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What are the use cases for write-only directory permissions (user, group or world)?

I've only ever seen world write-only permission, and that was for dropboxes/public folders on ftp servers. (I guess the same might go for group, on a network, if the group was large. But I've never seen that.)

It just occurred to me in all my years using Linux I'd never seen user- or group- write-only directory permissions.

EDIT: since -w is extremely rare, each pair of rw bits really only convey 1.58 bits of information; hence the 6 bits for ugw-rw really only need 4.75 bits. Come to think of it, it's rare to have execute permission on a file without r permission. So, 3 of the 8 encodings for rwx are nearly never used, and hence the 3 rwx bits contain ~ 2.32 bits of information.

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The "dropbox" is the only use case for write-only directories I'm aware of. I've seen it used for homework/project submission scripts, where it's write-only for group (e.g., "cs101") and other.

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I suspect backup drop directories could be another useful situation which falls into basically the same category. – Michael Kjörling Sep 5 '14 at 13:02

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