Chmod

chmod
Change access permissions (mode) for files and directories.

Syntax

chmod [Options]... Mode [,Mode]... file...
chmod [Options]... Numeric_Mode file...
chmod [Options]... --reference=RFile file...

Options

• -f, --silent, --quiet
  Suppress most error messages.

• -v, --verbose
  Output a diagnostic for every file processed.

• -c, --changes
  Similar to verbose, but report only when a change is made.

• --reference=RFile
  Use RFile's mode instead of specifying Mode.

• -R, --recursive
  Change files and directories recursively. Avoid applying recursive chmod on system directories or root /.

• --help
  Display help and exit.

• --version
  Output version information and exit.

Understanding chmod

The chmod command changes the access permissions of files and directories. Permissions can be modified using either symbolic or numeric modes.

Permissions

• Owner (User): r (Read), w (Write), x (Execute)
• Group: r (Read), w (Write), x (Execute)
• Others: r (Read), w (Write), x (Execute)

Set User ID (SUID), Set Group ID (SGID), and Sticky Bit

• SUID (Set User ID): 4xxx
• SGID (Set Group ID): 2xxx
• Sticky Bit: 1xxx

These attributes can be added by prepending the numeric mode.

Numeric Mode

Numeric mode consists of up to four octal digits (0-7), each representing specific permissions.

• First digit (optional): SUID (4), SGID (2), Sticky Bit (1)
• Second digit: Owner permissions (r=4, w=2, x=1)
• Third digit: Group permissions (r=4, w=2, x=1)
• Fourth digit: Others permissions (r=4, w=2, x=1)

Examples:

• chmod 777 file
  Grants read, write, and execute permissions to owner, group, and others.

• chmod 755 file
  Grants read, write, and execute to the owner; read and execute to group and others.

• chmod 644 file
  Grants read and write to the owner; read-only to group and others.

Symbolic Mode

Symbolic mode uses letters (r, w, x, u, g, o, a) to represent file permissions.

• u: User (owner)
• g: Group
• o: Others
• a: All (user, group, others)

Operators:

• +: Add permission
• -: Remove permission
• =: Set exact permission

Examples:

• chmod u+x file
  Adds execute permission for the owner.

• chmod go-w file
  Removes write permission from group and others.

• chmod a+r file
  Adds read permission for everyone (user, group, and others).

• chmod g+s file
  Sets the SGID bit on a file.

• chmod +x myscript.sh
  Makes a file executable.

Examples:

1. Deny execute permission to everyone:

   chmod a-x file

2. Allow read permission to everyone:

   chmod a+r file

3. Make a file readable and writable by the group and others:

   chmod go+rw file

4. Make a shell script executable by the user/owner:

   chmod u+x myscript.sh

5. Allow everyone to read, write, and execute the file and turn on the set group-ID:

   chmod =rwx,g+s file