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Mastering File and Directory Permissions with the chmod Command in Linux
Table of contents
Have you ever encountered a "permission denied" error while trying to access or modify a file or directory in Linux? This error is usually caused by inadequate file permissions.
In Linux, every file and directory has three sets of permissions - read, write, and execute assigned to three different groups of users - owner, group, and others.
The chmod command in Linux is used to change these permissions, and it is an essential tool for any Linux user who wants to take control of their system.
Syntax: chmod [options] mode file/directory
The "mode" is the permission setting you want to apply to the file or directory, and the "options" modify the command's behavior.
Now let's talk about the two ways to specify the mode.
Symbolic Notation
In symbolic notation, the mode is a combination of letters and symbols. The letters represent the three groups of users and the symbols represent the permissions. The following table shows the possible values for each:
| Letter | Group |
| u | User (owner) |
| g | Group |
| o | Others |
| a | All |
| Symbol | Permission |
| r | Read |
| w | Write |
| x | Execute |
chmod u+rwx file.txt: Add read, write, and execute permissions for the owner of file.txt.
chmod go-w file.txt: Remove write permission for the group and others of file.txt.
chmod a=x file.txt: Set execute permission for all users of file.txt.
Numeric Notation
In numeric notation, the mode is specified as a three-digit number, where each digit represents the permissions for one of the three groups of users. The digits are calculated by adding up the values for each permission, as shown in the following table:
| Permission | Value |
| --- | 0 |
| --x | 1 |
| -w- | 2 |
| -wx | 3 |
| r-- | 4 |
| r-x | 5 |
| rw- | 6 |
| rwx | 7 |
chmod 600 file.txt: Set read and write permissions for the owner of file.txt and no permissions for anyone else.
chmod 755 directory: Set read, write, and execute permissions for the directory owner and read and execute permissions for the group and others.
chmod 644 file.txt: Set read and write permissions for the owner of file.txt and read-only permissions for the group and others.
Now let's explore some of the most commonly used options for the chmod command:
-R: Recursively change the permissions of a directory and all its contents.
-v: Display a message for each file or directory that is modified.
-c: Display a message only for files or directories that are actually modified or changed.
Here are some examples of using the chmod command with options:
chmod -R 600 directory: Recursively set read and write permissions for the owner of the directory and no permissions for anyone else.
chmod -v a+r file.txt: Add read permission for all users of file.txt and display a message for each file modified.
chmod -c go+w file.txt: Add write permission for the group and others of file.txt and display a message only if a file is actually modified.
In conclusion, the chmod command is a powerful tool that allows users to modify file and directory permissions in Linux. Understanding how to use the chmod command is essential for any Linux user who wants to take control of their system's security.