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Basic File Permissions


Basic File Permissions



In Red Hat Enterprise Linux, all files have file permissions that determine whether a user is allowed to read, write, or execute them.


Basic File Access Permissions

Each file and directory has three user based permission groups:

1) owner (Users)- The Owner permissions apply only the owner of the file or directory, they will not impact the actions of other users.


2) group - The Group permissions apply only to the group that has been assigned to the file or directory, they will not effect the actions of other users.



3) all users (Others)- The All Users permissions apply to all other users on the system, this is the permission group that you want to watch the most.


Permission Types

Each file or directory has three basic permission types:

1) read - The Read permission refers to a user's capability to read the contents of the file.

2) write - The Write permissions refer to a user's capability to write or modify a file or directory.

3) execute - The Execute permission affects a user's capability to execute a file or view the contents of a directory.


When you issue the command ls -l, the first column of information contains these file permissions. Within this first column are places for 9 letters or hyphens.




1 A. The first space is either a hyphen, the letter d, or the letter l.

a)hyphen ( _ ) means it is a file.

b) A letter d means it is a directory.

c) A letter l means it is a symbolic link to a directory somewhere else on the file system.


1 B.The next nine spaces are divided into three sets of permissions are as follows :-

a) rwx – Read, Write and Execute permission for  the owner of the file or directory.

b) r-x – Read and Execute permissions for the group owing file or directory.

c) r-w – Read and Execute permissions for all other users for  file or directory.


2. 3 – its link

3. root = Owner name of the file or Directory.

4. root = Group name of the file or Directory.

5. 4096 = File or Directory size.

6. may = Month

7. 25 = Date

8. 2011 = Year

9. Documets = File or Directory name


Methods of Implementing Permission

1. Symbolic Mode :-  in Symbolic Mode file or directory permissions are denotes as follows :-

Read Permission = r

Write Permission = w

Execute Permission = x


Example :

Digits               Permission

x                      execute

w                     write

r                       read

wx                   write + execute

rx                     read + execute

rw                    read + write

rwx                  read + write + execute


2. Absolute Modeor Octal Value :-  in Absolute Mode file or directory permissions are denotes as follows :-

Read Permission = 4

Write Permission = 2

Execute Permission = 1


Example :

Digits               Permission

0                      none

1                      execute

2                      write

4                      read

3 (2+1)             write + execute

5 (4+1)             read + execute

6 (4+2)             read + write

7 (4+2+1)         read + write + execute


Default File Permission :- When the file is get created with the help of cat, vi, or touch command it will get  the permission for the as –rw-r—r--  or   644



1. rw- = read-write permission for the owner of the file.

2. r-- = read permission for the owner’s gorup of the file.

3. r-- = read  permissionfor the others.


Default Directory Permission :- When the directory is get created with the mkdir command it will get  the permission for the as drwxr-xr-x  or   755



1. rwx = read-write-execute permission for the owner of the directory.

2. r-x = read-execute permission for the owner’s gorup of the directory.

3. r-x = read-execute permission for the others.


Some examples of this permissions.

Permissions :-

Read (r = 4)    Write (w = 2)  Others (x = 1)


Owner             Group                         Other                         

1. rwx              rwx                  rwx                             

    7(4+2+1)      7(4+2+1)          7(4+2+1)

2. rwx              rwx                  rw

    7(4+2+1)      7(4+2+1)          6(4+2)

3. rwx              rwx                  rx

    7(4+2+1)      7(4+2+1)          5(4+1)

4. rwx              rwx                  r

    7(4+2+1)      7(4+2+1)          4

5. rwx              rwx                  wx

    7(4+2+1)      7(4+2+1)          3

6. rwx              rwx                  w

    7(4+2+1)      7(4+2+1)          2

7. rwx              rwx                  x

    7(4+2+1)      7(4+2+1)          1

8. rwx              rw                    rwx

    7(4+2+1)      6(4+2)              7(4+2+1)

9. rwx              rw                    rw

    7(4+2+1)      6(4+2)              6(4+2)

10. rwx                        rw                    rx

      7(4+2+1)    6(4+2)              5(4+1)


Umask :-The user file-creation mode mask (umask) is use to determine the file permission for newly created files. It can be used to control the default file permission for new files. Only the root user can set UMASK. It is a four-digit octal number. A umask can be set or expressed using:

1. Symbolic values = u=rwx,g=rx,o=rx

2. Octal values = 0022


[root@server1 ~]#umask                     ………

0022

[root@server1 ~]#umask -S                 ………

u=rwx,g=rx,o=rx


[root@server1 ~]#umask –S u=rwx,g=r,o=r                 ………

u=rwx,g=r,o=r

[root@server1 ~]#umask                     ………

0033

[root@server1 ~]#touch 1                    ………

[root@server1 ~]#ll                             ………

-rw-r—r--.  1    root   root  0     jun   12     21:28   1


[root@server1 ~]#umask –S u=rwx,g=w,o=w              ………

u=rwx,g=w,o=w

[root@server1 ~]#umask                     ………

0055

[root@server1 ~]#touch 2                    ………

[root@server1 ~]#ll                             ………

-rw--w--w-.  1  root   root  0     jun   12     21:28   2


[root@server1 ~]#umask –S u=rwx,g=x,o=x               ………

u=rwx,g=x,o=x

[root@server1 ~]#umask                     ………

0066

[root@server1 ~]#touch 3                    ………

[root@server1 ~]#ll                             ………

-rw-------.  1     root   root  0     jun   12     21:28   3


[root@server1 ~]#umask –S u=rwx,g=rw,o=rw                       ………

u=rwx,g=rw,o=rw

[root@server1 ~]#umask                     ………

0011

[root@server1 ~]#touch 4                    ………

[root@server1 ~]#ll                             ………

-rw-rw-rw-.  1  root   root  0     jun   12     21:28   4

20 comments:

  1. Can i change the file permission without using chmod command ?

    ReplyDelete
    Replies
    1. Sure, just write a C program that does that, give it a name different from "chmod", and call it. Or use a graphical file manager, right click, "properties" -> "permissions..." and change the permissions

      Delete
  2. we can change the permission by graphically if you don't want to use chmod command.

    ReplyDelete
  3. For example if user create a file by default the permission get rw r r right
    And the root user can also access that file. But if i dont want to make root user to access those file .. then what to do ?

    ReplyDelete
    Replies
    1. No, this cannot be done. The whole concept of the root user relies on unfettered access. You can quite easily protect your files from other regular users, using either the user/group/other paradigm or access control lists but, once someone has root powers, you cannot restrict them at all.

      There's a good reason for this - if you lost access to your files somehow, how would you expect the administrators to recover them for you?

      Delete
    2. yes,root has all the power , but you can provide extra level of protection by setting it as immutable file.

      you need to have root access to do this:

      chattr +i filename
      but remember root can unset this by running the command

      chattr -i filename
      and edit/delete your file.

      for more check
      man chattr

      Delete
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