What Is chmod?

In Linux, this command changes file permissions. It is short for change mode. You can change permissions for files and directories using octal mode or symbolic mode. In practice, octal mode is more widely used because it is simpler.


Usage

The basic form of the chmod command is below. Let’s look at options and modes.

$ chmod [option] [mode] [file]

Options

You can use options as follows.

  • R: Changes permissions for files and directories under the target.
  • v: Lists all files currently being processed.
  • c: Prints only files that were actually changed, with details.

Symbolic Mode

chmod can assign permissions using symbols. Compared with octal mode, symbolic mode is more intuitive.

$ chmod [reference] [operator] [mode] [target name]

References are:

  • u: user (owner), the file owner.
  • g: group, users who are members of the file’s group.
  • o: others, users who are neither owner nor group members.
  • a: all of the above, same as ugo.

Operators are:

  • +: adds permission.
  • -: removes permission.
  • =: sets permission exactly to the specified value.

Modes are:

  • r: read, can read a file or list directory contents.
  • w: write, can write to a file or directory.
  • x: execute, can execute a file.

Octal Mode

This is the common permission-setting method using octal numbers. It is less intuitive, but simple to use.

$ chmod [option] [octal number] [target name]

File permissions are displayed like -rwxrwxrwx. Ignore the first character, which indicates file type (d for directory, l for symbolic link, etc.), and split the next 9 characters into 3 groups for user, group, and other permissions.

Map read r to 4, write w to 2, and execute x to 1. You can set permissions by adding these values. For example, -rwxr-xr-x maps to 755.


Examples

Now let’s look at examples using the methods above.

# Grant read and write permissions on test to user
$ chmod u+rw test


# Grant only write permission on test to user
# Grant read permission on test to group
$ chmod u=w, g+r test


# Grant read, write, execute permissions to user
# Grant read and execute permissions to group and other
$ chmod 755 test