Acronym for User ID.
The numerical id that corresponds to a user. For example 0 is usually root. bobs UID might be 500. UID to name mappings are stored in /etc/passwd (see passwd(5))
A program is said to be "setuid" when the file has the "+s" user permission bit set, so the program will run as the permissions of the user that owned the file, not the user that ran the file.
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