File Permissions and Ownership (chmod, chown)

In Linux, every file and directory is assigned specific permissions and ownership. These define who can access the file, what operations can be performed on it, and by whom. Understanding how to manage file permissions and ownership is crucial for maintaining system security and ensuring that files are protected from unauthorized access.

File Permissions Overview

Each file and directory in Linux has three basic types of permissions:

• Read (r): Allows the user to open and view the file's content. For directories, it allows listing the files within.
• Write (w): Allows the user to modify the content of the file. For directories, it allows adding or deleting files within the directory.
• Execute (x): Allows the user to execute the file as a program. For directories, it allows entering or accessing the directory (i.e., change to that directory).

These permissions are assigned to three categories:

• User (u): The file's owner (also known as the file's user).
• Group (g): A group of users who share the same access rights to the file.
• Others (o): All other users who are not the owner or part of the group.

Understanding the Permission String

When you use the ls -l command in the terminal to list files, you'll see a string that represents the file’s permissions, which looks like this:

-rwxr-xr--

This string is broken into four parts:

• First character: Represents the file type (for regular files, it is -; for directories, it is d).
• Next three characters: Represents the user (owner) permissions.
• Next three characters: Represents the group permissions.
• Next three characters: Represents the permissions for others.

In this case:

• rwx means the user (owner) has read, write, and execute permissions.
• r-x means the group has read and execute permissions, but cannot modify the file.
• r-- means others have read-only permissions.

Numeric Representation of Permissions

Permissions can also be represented numerically. Each permission type is assigned a number:

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

To set permissions, you sum the values of the permissions for the user, group, and others:

• rwx = 4 + 2 + 1 = 7
• r-x = 4 + 0 + 1 = 5
• r-- = 4 + 0 + 0 = 4

Thus, the permission string -rwxr-xr-- can be represented as 755.

Changing Permissions with chmod

The chmod command is used to change file permissions in Linux. There are two ways to use chmod: symbolic mode and numeric mode.

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Symbolic Mode

In symbolic mode, you specify who (user, group, others) the permission change applies to, what operator (+, -, =) to use, and the permissions to set (r, w, x).

Syntax:

chmod [who][operator][permission] [file]

• who: Specifies the user(s) to apply the change to (u for user, g for group, o for others, or a for all users).
• operator: Specifies how the permissions should be changed (+ to add, - to remove, = to set exactly).
• permission: Specifies which permission(s) to modify (r for read, w for write, x for execute).

Examples:

1. Add execute permission for the user (owner):

chmod u+x file.txt

This command adds the execute (x) permission to the user (owner) of the file file.txt.

2. Remove write permission from the group:

chmod g-w file.txt

This command removes the write (w) permission from the group associated with file.txt.

3. Set read and write permissions for the user, and read-only for others:

chmod u=rw,o=r file.txt

This command sets the user permissions to read and write (rw), while setting others to read-only (r).

4. Add execute permission for everyone (user, group, others):

chmod a+x file.txt

This command adds the execute (x) permission for all users (a stands for all: user, group, others).

5. Set all permissions to rwx for the user, and r-x for the group and others:

chmod u=rwx,g=rx,o=rx file.txt

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Numeric Mode

In numeric mode, you specify the permission as a three-digit number, where each digit corresponds to the user, group, and others. The number is the sum of the individual permissions (read = 4, write = 2, execute = 1).

Examples:

1. Set permissions to 755** (User has full permissions, group and others can read and execute):

chmod 755 file.txt

This command sets the user permissions to rwx (7), group to r-x (5), and others to r-x (5).

2. Set permissions to 644 (User can read and write, group and others can only read):

chmod 644 file.txt

This command sets the user permissions to rw- (6), group to r-- (4), and others to r-- (4).

3. Set permissions to 777 (Everyone has full access to read, write, and execute):

chmod 777 file.txt

This command grants full permissions (read, write, execute) to everyone.

4. Set permissions to 700 (Only the user has full access, others have no access):

chmod 700 file.txt

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Changing Ownership with chown

The chown command changes the owner and/or group of a file. This is useful if you need to transfer file ownership or change the group associated with a file.

Syntax:

chown [owner][:group] [file]

• owner: The user who will become the new owner of the file.
• group: The new group (optional).
• file: The file or directory whose ownership is being changed.

Examples:

1. Change the owner of a file:

chown user1 file.txt

This command changes the owner of file.txt to user1.

2. Change both the owner and the group:

chown user1:group1 file.txt

This command changes the owner of file.txt to user1 and the group to group1.

Change only the group:

chown :group2 file.txt

This command changes only the group of file.txt to group2, leaving the owner unchanged.

4. Recursively change the owner and group of all files in a directory:

chown -R user1:group1 /home/user1/

This command changes the ownership of all files within the /home/user1/ directory and its subdirectories.

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Key Takeaways:

• Permissions in Linux control who can read, write, and execute a file.
• Use chmod to change permissions, either in symbolic or numeric format.
• chown is used to change the owner and/or group of a file.
• Correct management of file permissions and ownership is essential for securing files and directories in a Linux system.