Linux File Permissions

File permissions are assigned to:
1. the owner of a file
2. the members of the group the file is assigned to
3. all other users
4. Permissions under Linux are configured for each file and directory.

There are three levels of permissions:
1. The permissions that apply to the owner of the file. The owner of a file is by default the user that created the file1.
2. The permissions that apply to all members of the group that is
associated with the file.
3. The permissions that apply to all other users on the system.
4. Permissions can only be changed by the owner, and root of course.

For a file, these permissions mean the following:

• read allow the user to read the contents of the file, for instance with cat or less.
• write allow the user to modify the contents of the file,for instance with vi.
• execute allow the user to execute the file as a program, provided that the file is indeed an executable program (such as a shell script).

For a directory, these permissions have a slightly different meaning:

• read allow the user to view the contents of the directory, for instance with ls.
• write allow the user to modify the contents of the directory. In other words: allow the user to create and delete files, and to modify the names of the files. Note: Having write permissions on a directory thus allows you to delete files, even if you have no write permissions on that file!
• execute allow the user to use this directory as its current working directory. In other words: allow the user to cd into it.

r - read
w - write
x – execute
• u for the owner (user) of the file
• g for the group assigned to the file
• o for all other users
• a for all (owner+group+others)

<operator> can be:
• + to add permissions
• - to delete permissions
• = to clear all permissions and set to the permissions specified

Symbolic way
$ useradd sachin
$ passwd sachin
$ useradd dhoni
$ passwd dhoni
$ groupadd market;usermod –G market dhoni
$ useradd shewag
$ passwd shewag
$ groupadd market;usermod –G market shewag
$ mkdir /opt/perm/;touch /opt/perm/file{1..6}
$ mkdir /opt/perm/{data1,data2}
$ cd /opt/perm
$ ll –d data1
drwxr-xr-x 2 root root 4096 Jul 29 20:15 data1
$ chown sachin data1
$ ll –d data1
$ chgrp market data1
$ ll –d data1
$ chmod u-w data1
$ ll –d data1
$ chmod g+w data1
$ ll –d data1
$ chmod o+w,o-rx data1
$ ll –d data1
$ ll –d data2
drwxr-xr-x 2 root root 4096 Jul 29 20:15 data2
$ chown –Rv sachin.market data2
$ ll –d data2
$ chmod u-rwx data2
$ ll –d data2
$ chmod g+w,g-x data2
$ ll –d data2
$ chmod –Rv o+w,o-r data2
$ ll –d data2
Octal way
$ ll file1
-rw-r--r—- 1 root root 0 Jul 29 20:15 file1
$ chmod 777 file1
$ ll file1
$ chmod 666 file2
$ ll file1
$ chmod 467 file3
$ ll file1
$ chmod 541 file4
$ ll file1
$ chmod 724 file5
$ ll file1
$ chmod 000 file6
$ chmod 0 file6

This table shows what numeric values mean:
Octal
Digit Text Binary            Meaning
-----------------------------------------------------

  0   ---  000    All types of access are denied
  1   --x  001    Execute access is allowed only
  2   -w-  010    Write access is allowed only
  3   -wx  011    Write and execute access are allowed
  4   r--  100    Read access is allowed only
  5   r-x  101    Read and execute access are allowed
  6   rw-  110    Read and write access are allowed
  7   rwx  111    Everything is allowed 

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