- umask
Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, merely returns the current umask.
The Unix permission
rwxr-x---is represented as three sets of three bits, or three octal digits:0750(the leading 0 indicates octal and isn't one of the digits). Theumaskvalue is such a number representing disabled permissions bits. The permission (or "mode") values you passmkdirorsysopenare modified by your umask, so even if you tellsysopento create a file with permissions0777, if your umask is0022then the file will actually be created with permissions0755. If yourumaskwere0027(group can't write; others can't read, write, or execute), then passingsysopen0666would create a file with mode0640(0666 &~ 027is0640).Here's some advice: supply a creation mode of
0666for regular files (insysopen) and one of0777for directories (inmkdir) and executable files. This gives users the freedom of choice: if they want protected files, they might choose process umasks of022,027, or even the particularly antisocial mask of077. Programs should rarely if ever make policy decisions better left to the user. The exception to this is when writing files that should be kept private: mail files, web browser cookies, .rhosts files, and so on.If umask(2) is not implemented on your system and you are trying to restrict access for yourself (i.e., (EXPR & 0700) > 0), produces a fatal error at run time. If umask(2) is not implemented and you are not trying to restrict access for yourself, returns
undef.Remember that a umask is a number, usually given in octal; it is not a string of octal digits. See also "oct", if all you have is a string.

