In Unix-like operating systems, the chmod command sets the permissions of files or directories. On Unix-like operating systems, a set of flags associated with each file determines who can access that file, and how they can access it. These flags are called file permissions or modes, as in "mode of access." The command name chmod stands for "change mode." It restricts the way a file can be accessed.
In general, chmod command take the form: chmod options permissions file name
If no options are specified, chmod modifies the permissions of the file specified by file name to the permissions specified by permissions. Permissions defines the permissions for the owner of the file (the "user"), members of the group who owns the file (the "group"), and anyone else ("others"). There are two ways to represent these permissions: with symbols (alphanumeric characters), or with octal numbers (the digits 0 through 7).
Syntax :
chmod [reference][operator][mode] file...
Example: chmod u=rwx,g=rx,o=r myfile
The letters u, g, and o stand for "user", "group", and "other". The equals sign ("=") means "set the permissions exactly like this," and the letters "r", "w", and "x" stand for "read", "write", and "execute", respectively. The commas separate the different classes of permissions, and there are no spaces in between them.
The equivalent command using octal permissions notation:
chmod 754 myfile
Here the digits 7, 5, and 4 each individually represent the permissions for the user, group, and others, in that order. Each digit is a combination of the numbers 4, 2, 1, and 0:
4 stands for "read",
2 stands for "write",
1 stands for "execute", and
0 stands for "no permission."