In Unix and Unix-like operating systems, chmod is the command and system call which is used to change the access permissions of file system objects files and directories. It is also used to change special mode flags. It also may refer to a function that sets the mask, or it may refer to the mask itself, which is formally known as the file mode creation mask. The mask is a grouping of bits, each of which restricts how its corresponding permission is set for newly created files. The bits in the mask may be changed by invoking the umask command