File Permissions in Linux

File permissions in Linux control the access rights to files and directories. These file permissions as crucial in controlling who or what can read, write, or execute a file. These permissions apply to the user or file owner, the group, and others.

Categories of Users:

1. Owner (User): The user who owns the file.

2. Group: A group of users who share the same permissions.

3. Others: All other users who are not the owner and do not belong to the group.

Types of Permissions:

1. Read (r): Permission to read the file or list the directory contents.

2. Write (w): Permission to modify the file or directory.

3. Execute (x): Permission to execute the file or access the directory.

Understanding the Permission Notation:

Permissions are displayed using a 10-character string (e.g., -rwxr-xr–):

1. First Character:

   – -: Indicates a regular file.

   – d: Indicates a directory.

   – l: Indicates a symbolic link.

   – b: Indicates a block device.

   – c: Indicates a character device.

   – s: Indicates a socket.

   – p: Indicates a named pipe.

2. Next Nine Characters:

Represent the permissions for the owner, group, and others, in three sets of three characters:

   – Owner: The next three characters (rwx in this example) show the permissions for the file owner.

   – Group: The following three characters (r-x in this example) show the permissions for the group.

   – Others: The last three characters (r– in this example) show the permissions for others.

Example: -rwxr-xr–

– -: Regular file.

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

– r-x: Group can read and execute.

– r–: Others can only read.

Changing Permissions:

Permissions can be changed using the chmod command. There are two ways to use chmod:

1. Symbolic Mode:

   – Use symbols to modify permissions.

   – Example: chmod u+x file.txt adds execute permission for the owner.

2. Numeric Mode:

– Use octal numbers to set permissions.

   – Each permission type is represented by a number: read (4), write (2), execute (1).

   – Example: chmod 755 file.txt sets the permissions to rwxr-xr-x:

     – 7 (4+2+1) for the owner (read, write, execute).

     – 5 (4+0+1) for the group (read, execute).

     – 5 (4+0+1) for others (read, execute).

Special Permissions:

1. Setuid (Set User ID):

   – When set on an executable file, this allows users to execute the file with the permissions of the file owner.

   – Represented by an s in the owner’s execute field (rws).

2. Setgid (Set Group ID):

   – When set on a directory, new files created within the directory inherit the group of the directory.

   – When set on an executable file, users execute the file with the permissions of the file’s group.

   – Represented by an s in the group’s execute field (r-s).

3. Sticky Bit:

   – When set on a directory, it ensures that only the file owner, directory owner, or root can delete or rename files within the directory.

   – Represented by a t in the others’ execute field (rwt).

 Examples of Using chmod:

– Symbolic Mode:

  – Add execute permission for the owner: chmod u+x file.txt

  – Remove write permission for the group: chmod g-w file.txt

  – Add read permission for others: chmod o+r file.txt

– Numeric Mode:

  – Set permissions to rwxr-xr–: chmod 754 file.txt  

– Set permissions to rw-r–r–: chmod 644 file.txt

Having an understanding of managing file permissions is a key part of ensuring that proper security is maintained in a Linux system.