, you have to give the command wq and hit enter. This command saves the file and quit the editor.

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Now verify the content of the file using cat command. Ok now let us examine some vi commands executed in command mode. In command mode, almost all commands do not require to press [ Enter ] key.  

  1.  i – switch to insert mode, content is inserted just before the current cursor position.
  2. a – switch to insert mode, text is appended just after the current cursor position.
  3. A – switch to insert mode, start appending text after the end of the last character.
  4. x – deletes single character in the current cursor position.
  5. dw – deletes word in the current cursor position.
  6. dd – delete current line.u – undo last change.
  7. h – move cursor to left like left arrow key.
  8. l – move cursor to right like right arrow key.
  9. j – move cursor down to next line similar to down arrow key.
  10. k – move cursor up to next line similar to up arrow key.

Some commands useful in line mode. Almost all commands in this mode are followed by pressing [ Enter ] key. You go to line mode from command mode by pressing [ : ] key.   

  1. w – save file, got back into command mode.
  2. wq – save and quit editor.
  3. q – exit editor.
  4. q! – quit editor without saving last session changes.
  5. /[pattern] – search for pattern in the file. It go back to command prompt and if any match is found, it keeps the cursor in the first match line.
  6. $ – move the cursor to the last line in the file.

 

  • man - man <command name> is used to get help regarding a particular command in Liux. If you want to know more about a command you should use this command. For eg, man ls is used to get help for list command. Type the command in shell prompt and hit [ Enter ].

 

  • more – Suppose you are making a detailed listing of large number of files in a directory. If you want to see the listing page by page, more command comes at your help. Just type the following command and hit [ Enter ].

 ls -lt /etc | more, here the listing of ls command is used as input for more command using pipe symbol. Here the number of records is showing in a page and waits for another key press. If you want to go to next page, you may press [ Space ] key .  

  • tailtail command is used to print the last lines of input text. The default value of number of lines is 10. But you can limit the number of lines using -n or –lines argument. Go to your home directory and type,

ls -lt | tail -n 1, this command prints the last line of listing. You may use the command to print the last portion of file also. For eg, tail -n 1 example prints the last line of example file.

  • wc – The word count command gives count about words, lines and characters in input text. -l argument gives number of lines, -w gives number of words, -m gives number of characters. For eg,

wc -w example gives number of words in example file.

Ok, let us stop here, because the number of commands in Linux is endless. I am planning to write another tutorial on basic admin commands in Linux, but before that let us take a break.

cheers, 

7 Responses to “Some basic Linux commands everyone must know”

  1. Ambadas says:

    “$ – move the cursor to the last line in the file”
    correction:
    $ – move the cursor to the last word in the line

  2. SriniReddy says:

    Fantastic stuff. Keep going on & on.

  3. aldi says:

    Excellent…!!!

  4. Abhisek says:

    I just can’t help bookmarking! Excellent!

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