1. Name

chmod [man page] - change file access permissions

2. Synopsis

chmod [OPTION]... MODE[,MODE]... FILE...
chmod [OPTION]... OCTAL-MODE FILE...
chmod [OPTION]... --reference=RFILE FILE... 

3. Frequently used options

-R, --recursive
      change files and directories recursively
-c, --changes
      like verbose but report only when a change is made
-v, --verbose
      output a diagnostic for every file processed 

4. Basics

• r : read (user can read the file, or list a given directory)
• w : write (user can write to the file, or create file or directory in the directory)
• x : execute (user can execute the file, or enter directory)

$ ls -ld dir/ 

5. Examples

5.1. Octal Mode

Let's see how we can change file.txt permissions to rwxr-xr-- with octal mode:

$ chmod 764 file.txt 

5.2. Symbolic Specification

Symbolic specification is just another way to change file and directory permissions.  In the next example we will change file.txt's permissions to rwxr-xr-- with a following symbolic specification:

$ chmod u=rwx,g=rx,o=r file.txt