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Spacen Jasset
Asked: 2011-07-03 16:05:59 +0800 CST

Why is umask a AND NOT mask operation

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Why is umask a $perms AND NOT $mask and not simply $perms AND $mask instead?

In other words why is there a NOT in there; why doesn't wasn't it implemented as an and mask?

As an example, 666 and 700, to give 600?

unix umask

2 Answers

  1. Best Answer

    The key is that the umask ("user mask") is intended to keep processes from creating files with permission bits they're not supposed to use. If you look at it from that perspective, the concept of a umask may make more sense; in particular, the common umask of 022 prevents processes from creating files that are writable by group or world, which is usually what you want.

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  2. The input value is octal which may require a leading 0 to be parsed correctly. As the owner mask is almost always 0 the mask can be entered with three digits. As an AND mask, it would need to be entered as 0755 or 0750.

    Defining it as an AND NOT operation makes it safer if characters are missed in the mask. If umask was and AND operation umask 5 would allow only limited world access and umask 0 would be even worse. umasq 75 parsed as a decimal value would be be effectively umask 113 which would not allow read acces.

    EDIT: From another perspective, the mask is a list of the bits you want masked off. Therefore you want masks like 026 rather than 0751. Internal representation may well be 07751 or what ever is appropriate for ANDing. If not, the conversion is relatively trivial, and in the grand scheme of things the mask doesn't get applied that often.

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