Linux File Permissions
If you run the command
$ ls -l
you will get a list of files like below
-rw-r--r-- 1 dev dev
1892 Jul 10 18:30 linux.txt
Here first part is the file permission symbol, 1 is the no
of links of the file or in simple words showing there is one file linking to
linux.txt. The file belongs to user dev and group dev or in simple word dev
user and dev group owns the file. 1982 is the file size, after that the date
and time the file has created. Last one is the file name.
Each file and directory has three user based permission
groups:
Owner – It apply only the owner of the file or directory.
Group - The Group permissions apply only to the group that
has been assigned to the file or directory.
All other users - The All other Users permissions apply to
all other users on the system, this is the permission group that you want to
watch the most.
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| File permissions in Linux |
File Permission Types:
Each file or directory has three basic permission types:
Read – You can read the contents of the file.
Write – You can write or modify a file or directory.
Execute – You can execute a file and for directory you can
view the contents of that directory.
Now let’s see the first part of the ls –l output, it is
-rw-r--r--
The first char shows the file type in the example –
indicates it is a file.
Following three characters are for user or owner permission
(rwx).
Next three characters are for group permission (rwx).
The next three characters are for all other users permission
(rwx).
Here r indicates for Read permission, w indicates Write
permission and x indicates Execute permission.
If the r, w or x characters are there it means permission is
on or you have that specific permission, if there is – in place of any character then shows it doesn’t have that
specific permission.
For example
-rw-r--r-- says the file have read and write permission
to owner, read permission to group and read permission to all others.
Modifying (Changing) the Permissions:
The permissions can be modified by using the chmod command.
Permissions can be assigned explicitly or by using a binary reference.
Explicitly Defining Permissions:
For explicitly defining we use Permission Groups that are
u - Owner
g - Group
o or a – All other Users
And Permission Types that are
r - Read
w - Write
x - Execute
To add specific permission use + (plus) sign and to remove
use – (minus) sign.
Example
To add the write permission to other user on linux.txt file
Chmod a+w linux.txt
To remove the same permission
Chmod a-w linux.txt
To remove both read write permission we can use single
command
Chmod a+rw linux.txt
Binary References permissions:
Binary references permission can done by entering three
integers or numbers.
The first number represents the Owner permission, the second
represents the Group permissions and the last number represents the permissions
for all other users. The numbers are a binary representation of the rwx string.
r = 4
w = 2
x = 1
You add the numbers to get the integer representing the
permissions. You will need to include the binary permissions for each of the
three permission groups.
Example
To set a permission –rwxrw-r—on file linux.txt use command
Chmod 764 linux.txt
Here 7 used for user for read write and execute permission,
6 for group for read and write permission and last 4 for read permission to
others.
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