Linux-standards-permissions
Linux

Introduction to Linux standard file permissions

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Introduction to Linux standard file permissions

Hi and welcome . In this tutorial we will introduce linux standard file permissions. We will discuss how to determine whether linux content in directory is file, or subdirectory or symbolic link. And we will talk about chgrp and chown commands to change group and change owner of any linux file.

file ownership

user owner and group owner

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we can manager users and groups in linux locally in two configuration files /etc/passwd and /etc/group or if on network they can be managed in NIS, LDAP or samba domain. These groups and users can own files/directories/ In Linux every file or directory has owner and group assigned to it.See below example

[root@shahid security]# ls -lh
total 0
-rw-rw-r--. 1 rob     shahid  0 Feb 20 18:54 file
-rw-rw-r--. 1 shahid  shahid  0 Feb 20 18:53 file1
-rw-rw-r--. 1 tim     tim     0 Feb 20 18:53 file2
-rw-r--r--. 1 michael michael 0 Feb 20 18:57 file3
-rw-r--r--. 1 root    root    0 Feb 20 19:00 testfile
[root@shahid security]#

user rob owns file but file has group permission for shahid. file1 has owner shahid and also has group permission of shahid. file2 has owner tim and group tim
file3 owner and group is michael. testfile has root user and root group permission

Displaying user accounts

You can use the following command to display all local user accounts.

[root@shahid security]# cut -d: -f1 /etc/passwd | column
root                    abrt                    gdm
bin                     tss                     gnome-initial-setup
daemon                  unbound                 avahi
adm                     colord                  postfix
lp                      usbmuxd                 sshd
sync                    libstoragemgmt          tcpdump
shutdown                saslauth                shahid
halt                    rpc                     john
mail                    rtkit                   michael
operator                radvd                   alex
games                   rpcuser                 khalid
ftp                     nfsnobody               david
nobody                  ntp                     rob
avahi-autoipd           qemu                    tim
dbus                    chrony
polkitd                 pulse
[root@shahid security]#

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chgrp

To change the group owner of a file use the chgrp command.

[root@shahid security]# ls -lh
total 0
-rw-rw-r--. 1 rob     shahid  0 Feb 20 18:54 file
-rw-rw-r--. 1 shahid  shahid  0 Feb 20 18:53 file1
-rw-rw-r--. 1 tim     tim     0 Feb 20 18:53 file2
-rw-r--r--. 1 michael michael 0 Feb 20 18:57 file3
-rw-r--r--. 1 root    root    0 Feb 20 19:00 testfile
[root@shahid security]# chgrp rob file
[root@shahid security]# ls -lh
total 0
-rw-rw-r--. 1 rob     rob     0 Feb 20 18:54 file
-rw-rw-r--. 1 shahid  shahid  0 Feb 20 18:53 file1
-rw-rw-r--. 1 tim     tim     0 Feb 20 18:53 file2
-rw-r--r--. 1 michael michael 0 Feb 20 18:57 file3
-rw-r--r--. 1 root    root    0 Feb 20 19:00 testfile
[root@shahid security]#

root@rhel65:/home/paul/owners# ls -l file2
-rw-r–r–. 1 root tennis 185 Apr 8 18:46 file2
root@rhel65:/home/paul/owners# chgrp snooker file2
root@rhel65:/home/paul/owners# ls -l file2
-rw-r–r–. 1 root snooker 185 Apr 8 18:46 file2
root@rhel65:/home/paul/owners#

chown

To change owner of a file you can use chown command.

[root@shahid security]# ls -l file1
-rw-rw-r--. 1 shahid shahid 0 Feb 20 18:53 file1
[root@shahid security]# ls -l file1
-rw-rw-r--. 1 rob shahid 0 Feb 20 18:53 file1

To change both user and group for a file use below method

[root@shahid security]# chown rob:rob file1
[root@shahid security]# ls -l file1
-rw-rw-r--. 1 rob rob 0 Feb 20 18:53 file1
[root@shahid security]# chown rob:shahid file1

list of special files

When you execute command ls -l, for each file you see ten characters before the user and group owner.
The first character represents the type of file.hyphen (-) represents normal regular file,d means file is direectory, symbolic
links are represented with letter l, pipes get a p, character devices gets c, block devices represented by b, and sockets are shown as
an s.
Unix special files
first character file type
– normal file
d directory
l symbolic link
p named pipe
b block device
c character device
s socket
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Below comamnd will display a character device (the console)

[root@shahid security]#  ls -ld /dev/console
crw-------. 1 root root 5, 1 Feb 19 11:44 /dev/console
[root@shahid security]#

Below comamnd will display a block device (hard disk)

[root@shahid security]#  ls -ld /dev/sda
brw-rw----. 1 root disk 8, 0 Feb 19 11:44 /dev/sda
[root@shahid security]#

[root@shahid security]#

I created a sampledirectory and below command will display its directory and will see first letter as d

mkdir sampledirectory
[root@shahid security]# ls -ld sampledirectory/
drwxr-xr-x. 2 root root 6 Feb 20 20:37 sampledirectory/
[root@shahid security]#

In below example you will see normal files have – as first character (file1, file2,file3, file)

[root@shahid security]# ls -lh
total 0
-rw-rw-r--. 1 rob     rob     0 Feb 20 18:54 file
-rw-rw-r--. 1 rob     shahid  0 Feb 20 18:53 file1
-rw-rw-r--. 1 tim     tim     0 Feb 20 18:53 file2
-rw-r--r--. 1 michael michael 0 Feb 20 18:57 file3
drwxr-xr-x. 2 root    root    6 Feb 20 20:37 sampledirectory

below command show symbolica link my_test_link represented by letter l

[root@shahid security]# ls -lh
total 0
-rw-rw-r--. 1 rob     rob      0 Feb 20 18:54 file
-rw-rw-r--. 1 rob     shahid   0 Feb 20 18:53 file1
-rw-rw-r--. 1 tim     tim      0 Feb 20 18:53 file2
-rw-r--r--. 1 michael michael  0 Feb 20 18:57 file3
lrwxrwxrwx. 1 root    root    15 Feb 20 20:43 my_test_link -> sampledirectory
drwxr-xr-x. 2 root    root     6 Feb 20 20:37 sampledirectory

This is end of this tutorial in next tutorial we will discuss about next 9 characters , their meanings and how they can be represented by bits.

In this tutorial we have covered basics of chgrp and chown commands and its usage. I hope you have enjoyed tutorial.

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