Understanding chmod 777 ❓

NOTE: The is a contains small portion of Linux file permission blog. Contents from the parent blog are pruned and customize for this article. Thank you for reading.

In Linux, permissions define the access rights for three categories of users: 👤User, 👥Group, and Others.

• The User is the file or directory owner.
• The Group consists of users belonging to the same group as the owner.
• Others refers to the general public.

Each of these users can have three types of permissions: 📕Read, ✍️Write, and Execute🥬.

• Read ( r ): Allows viewing the content of the file. Represented by an octal value of 4.
• Write ( w ): Allows modifying or editing the file. Represented by an octal value of 2.
• Execute ( x ): Allows running the file (if it's a script) or traversing a directory. Represented by an octal value of 1.

These permissions are presented as a string of nine characters for each file or directory:

Example:

rw-r--r--

• rw- means the owner has read and write permissions.
• r-- means the group has read-only permissions.
• r-- means others have read-only permissions.

Octal Mode in chmod

In advanced file permission manipulation, numeric values are used to represent permissions:

• Read ( r ) = 4
• Write ( w ) = 2
• Execute ( x ) = 1

To set permissions using octal values, you assign a three-digit number representing the permissions for User, Group, and Others. For example, 777 gives read, write, and execute permissions to everyone.

Note: The maximum permission 777 means User, Group, and Others all have full read, write, and execute permissions. However, granting such permissions can pose a security risk.

Example: Modifying File Permissions with Octal Values

Let’s say we want to set specific permissions:

• User: Read (4) and Execute (1) ⇒ 4 + 1 = 5
• Group: Read-only (4)
• Others: Read-only (4)

The combined permission value is 544, meaning the user can read and execute, and both the group and others can only read.

chmod 544 myfile.txt

This sets the permission to r-xr--r--, which breaks down into three blocks:

• r-x for the User
• r-- for the Group
• r-- for Others

Note: The - before the permissions indicates the file type, where - is for regular files and d is for directories.

Example 2: Complex Permission Setup

Let’s give the following permissions:

• User: Read (4) and Write (2) ⇒ 4 + 2 = 6
• Group: Write (2) and Execute (1) ⇒ 2 + 1 = 3
• Others: Read (4)

The combined value is 634, meaning:

chmod 634 myfile.txt

Changing File Ownership

To change the ownership of a file or directory, use the chown command:

chown username myfile.txt

Note: You might require root privileges for this command.

Changing File Group Ownership

To change the group ownership of a file, use the chgrp command:

chgrp groupname myfile.txt

Important Tips

• Avoid using 777: It’s a significant security risk.
• Use ls -l to display detailed file information, including permissions.
• Don’t forget about umask_, which affects default file permissions.

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