chmod [options] mode[,mode] file1 [file2 ...]
change the permission for one or several files

Examples
Add execute-by-user permission to file:
chmod u+x file

Either of the following will assign read/write/execute permission by owner (7), read/execute permission by group (5), and execute-only permission by others (1) to file:

chmod 751 file chmod u=rwx,g=rx,o=x file

Any one of the following will assign read-only permission to file for everyone:

chmod =r file chmod 444 file chmod a-wx,a+r file

The following makes the executable setuid, assigns read/write/execute permission by owner, and assigns read/execute permission by group and others:

chmod 4755 file

chmod 777 it can be read/write and execute

Permissions

r Read.

w Write.

x Execute.

s Set user (or group) ID.

t Sticky bit; used on directories to prevent removal of files by non-owners.

u User’s present permission.

g Group’s present permission.

o Other’s present permission.

Options

-c, –changes

Print information about files that are changed.

-f, –silent, –quiet
Do not notify user of files that chmod cannot change.

–help
Print help message and then exit.

-R, –recursive
Traverse subdirectories recursively, applying changes.

–reference=filename
Change permissions to match those associated with filename.

-v, –verbose
Print information about each file, whether changed or not.

–version
Print version information and then exit.

users
(the fist letter)
u ser
g roupe
o ther
a ll (default)

Opcode
+ for Add permission.
- for remove permission
= for Assign permission (and remove permission of the unspecified fields).