File permissions in Linux

Question 01:

While connecting to a remote server using SSH, the first step is to change the permissions of the key-pair file using the command :

chmod 400 Instance2.pem 

Here, chmod command is used to change permissions of the specified file. But what's the use of 400?

Question 02:

When you run ls -la it lists all directories and files present in the root, but what's with the drwxr-xr-x@ and -rw-r--r--@?

...and the solution is (drumroll) File system in Linux.

Each directory/file in Linux has associated permissions for users. Users are categorized as follows :

1. owner : The owner of the file/dir.
2. group : The group that owns the file/dir.
3. others : Anyone except the owner and group.

Each type of user has three permissions for any file/directory :

1. read(r)
2. write(w)
3. execute(x)

Okay! But what does drwxr-xr-x@ or -rw-r--r--@ mean?

As there are 3 permissions for 3 types of users, we can allocate 3 cells to each users. Each cell can have a value 'r', 'w', 'x' or '-' if the user has no permissions.

Note that for each user the sequence is same rwx.

Then, for each cell we can store 1 if the user has that permission else 0. Thus we convert the chars to binary system.

In 3 bits, we can store values from 0-7 in decimal system, and for three types of user it turns combinations of three values that represent permissions.

As shown in the figure above:

1. owner: permissions rwx which in binary system is represented by 111 and in decimal system is 7.
2. group: permissions r-x which in binary system is represented by 101 and in decimal system is 5.
3. others: permissions r-x which in binary system is represented by 101 and in decimal system is 5.

Combining the result for all types of users gives the three-digit number 755.

Thus, all combinations for permissions can be signified with a 3-digit number instead of drwxr-xr-x@.

Combining back to Question 01, can you find out which permissions does a file have with code 400?

Observe that, code 400, implies that only owner has read access to the file. It's significance is that, the Instance2.pem contains SHH key-pair which should be stored securely, hence it has only read access.

It's amazing, how much of thought must have went into designing a file system that supports multiple users using the same machine simultaneously with multiple file permissions. And the complexity is reduced to three digit numbers. It's secure, flexible and user-friendly.

Okay! Great! But you don't know what chmod command is used for, right? :p