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APPENDIX B - Setting File Permissions
 
The UNIX operating system stores a set of permissions with every file. These permissions help to keep files secure on a multiuser system by determining who can access a file or a directory, who can change a file, and who can execute a program.

There are three types of permissions: USER, GROUP and OTHERS in UNIX files. Each permission consists of three numbers - 4 for read, 2 for write, and 1 for execute access. By adding these numbers, the permissions make up one digit. For example, 4 + 2 + 1 = 7 grants read, write, and execute right while 4 + 1 = 5 grants read and execute right.

The following command makes the file filename readable, writable, and executable by the owner (USER) of the file while making it only readable and executable by the group you are (GROUP) and the world (OTHERS) .

    chmod 755  filename

The first digit corresponds to the access right of user, the second digit is that of group, and the final digit is that of others. In the above example, (4 + 2 + 1 = 7)=USER, (4 + 1 = 5) =GROUP, (4 + 1 = 5) = OTHERS.

The following are a quick reference of file permissions :

 PERMISSION  COMMAND
  U   G   O
 rwx rwx rwx chmod 777 filename
 rwx r-x r-x chmod 755 filename
 rw- r-- r-- chmod 644 filename

 U = User
 G = Group
 O = Others

 r = Read
 w = write
 x = execute
 - = no permission

Note: Please set the file permissions correctly. It is the main reason why the program cannot run.


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