The “chmod” Command in Linux [6 Practical Examples]

The chmod command in Linux is a very useful command. It is used for changing the permission of access. In the Linux distribution Ubuntu or in any other system there might be files that you don’t want others to access. Changing access permission is the solution to that. In this article, I will show you how to use the chmod command and I will show you some useful examples too.

A. Description

The word chmod is the short form of Change Mode. This means the chmod command can make a file Readable, Writable, and Executable for users other than you. Or it can restrict permission for everyone or a selected group of people. All these will be discussed here in detail.

B. Syntax

The syntax for the chmod command is very simple. But there are multiple syntaxes for the chmod command. All of the syntaxes do the same thing which is altering the permission of a file or directory.

• Syntax 1

chmod [OPTION]... MODE[,MODE]... FILE…

• Syntax 2

chmod [OPTION]... OCTAL-MODE FILE…

• Syntax 3

chmod [OPTION]... - -reference=RFILE FILE…

C. Options

Many useful options are available for the chmod command. I have listed some of the most used ones here. However, if you want to know more about options for the chmod command you can always check the man page for the chmod command.

man chmod

Useful Options

View Permission of a File or Directory

Knowing the existing permission of a file or directory is mandatory before changing the permissions. Now follow the steps below to view the permission of a file or directory.

Steps to Follow >

➊ At first open the Ubuntu Terminal.

➋ Type the following command in the command prompt:

ls -l

➌ Now, press the ENTER button.

Output >

In the picture below first 10 characters define the file type and permissions.

Altering Permission of a File or Directory

There are three ways that permission can be altered. Three types of argument are given below.

Practical Examples of the “chmod” Command in Linux

The chmod command is very easy to use. It is one of the most important commands of the Linux CLI. Here I have given some basic examples of the chmod command. I have used some Files and Directories that I have created on my machine. You can download these contents if you want to work with the same Files and directories.

Example 1: Giving Permissions Using Normal Expression of the “chmod” Command in Linux

I have a file named File1.txt which doesn’t have permission granted for other users to write or execute. Look at the picture below to understand clearly.

Now to alter these permissions follow the steps below.

Steps to Follow >

➊ At first open the Ubuntu Terminal.

➋ Type the following command in the command prompt:

chmod o=rwx File1.txt

➌ Now, press the ENTER button.

Output >

Here File1.txt didn’t have write or execute permission for others. But I used the chmod command to change that permission.

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Example 2: Revoking Permissions Using Normal Expression of the “chmod” Command in Linux

I have a file named File2.txt which has permission granted for the members of the file group to read or write. Look at the picture below to understand clearly.

Now follow the steps below to revoke these permissions.

Steps to Follow >

➊ At first open the Ubuntu Terminal.

➋ Type the following command in the command prompt:

chmod g= File2.txt

➌ Now, press the ENTER button.

Output >

As you can see, all the permissions for the group members were nullified.

Example 3: Giving Permissions Using Logical Expression of the “chmod” Command in Linux

I will give the permission back to the group users of File2.txt, which I revoked in the previous Example. But now I will use the Logical Expressions to the chmod command. Now follow the steps below to see how this works.

Steps to Follow >

➊ At first open the Ubuntu Terminal.

➋ Type the following command in the command prompt:

chmod g+r File2.txt

➌ Now, press the ENTER button.

➍ Type the following command in the command prompt:

chmod g+w File2.txt

➎ Now, press the ENTER button.

➏ Type the following command in the command prompt:

chmod g+x File2.txt

➐ Now, press the ENTER button.

Output >

As you can see I used the chmod command 3 times to grant three permissions. All three characters were dash (-) but they are now changed to r, w, and x by the chmod command.

Example 4: Revoking Permissions Using Logical Expression of the “chmod” Command in Linux

I will use the same file File2.txt to demonstrate this example. As you can see from the picture below, this file has permission to be read by other users.

But I don’t want others to read this file. So I will revoke that permission using the chmod command. Now follow the steps below to see how this works.

Steps to Follow >

➊ At first open the Ubuntu Terminal.

➋ Type the following command in the command prompt:

chmod o-r File2.txt

➌ Now, press the ENTER button.

Output >

As you can see from the picture others don’t have read permission anymore. The “r” character is replaced by dash(-).

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Example 5: Changing Permissions for All the Users

In previous examples, you must have noticed that I was only altering the permission of one group of users at a time. It was either Owner or the User of the File Group. But I didn’t alter their permission together. Because there is a different way to do that. For this example, I will use the File1.txt file. Which has read permission for all the users. You can affirm that from the picture below.

I will revoke read permission for all users with just one command. Now follow the steps below to see how you can alter permission for all the users at one go.

Steps to Follow >

➊ At first open the Ubuntu Terminal.

➋ Type the following command in the command prompt:

chmod a-r File1.txt

➌ Now, press the ENTER button.

Output >

As you can see all the “r” characters are replaced with dash(-). So it’s safe to say that revoking permission was successful.

Example 6: Giving Permissions Using Octal Notation of the “chmod” Command in Linux

I will give back the permissions to all the users which I have revoked in the previous example. First, take a look at the picture below to understand what permission File1.txt has as of now.

So from the picture, it’s evident that none of the users have read permission. So let’s give read permission to everyone. But I will use Octal Notation rather than using other expressions. Now, I will give the octal expression for that.

Now follow the steps below to see how this works.

Steps to Follow >

➊ At first open the Ubuntu Terminal.

➋ Type the following command in the command prompt:

chmod 667 File1.txt

➌ Now, press the ENTER button.

Output >

As you can see all the dash(-) characters are replaced with “r” characters. So it’s safe to say that giving permission back was successful.

Conclusion

In this article, I have shown you how the chmod command works. And I have also included some useful examples. If you go through this article properly, you should be able to use the command chmod properly on your own.

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