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Showing content from https://www.geeksforgeeks.org/linux-unix/set-file-permissions-linux/ below:

How to Set File Permissions in Linux

Linux file permissions form the foundation of the system’s security model. They define who can read, write, or execute files and directories, ensuring only authorized users or processes can access sensitive data. You can modify these permissions using the chmod command.

• chmod +rwx filename – Adds read, write, and execute permissions.
• chmod -rwx directoryname – Removes all permissions.
• chmod +x filename – Grants executable permission.
• chmod -wx filename – Removes write and execute rights.

Every file or directory has three types of permissions:

• Read (r): View the file’s contents or list a directory’s files.
• Write (w): Modify a file or add/delete files in a directory.
• Execute (x): Run a file as a program/script or enter a directory.

Permissions are assigned to three categories of users:

• User (Owner): The person who created the file.
• Group: Users belonging to a shared group (e.g., "developers" or "admins").
• Others: Everyone else on the system.

Note: All these permissions are being granted at three different levels based on their group.

First, you must think of those nine characters as three sets of three characters (see the box at the bottom). Each of the three "rwx" characters refers to a different operation you can perform on the file.  

1. Owners: These permissions apply exclusively to the individuals who own the files or directories.
2. Groups: Permissions can be assigned to a specific group of users, impacting only those within that particular group.
3. All Users: These permissions apply universally to all users on the system, presenting the highest security risk. Assigning permissions to all users should be done cautiously to prevent potential security vulnerabilities.

---     ---     ---
rwx     rwx     rwx
user    group   other 

Let's dive in to understand the possible methods to check all the desired details of a file including "File Permission"

Here's the command to execute it within the terminal. Let's show you with an example:

Input:

We're taking 'NarX' as a default file name:

ls -l NarX.txt

Output:

-rw-r--r-- 1 user group 46 Apr 14 16:37 NarX.txt

The above command represents these following information:

1. The first character = '-', which means it's a file 'd', which means it's a directory.
2. The next nine characters = (rw-r--r--) show the security
3. The next column shows the owner of the file.
4. The next column shows the group owner of the file. (which has special access to these files)
5. The next column shows the size of the file in bytes.
6. The next column shows the date and time the file was last modified.

The 'namei' command is used to check the file path through layer of folder's path. Here's the command to execute it within Terminal:

Here, we've taken 'path' as " root@anonymous-VirtualBox:~# " and file name as "hoops"

namei -l /path/to/your/file

Unlike 'ls -l' command, the "stat" command is used to pin point the file location. Here's how you can do it:

We're taking file name as "hoops"

stat hoops

Output:

  File: example.txt
  Size: 2210       Blocks: 8          IO Block: 4096  regular file
Device: 802h/2050d    Inode: 1288496     Links: 1
Access: 2024-11-18 10:50:56.000000000 +0000
Modify: 2024-11-18 10:50:56.000000000 +0000
Change: 2024-11-18 10:50:56.000000000 +0000
 Birth: -

The command you use to change the security permissions on files is called "chmod", which stands for "change mode" because the nine security characters are collectively called the security "mode" of the file. You can modify permissions using symbolic notation or octal notation.

Symbolic notation allows you to add, remove, or set permissions for specific users. Let's understand this using different example below:

Example 1: To Change File Permission in Linux

If you want to give "execute" permission to the world ("other") for file "xyz.txt", you will start by typing. 

chmod o

Now you would type a '+' to say that you are "adding" permission. 

chmod o+

Then you would type an 'x' to say that you are adding "execute" permission. 

chmod o+x

Finally, specify which file you are changing.  

chmod o+x xyz.txt

You can see the change in the picture below.  

You can also change multiple permissions at once. For example, if you want to take all permissions away from everyone, you would type. 

chmod ugo-rwx xyz.txt

The code above revokes all the read(r), write(w), and execute(x) permission from all user(u), group(g), and others(o) for the file xyz.txt which results in this. 

Example 2:

The code adds read(r) and write(w) permission to both user(u) and group(g) and revoke execute(x) permission from others(o) for the file abc.mp4. 

chmod ug+rw,o-x abc.mp4

Something like this: 

chmod ug=rx,o+r abc.c

Assigns read(r) and execute(x) permission to both user(u) and group(g) and add read permission to others for the file abc.c. 

There can be numerous combinations of file permissions you can invoke revoke and assign. You can try some on your Linux system. 

The octal notation is used to represent file permission in Linux by using three user group by denoting 3 digits i.e.

• user
• group
• other users

Here's how to permissions are mapped:

• Read (r) = 4
• Write (w) = 2
• Execute (x) = 1

Permissions for owner, group, and others are represented by a three-digit octal value. The sum of permissions for each group gives the corresponding number.

Reference:

chmod o

Now you would type a '+' to say that you are "adding" permission.  

chmod o+

Then you would type an 'x' to say that you are adding "execute" permission. 

chmod o+x

Finally, specify which file you are changing.  

chmod o+x xyz.txt

You can see the change in the picture below.  

You can also change multiple permissions at once. For example, if you want to take all permissions away from everyone, you would type. 

chmod ugo-rwx xyz.txt

The code above revokes all the read(r), write(w), and execute(x) permission from all user(u), group(g), and others(o) for the file xyz.txt which results in this. 

Example:

The code adds read(r) and write(w) permission to both user(u) and group(g) and revoke execute(x) permission from others(o) for the file abc.mp4. 

chmod ug+rw,o-x abc.mp4

Something like this: 

chmod ug=rx,o+r abc.c

assigns read(r) and execute(x) permission to both user(u) and group(g) and add read permission to others for the file abc.c. 

There can be numerous combinations of file permissions you can invoke revoke and assign. You can try some on your Linux system. 

You can also use octal notations like this. 

Using the octal notations table instead of 'r', 'w', and 'x'. Each digit octal notation can be used for either of the group 'u', 'g', or'o'. 

So, the following work is the same. 

chmod ugo+rwx [file_name]
chmod 777 [file_name]

Both of them provide full read write and execute permission (code=7) to all the group. 

The same is the case with this. 

chmod u=r,g=wx,o=rx [file_name]
chmod 435 [file_name]

Both the codes give read (code=4) user permission, write and execute (code=3) for the group and read and execute (code=5) for others. 

And even this... 

chmod 775 [file_name]
chmod ug+rwx,o=rx [file_name]

Both the commands give all permissions (code=7) to the user and group, read and execute (code=5) for others. 

The combination for the permissions are r,w,x, and -. Let's understand this briefly in elaborative way:

• "rw-": the first three characters `rw-`. This means that the owner of the file can "read" it (look at its contents) and "write" it (modify its contents). We cannot execute it because it is not a program but a text file. 
• "r-x": the second set of three characters "r-x". This means that the members of the group can only read and execute the files. 
• "r--": The final three characters "r--" show the permissions allowed to other users who have a UserID on this Linux system. This means anyone in our Linux world can read but cannot modify or execute the files' contents.

Besides usual methods, Linux also offers special permission types to have more complex control over files.

The SET User ID permission allows user to execute programs with the previledges of its owner. Below is the example for the same:

chmod u+s program

The Set Group ID permission allows files to run under fule's group permissions (or ensures the files created in a directory inherits the group of the directory). Here's the command for the same:

chmod g+s directoryname

This allows the user (only owner) to delete or rename files within the directory (regardless of other user's permissions). Here's a command for the same:

chmod +t directoryname

To set permissions for a specific user or group:

Use chown to change file ownsership:

chown user:group file.txt

Use chmod to modify permissions:

chmod 755 file.txt

File and Directory Permissions in Operating System (OS) | Linux Tutorial

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