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File Permissions with Linux

There are three types of access restrictions:

Permission     Action     chmod option
=======================================
read           (view)      r or 4
write          (edit)      w or 2
execute        (execute)   x or 1

There are also three types of user restrictions:

User     ls output
===================
owner    -rwx------
group    ----rwx---
other    -------rwx

Folder/Directory Permissions

Permission     Action                              chmod option
===============================================================
read           (view contents: i.e., ls command)   r or 4
write          (create or remove files from dir)   w or 2
execute        (cd into directory)                 x or 1

Numeric notation

Another method for representing Linux permissions is an octal notation as shown by stat -c %a. This notation consists of at least three digits. Each of the three rightmost digits represents a different component of the permissions: owner, group, and others.

Each of these digits is the sum of its component bits in the binary numeral system:

Symbolic Notation     Octal Notation     English
============================================================
----------            0000               no permissions
---x--x--x            0111               execute
--w--w--w-            0222               write
--wx-wx-wx            0333               write & execute
-r--r--r--            0444               read
-r-xr-xr-x            0555               read & execute
-rw-rw-rw-            0666               read & write
-rwxrwxrwx            0777               read, write & execute

Now, what does 755 mean?

7=rwx 5=r-x 5=r-x

This means that the directory has the default permissions -rwxr-xr-x (represented in octal notation as 0755).

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