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Linux uses the file system user id (fsuid) instead of the effective user ID (euid), and the file system group ID (fsgid) instead of the effective group ID (egid) in permission checks.

Is Linux the only operating system in the Unix-like/Unix category of operating systems that uses the fsuid and the fsgid? for example, do operating systems like BSD, macOS, Solaris use the fsuid and the fsgid?

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All mentions of the fsuid I can find seem to say "Linux only" or something to that effect.

The man page setfsuid(2):

setfsuid() is Linux-specific and should not be used in programs intended to be portable.

This comparison on UID setting functions of different Unixes:

setfsuid(uid) Sets the process's filesystem UID to a new value. [...] Linux only.

As noted by the man page, the fsuid was only used to work around a potential issue with (userspace) NFS servers and the signal handling rules in old versions of Linux; it's not likely to be used much any more.

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