I'd like to know what the maximum username length is for current GNU/Linux systems, e.g. Ubuntu 11.04.

8 characters appears to be some historical standard, but I've already noticed on my current Ubuntu system that this limit does not apply.


The current limit is 32 characters (according to useradd man page).

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    I think Dr. Edward Morbius's answer is better, because the useradd limit is not the only limit that matters. – Edward Ned Harvey Jan 9 '19 at 12:18

The answer varies somewhat.

useradd(1) references a limit of 32 characters. This is based AFAIU on libc6.

Some utilities or systems may impose shorter names or behave inconsistently when presented with longer names, including top, ps, w/who, finger, NFS, and various multi-platform directory systems (NIS/NIS+, SMB, CIFS, Kerberos), potentially based on limitations of other/remote platforms. Many of the various psutil commands will display a UID rather than username if the latter exceeds 8 characters.

Some utilities and applications may impose their own arbitrary limitations. E.g.: IBM's DB2 apparently won't allow logins from users with usernames exceeding 8 characters: http://database.ittoolbox.com/groups/technical-functional/db2-l/length-of-username-permitted-on-db2-95-aix-6-3248147

8 characters is a generally sane limit, and saves typing.

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    Thank you for this answer. Just one note: If you use linux users for isolated computing environments, 8 characters is often too small. At least if you want the names to be readable. – guettli Mar 20 '14 at 12:19

As other answers have explained, longer usernames are possible, but another practical reason to try to limit to 8 chars maximum is that ps(1) reports numeric uids instead of usernames beyond 8 chars.

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