linux-file-permissions.md

Linux File Permissions Tutorial

In Linux, file permissions dictate who can read, write, or execute a file or directory.

Understanding File Permissions

To view file permissions, use the ls -l command, which lists the contents of a directory along with their permissions. Each entry is preceded by a string representing the file type and permission settings.

• The first character in the string represents the file type:

  • -: Regular file
  • d: Directory
  • l: Symbolic link
  • c: Character device file
  • b: Block device file
  • s: Socket
  • p: Named pipe (FIFO)

• The next nine characters are grouped into three sets of three, representing permissions for the file owner, group, and others:

  • The first set (positions 2-4) represents the owner's permissions.
  • The second set (positions 5-7) represents the group's permissions.
  • The third set (positions 8-10) represents others' permissions.

  Each set can include:

  • r: Read permission
  • w: Write permission
  • x: Execute permission
  • -: No permission

For example, in drw-r-xr-x:

• d: This is a directory.
• rw-: The owner has read and write permissions.
• r-x: The group has read and execute permissions.
• r-x: Others have read and execute permissions.

Numerical Representation of Permissions

Permissions can also be represented numerically using octal notation. Each permission set (owner, group, others) is represented by a number from 0 to 7, which corresponds to a combination of read (4), write (2), and execute (1) permissions:

Number	Permissions	Binary
0	None	000
1	Execute	001
2	Write	010
3	Write and Execute	011
4	Read	100
5	Read and Execute	101
6	Read and Write	110
7	Read, Write, Execute	111

For example, a permission of 755 means:

• Owner: 7 (Read, Write, Execute)
• Group: 5 (Read, Execute)
• Others: 5 (Read, Execute)

How to Change File Permissions

The chmod command is used to change the permissions of files and directories. It can be used with symbolic or numeric (octal) notation.

Using Symbolic Notation

Symbolic notation allows you to modify specific permission sets (user, group, others):

$ chmod u+rwx,g+rx,o-r myfile

This command grants the owner (u) read, write, and execute permissions, the group (g) read and execute permissions, and removes read permission from others (o).

Using Numeric (Octal) Notation

$ chmod 755 myfile

This command sets the permissions to rwxr-xr-x, meaning the owner has full permissions, while the group and others have read and execute permissions.

How to Change File Ownership

You can change the ownership of a file or directory using the chown command:

$ sudo chown user:group filename

This command changes the ownership of filename to user and the group to group. Add -R to the command to change the ownership of all the contents of a directory.

Special Permissions

• Setuid (s): Allows a file to be executed with the privileges of the file's owner. Set using chmod u+s.
• Setgid (g): Similar to setuid, but for the group. Set using chmod g+s.
• Sticky Bit (t): Ensures that only the file's owner, directory owner, or root can delete or modify a file. Set using chmod +t.