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On Unix-like systems, users are represented by a user identifier, often abbreviated UID. The range of values for a UID varies amongst different systems; at the very least, a UID can be between 0 and 32767, with some restrictions:
- The Superuser must always have UID 0.
- On systems where the highest possible UID is greater than 32767 the Nobody user may either be assigned that maximum value or 32767 for compatibility. On systems for which 32767 is the highest possible UID, it is usually assigned to Nobody.
- UIDs from 1 to 100 are otherwise reserved for system use by convention; some manuals recommend that UIDs from 101 to 499 be reserved as well.
The UID value references users in the /etc/passwd file. Shadow password files and Network Information Service also refer to numeric UIDs. The user identifier is a necessary component of Unix file systems and processes.
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