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chmod

chmod

Check permissions

In terminal: ls –l [file_name]

For example:

$ ls -l

-rw-r--r--  1 EckoTan  staff  12 Jan  5 18:02 hello-world.txt
  • -rw-r--r-- : file permission
  • EckoTan : owner/creator of the file
  • staff : the group to which that owner belongs to
  • Jan 5 18:02 : date of creation
  • hello-world.txt : the file

Permissions

The permission settings, grouped in a string of characters (-, r, w, x), are classified into four sections:

  1. File type. There are three possibilities for the type. It can either be a regular file (–), a directory (d) or a link (i).
  2. File permission of the user (owner)
  3. File permission of the owner’s group
  4. File permission of other users

file-permission-syntax-explained

In permission of user, group, and others, the characters -, r, w, and x stand for None, read, write, and execute.

  • -: permission is NOT granted
  • r: Read permissions. The file can be opened, and its content viewed. But it can not be modified (nor added/removed)
  • w: Write permissions. The file can be edited, modified, and deleted.
  • x: Execute permissions. If the file is a script or a program, it can be run (executed).This option is mainly used for running scripts.

Example

-rw-r--r--  1 EckoTan  staff  12 Jan  5 18:02 hello-world.txt

hello-world.txt is a regular file with read and write permission assigned to the owner, but gives read-only access to the group and others

Change permissions

The command that executes such tasks is the chmod command.

Syntax:

$ chmod [permission] [file_name]

There are two ways to define permission:

  1. using symbols (alphanumerical characters)
  2. using the octal notation method

Define File Permission with Symbolic Mode

To use chmod to set permissions, we need to tell it:

  • Who: Who we are setting permissions for?

    • u : User, meaning the owner of the file.
    • g : Group, meaning members of the group the file belongs to.
    • o : Others, meaning people not governed by the u and g permissions.
    • a : All, meaning all of the above.

    (If none of these are used, chmod behaves as if “a” had been used.)

  • What: What change are we making? Are we adding or removing the permission?

    • – : Removes the permission.
    • + : Grants the permission. The permission is added to the existing permissions.
    • = : Set a permission and remove others.

  • Which: Which of the permissions are we setting?

    • r
    • w
    • x

Example

$ ls -l

-rw-r--r--  1 EckoTan  staff  12 Jan  5 18:02 hello-world.txt

We want to give group members to have write permission on hello-world.txt:

$ chmod g+w hello-world.txt

$ ls -l

-rw-rw-r--  1 EckoTan  staff    12B Jan  5 18:02 hello-world.txt

We want the permission settings look like this

  • u: read, write, execute
  • g: read, write
  • o: read, write
$ chmod u=rwx,g=rw,o=rw hello-world.txt

$ ls -l

-rwxrw-rw-  1 EckoTan  staff    12B Jan  5 18:02 hello-world.txt

Define permission using numerical shorthand

Another way to specify permission is by using the octal/numeric format. This option is faster, as it requires less typing, although it is not as straightforward as the previous method.

The following example shows how it works:

img

Using this method, each of the three permissions (r, w, x) is represented by one of the bits in the binary equivalent of the decimal number.

In this example,

  • The permission of user/owner is rwx. This could be represented as 111, which equals 7 in decimal.
  • The permission of group is r-x. This could be represented as 101, which equals 5 in decimal.
  • The permission of others is r--. This could be represented as `1010, which equals 4 in decimal.

Therefore, to achieve the same permission settings as

$ chmod u=rwx,g=rx,o=r hello-world.txt

We can also type:

$ chmod 754 hello-world.txt

-rwxr-xr--  1 EckoTan  staff    12B Jan  5 18:02 hello-world.txt

Reference