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Linux permissions

  • Permission groups
    • superuser (su), sys admins
    • owner, the user who made the file/directory, ex. someone
    • group, a subset of users with a distinct name, ex. student
    • others, everyone else, public, guest, untrusted unsers
  • Types of permissions
    • w: Write, can modify
    • r: Read, can read
    • x: eXecute, run a program, or list a directory
    • -: none

It is specified who can do what, ex:

  • owner can write, read and execute
  • its group can read and execute, but not modify
  • others cannot do anything

The superusers (su) can do anything at any time, only a superuser can make an other user super.

These can be expressed with a set of three alpha-numeric charactes:

  • u: user, owner
  • g: group
  • o: others
  • a: all of the above
  • read: r or 4
  • write: w or 2
  • execute: x or 1
  • none: - or 0

The a is not the same as o, because it is possible that a group does not have a permission, but someone outside of the groups has.

The permissions can be represented with a 10 character string:

1 2 3 4 5 6 7 8 9 10
type owner group others
read write execute read write execute read write execute
$ ls -l ~
drwxr-xr-x 8 borbely student 4096 Aug 30 23:24 Desktop
drwxr-xr-x 2 borbely student 4096 Mar 27  2012 Downloads
drwxr-xr-x 2 borbely student 4096 Oct 20  2009 Drives
drwx------ 2 borbely student 4096 Apr 20 10:42 mail
drwxr-xr-x 7 borbely student 4096 Sep  6 13:01 public_html
$ _

The public_html folder is owned by borbely, its group is student, permissions: drwxr-xr-x

  • type: it is a directory
  • my permissions rwx means that owner can do anything
  • other students' permissions: r-x, they can read but not write
  • others: r-x, they can still read but not write

Numerically, you can encode this in 3 digits. The three numbers are (from left to right): owner, group and others. The munbers 4: read, 2: write, 1: execute. They can be added. Example: /home/student/borbely/public_html has permission: 755 meaning

  • owner (borbely) 7=4+2+1: read, write, execute
  • group (student) 5=4+1: read and execute
  • other 5=4+1: read and execute


You can change the permissions with chmod Examples:

  • chmod 700 ~/info_hazi: nobody can see anything, except me (of course superusers still can see it).
  • chmod 750 ~/important_work: In this folder one can share data with group members, but not others. Also group can only see it, not modify.
  • chmod 754 -R ~/public_html: The -R applies the permissions recursively in the subfolders of the folder, and every file in it.

More info

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