Programming Perl, 4th Edition

umask

umask EXPR
umask

This function sets the umask for the process and returns the old one using the umask(2) syscall. Your umask tells the operating system which permission bits to disallow when creating a new file, including files that happen to be directories. If EXPR is omitted, the function merely returns the current umask. For example, to ensure that the “user” bits are allowed and the “other” bits disallowed, try something like:

umask((umask() & 077) | 7);   # don't change the group bits

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. Remember also that the umask’s bits are complemented compared to ordinary permissions.

The Unix permission rwxr–x––– is represented as three sets of three bits, or three octal digits: 0750 (the leading 0 indicates octal and doesn’t count as one of the three digits). Since the umask’s bits are flipped, it represents disabled permissions bits. The permission (or “mode”) values you supply to 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, the file is created with permissions 0755. If your umask were 0027 (group can’t write; others can’t read, write, or execute), then passing ...