🎯How to Set File Permissions in Linux.🎉

1. File Permission Basics

Each file and directory in Linux has three permission categories:

• Owner (u): The user who owns the file.

• Group (g): Users in the group associated with the file.

• Others (o): Everyone else who can access the system.

Permissions control three operations:

• Read (r): View the contents of the file or list a directory.

• Write (w): Modify the file or add/delete files in a directory.

• Execute (x): Run a file as a program or access a directory.

2. Permission Syntax

To see permissions, use:

ls -l

Output example:

-rwxr-xr-- 1 user group  4096 Dec 27 12:00 file.txt

Breaking it down:

1. -rwxr-xr--: Permission string:

   • First character: File type:

     • -: Regular file

     • d: Directory

     • l: Symbolic link

   • Next 9 characters: Permissions in three groups of three:

     • rwx: Owner can read, write, and execute.

     • r-x: Group can read and execute, but not write.

     • r--: Others can only read.

2. 1: Number of links to the file.

3. user: File owner.

4. group: Group associated with the file.

5. 4096: File size in bytes.

6. Dec 27 12:00: Last modified date.

7. file.txt: File name.

Linux file permissions are essential for securing files and directories, determining who can read, write, or execute them. Here's an in-depth explanation of Linux file permissions:

3. Modifying File Permissions

Permissions can be changed using the chmod command.

Symbolic Mode

Assign permissions using letters:

• u: Owner

• g: Group

• o: Others

• a: All (u, g, o)

Operators:

• +: Add permission

• -: Remove permission

• =: Set specific permission

Examples:

chmod u+x file.txt               # Add execute permission for the owner.
chmod g-w file.txt               # Remove write permission for the group.
chmod o=r file.txt               # Set read-only permission for others.
chmod a+x script.sh              # Add execute permission for everyone.

Numeric Mode

Assign permissions using octal values:

• 4: Read (r)

• 2: Write (w)

• 1: Execute (x)

• 0: No permission

Combine values for each group:

• 7 (4+2+1): Read, write, and execute.

• 6 (4+2): Read and write.

• 5 (4+1): Read and execute.

• 4: Read-only.

Examples:

chmod 755 file.txt               # Owner: rwx, Group: r-x, Others: r-x
chmod 644 file.txt               # Owner: rw-, Group: r--, Others: r--
chmod 700 secret                 # Owner: rwx, Group: ---, Others: ---

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4. Ownership

File ownership is managed with two identifiers:

• User (owner): Specifies the user who owns the file.

• Group: Specifies the group associated with the file.

Change Ownership

Use the chown command:

chown user file.txt                      # Change owner to 'user'.
chown user:group file.txt                # Change owner to 'user' and group to 'group'.
chown :group file.txt                    # Change group only.

Example:

sudo chown alice:developers report.txt

5. Default Permissions

Umask

The umask (user mask) determines default permissions for newly created files/directories.
Default permissions:

• File: 666 (read and write for all).

• Directory: 777 (read, write, and execute for all).

Umask subtracts permissions:

umask 022                 # Removes write permission for group and others.

View or set umask:

umask                     # View current umask.
umask 027                 # Set umask to 027.

6. File Permission Commands Summary

7. Practical Examples

1. Make a script executable:

    chmod +x script.sh
   

2. Restrict directory access to the owner:

    chmod 700 secret
   

3. Allow a user to edit a file without changing ownership:

    setfacl -m u:username:rw file.txt
   

   8. Security Best Practices

   • Use least privilege: Assign only the permissions needed for a task.

   • Regularly audit permissions with ls -l .

   • Avoid using 777 permissions as it grants unrestricted access to all.

   • Secure sensitive files with chmod 600 or chmod 700.