Linux: File Permissions

Linux’s robust and secure nature is attributed to its sophisticated file permission system. This article serves as a comprehensive guide to Linux file permissions, aimed at beginners and experienced users alike. It covers the core concepts, terminology, and syntax of file permissions, providing step-by-step instructions and troubleshooting tips.

The guide explores the three permission categories: user, group, and others, along with their access levels. It also explains symbolic and numerical representations of permissions, including special permissions like setuid and setgid.

Practical examples and scenarios are included to facilitate practical application.

By the end, readers will have the knowledge and confidence to effectively manage file permissions, ensuring system integrity and data security in the Linux environment.

Steps

Navigate to your working directory(folder) or create a new one and navigate to it using the following command

mkdir file_permissions && cd file_permissions

Create a directory by the name sample_folder and a file called sample_file inside the file_permissions directory.. Use the following command

mkdir sample_folder && touch sample_file

You can verify that the two items are created by listing the contents of the folder using the following command

ls

Your output should be as follows

~$ ls
  sample_folder   sample_file

You are now set up to start learning about file permissions. Lets get started.

Lets start by executing our sample_file from the terminal. Run the following command on the terminal

./sample_file
Auto (Bash)./sample_file

You should get an error similar to the one shown below

bash: ./sample_file: Permission denied

Bash is the default shell on most linux versions.

You may be wondering why you are denied the permission to execute the file yet you created it yourself

Lets take a step back

Lets list the long version of the contents in the file_permissions directory. We use the following command to accomplish this

ls -l

The output should look as shown below

~$ ls -l
  total 4
  -rw-rw-r-- 1 sam sam    0 Jun  3 20:59 sample_file
  drwxrwxr-x 2 sam sam 4096 Jun  3 20:59 sample_folder

In the above output, we can see the permissions present on the file and folder, on the far right we can see the names of the file and folder.

The permissions on the file are represented as -rw-rw-r —- and the permissions on the folder are represented as drwxrwxr-x . What do these letters and dasshes mean? Lets explore their meaning

- represents absence of permission
r represents the read permission
w represents the write permission
x represents the execuution permission
d represents a directory

A — at the beginning of a permission represents a permission for a file

There are three groups of permissions for both files and directories:

1. User permissions
2. Group permissions and,
3. Other permissions

See the below image for more clarification of the groupings

Now with this understanding, we can investigate why it was not possible for us to execute our file.

After listing the long version of the contents of the file_permissions directory, we can see that our sample_file has the following output

-rw-rw-r-- 1 sam sam    0 Jun  3 20:59 sample_file

This indicates that it does not have execution permission on either user, group or other.

We can change the mode of the file to add the execution permission

We accomplish that using the following command

chmod +x sample_file

If we list the long version contents of the file_permissions directory, we can see that the sample_file now has the execution permission

~$ ls -l
  -rwxrwxr-x 1 sam sam    0 Jun  3 20:59 sample_file

This means that we can now execute this file without facing any issues. lets verify this. Use the following command

./sample_file

There is no output on the terminal implying that this empty file was executed successfully.

We can also remove permissions from a file using the — sign. Lets remove the execution permission for the group for our sample_file .

chmod g-x sample_file 

g represents the group

If we list the long version of the contents of the file_permissions folder, we can see that the group for sample_file no longer has the execution permissions. See output below

-rwxrw-r-x 1 sam sam    0 Jun  3 20:59 sample_file

We can also remove the read and execution permissions from the other as shown below

chmod o-rx sample_file

o represents other

Our output should now look as shown below once we list the contents of the file_permissions folder

-rwxrw---- 1 sam sam    0 Jun  3 20:59 sample_file

We no longer have read and execute permissions on other.

You can play around with the permissions for the different groups and see the outcome.

The read, write and execution permissions can also be represented numerically as follows

4 read permission for a specific group
2 write permission for a specific group
1 execute permission for a specific group

This implies that in order to have read, write and execute permissions on a specific group, you add the three numerical numbers 4 + 2 + 1 which will give you 7 .

6 represents both read(4) and write(2) permissions for a specific group. i.e 4 + 2

3 represents write(2) and execute(1) permissions for a specific file. i.e 2 + 1

We can change the permissions for our sample_file and give only the user read, write and execute permissions and remove all permissions from the rest of the files as shown below

chmod 700 sample_file

If we list the long version of the output, we can see that then permissions have been altered as shown below

-rwx------ 1 sam sam    0 Jun  3 20:59 sample_file

Congratulations!

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