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chmod

The chmod command allows you to change the permissions of a file or directory.
You can either use symbolic notation for permissions, or use the octal notation for specifying the permissions.

Octal notation is a concise way to set permissions on a file or directory. It's based on the octal (base-8) numbering system, which uses digits from 0 to 7.

Basics of Octal Notation

In octal notation, permissions are represented by a three- or four-digit number.
Each digit represents a permission set:

  1. The first digit sets permissions for the owner.
  2. The second digit sets permissions for the group.
  3. The third digit sets permissions for others (everyone else).

If present, a fourth digit at the beginning sets special permissions (like the setuid, setgid, and sticky bits).

Permission Values

Each digit in octal notation is the sum of its component permissions:

  • Read (r) has a value of 4
  • Write (w) has a value of 2
  • Execute (x) has a value of 1

Examples

  1. Read, Write, and Execute for Owner; Read and Execute for Group and Others 

    chmod 755 file

    • Owner: rwx (4 + 2 + 1 = 7)
    • Group: r-x (4 + 0 + 1 = 5)
    • Others: r-x (4 + 0 + 1 = 5)

  2. Read and Write for Owner; Read for Group and Others 

    chmod 644 file

    • Owner: rw- (4 + 2 + 0 = 6)
    • Group: r-- (4 + 0 + 0 = 4)
    • Others: r-- (4 + 0 + 0 = 4)

  3. Special Permissions To set the setuid, setgid, and sticky bits, you can use a fourth digit (at the beginning of the number set): 

    chmod 1755 file

    • Special bits: 1
    • Owner: rwx (7)
    • Group: r-x (5)
    • Others: r-x (5)

Special Permissions

Special permissions like setuid, setgid, and sticky bits are advanced file permissions that provide additional control over file execution and access.

setuid (Set User ID)

  • Octal Value: 4
  • Symbolic Notation: s in the owner's execute field
  • Effect: When a file with setuid is executed, it runs with the permissions of the file's owner, not the user who ran it.

Example: 

chmod 4755 my_script
This sets the setuid bit, making the script run as its owner.

setgid (Set Group ID)

  • Octal Value: 2
  • Symbolic Notation: s in the group's execute field
  • Effect: Similar to setuid but for the group. When a file with setgid is executed, it runs with the permissions of the file's group.

Example: 

chmod 2755 my_script
This sets the setgid bit, making the script run as its group.

sticky Bit

  • Octal Value: 1
  • Symbolic Notation: t in the others' execute field
  • Effect: Mostly used on directories. It allows a file to be deleted only by its owner, the directory owner, or the root user, even if the directory has write permissions for others.

Example: 

chmod 1755 my_directory
This sets the sticky bit on a directory, restricting file deletion as described.

Combining Special Permissions

You can combine these special permissions by adding their octal values:

  • Setuid + Setgid: 4 + 2 = 6
  • Setuid + Sticky Bit: 4 + 1 = 5
  • Setgid + Sticky Bit: 2 + 1 = 3
  • All three: 4 + 2 + 1 = 7

Example: 

chmod 6755 my_script
This sets both the setuid and setgid bits.