Linux: Chmod and Chown

This is a short introduction to basic Linux file permissions and how to set them correctly. As a beginner, junior or experienced developer and/or administrator, you are going to need this!

Chmod: Change File Mode Bits

15:45:18 user@example temporary 
total 12K
drwxr-xr-x  2 test users 4.0K Feb  1 14:29 .
drwx------ 33 test users 4.0K Feb  3 15:45 ..
-rw-r--r--  1 test users    0 Jan 21 12:14 1
-rw-r--r--  1 test users    0 Jan 21 12:14 3
-rw-r--r--  1 test users   16 Feb  1 14:29 file.txt

In the above example output from left to right we have: filemode, number of links, user, group, size, date last modified, name of file.

We will focus on the filemode for now.

drwxr-xr-x
0123456789 <-- this is our reference marker.

0: is either - meaning a file, or d signifying that it is a directory.

1-3: represents the user or owner of the file.

4-6: is reference to the group that owns the file.

7-9: everybody else, or other.

Change the filemodes:

The command chmod is used to change the mode files of files. Its basic syntax is as follows:

In chmod u+x file.txt the u signifies user and the + means add and the x refers to execute. This can be stacked, and used to takeaway permissions.

chmod -R g+rwx directory would recursively (-R) change all group files within directory to read, write and execute.

Importantly, if the user or group does not own the file or directory then the command sudo must be used. Otherwise it is not needed.

Octal:

In the above examples we used rwx to note what permissions we wanted to add or remove from a file. Another method is to use the octal notation; numeral.

chmod 777 file.txt would mean that we are giving rwx to user, group and other. How does 7 equal rwx?

This is because 4 gives read access, 2 is write and 1, execute. When using octal we add the numbers together, so if we wanted read, write and execute we simply add 4,2,1 which equals 7.

chmod 765 xxx.txt would mean: - user: read, write and execute -group: read and write - other: read and execute

Using this is very simple, but how do I remove permissions? Previously, we would chmod u-x i.e. we used the - to signify removal of privilege. Using octal we just set new filemode to what we want and it will add or subtract the mode accordingly. An example below:

chmod a+rwx == give read, write and execute to all (user,group a)nd other) or 777.

But if we only wanted the user to have rwx and everyone else read and write we could call: chmod 755. To use non-octal here would be chmod go-x.

Important note: As with all things linux there are many more advanced features. We are just touching the surface here.

Chown: Change File Owner and Group

The chown command deals with changing the ownership of files and directories.

drwxrwxr-x  1 test users 14.0K Jan 21 12:14 dir_1

-rw-r--r--  1   admin   root   16 Feb  1 14:29 file.txt
[filemode]     [owner] [grp]  

Above we have a break down of the important parts of our ls -la output.

In the faked output we have admin in the first position which is representing the owner/ creator of file.txt. In postilion two we have the group that file.txt belongs to, in this case its root.

The owner of a file can make changes to the filemode and ownership of a file without super user privileges. However, if another user wanted to change the ownership they would require this access.

To make a change is as simple as chown [user]:[group] [file/s].

$ sudo chown root file.txt # 1.
$ ls -l 
-rw-r--r--  1 root root   16 Feb  1 14:29 file.txt

The simplest usage of chown is the command plus the new owner and file or directory to be affected.

$ sudo chown -R admin dir_1 # 2.
$ ls -l
drwxrwxr-x  1 admin users 14.0K Jan 21 12:14 dir_1

If we wanted to make the changes to all files and directories inside a directory we can add -R. This is a recursive function with the same syntax as chmod.

$ sudo chown admin:admin file.txt 
$ ls -l
-rw-r--r--  1 admin admin   16 Feb  1 14:29 file.txt

To specify a new owner and group the use of : between the owner and group can be used. admin:users, root:root and so on.