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). The umask value is such a number representing disabled permissions bits. The permission (or "mode") values you pass mkdir or sysopen are modified by your umask, so even if you tell sysopen to create a file with permissions
0777, if your umask is
0022, then the file will actually be created with permissions
0755. If your umask were
0027(group can't write; others can't read, write, or execute), then passing sysopen
0666would create a file with mode
0666 &~ 027is
Here's some advice: supply a creation mode of
0666for regular files (in sysopen) and one of
0777for directories (in mkdir) and executable files. This gives users the freedom of choice: if they want protected files, they might choose process umasks of
027, or even the particularly antisocial mask of
077. 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), raises an exception. 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.
Portability issues: umask in perlport.