Linux file permissions in a nutshell

Every file and directory in linux have their own permissions. They decide what a user can do with them, inform the same to system.

In a nutshell, a file/directory lets a user do either of:
read (r), write (w) and execute (x).

For files:

• read -- user can read contents of file.
• write -- user can edit/write to file. Also, this lets user delete the file.
• execute -- user can execute the file as a script (something like bash command)

For directories:

• read -- user can read contents of directory.
• write -- user can create a new file in the directory or rename the file or delete the file. Also, this lets user delete the directory.
• execute -- user can cd to directory. Yes, user can't do it with only read permission.

Now, how do file/folder specify which users can access what?

Answer lies in users(u), groups (g) and others (o)

• User - The owner of file/directory.
• Group - All users added to these groups (yes, they can be multiple.) can do what they're permitted to do.
  Groups essentially determine how multiple users can access a file/directory.
• Other - Those who have nothing specifically assigned.

Example walkthrough

To look for permissions of all files in a folder, run the command ls -la <folder_path>

Let's have a peek through of /var/www/ on the server where this website is hosted.

$ ls -al /var/www/
drwxr-xr-x  6 root     root     4096 Jun  1 22:01 .
drwxr-xr-x 13 root     root     4096 Apr 16  2015 ..
drwxr-xr-x 10 ghost    ghost    4096 Sep 19 00:35 ghost
drwxr-xr-x  3 srujan   srujan   4096 Sep 23 06:52 html

• drwxr-xr-x translates to
  • d -- directory
  • "rwx" -- owner can read, write, execute.
  • "r-x" -- Group can read, can't write, execute.
  • "r-x" -- Others can read, can't write, execute.
• 6, 13, 10, 3 are respective number of hardlinks.
• root root/ ghost ghost / srujan srujan says owner group
  By default, user who creates the file owns it.
• 4096 -- size of file/directory in bytes
• The rest are timestamps.
• Filenames:
  • . -- Current directory.
  • .. -- Parent directory.
  • ghost -- Child directory.
  • html -- Another child directory

And other example where a file is present. Notice that d is not present in the rules.

$ ls -al /var/www/html/
total 16
drwxr-xr-x 3 srujan srujan   4096 Sep 23 06:52 .
drwxr-xr-x 6 root   root     4096 Jun  1 22:01 ..
-rw-rw-r-- 1 srujan srujan     94 Sep 23 06:52 404.html

How to change permissions

Go through the manual of command chmod for understanding how to achieve desired permissions to a file/directory.