What is Ubuntu's automatic UID generation behavior?

I'm interested in the case where a new user is generated and no UID is explicitly given, leaving Ubuntu to automatically assign a UID. I know that by default Ubuntu will generate a UID above 1000, but I want to know all about ubuntu's UID generation policy.

A good answer to this question will clear up the following points

• What if the following two UIDs are already used: 1001, 2001 - will the next auto-generated UID be 1002 or 2002?
• Is there a maximum UID? What does Ubuntu do if some account already has been assigned the maximum UID (but there are otherwise free UIDs)?

1 Answer

See /etc/adduser.conf:

# FIRST_SYSTEM_[GU]ID to LAST_SYSTEM_[GU]ID inclusive is the range for UIDs
# for dynamically allocated administrative and system accounts/groups.
# Please note that system software, such as the users allocated by the base-passwd
# package, may assume that UIDs less than 100 are unallocated.
FIRST_SYSTEM_UID=100
LAST_SYSTEM_UID=999
FIRST_SYSTEM_GID=100
LAST_SYSTEM_GID=999
# FIRST_[GU]ID to LAST_[GU]ID inclusive is the range of UIDs of dynamically
# allocated user accounts/groups.
FIRST_UID=1000
LAST_UID=29999
FIRST_GID=1000
LAST_GID=29999

And, reading the Perl script at $(type -p adduser) or /usr/sbin/adduser, we find this function:

 sub first_avail_uid { my ($min, $max) = @_; printf (gtx("Selecting UID from range %d to %d ...\n"),$min,$max) if ($verbose > 1); my $t = $min; while ($t <= $max) { return $t if (!defined(getpwuid($t))); $t++; } return -1; # nothing available
}

What this means is: adduser picks the first free UID between 1000 and 29999, or fails.

Exact answer: 1002, It will pick a free one.

There IS a maximum UID, 4294967295, because UIDs are 32 bit fields, but adduser uses a lower limit.

However, there is also /usr/sbin/useradd BEWARE adduser and useradd are easily mistaken/mistyped for each other.

man useradd tells me:

DESCRIPTION useradd is a low level utility for adding users. On Debian, administrators should usually use adduser(8) instead.
... -u, --uid UID The numerical value of the user's ID. This value must be unique, unless the -o option is used. The value must be non-negative. The default is to use the smallest ID value greater than or equal to UID_MIN and greater than every other user. See also the -r option and the UID_MAX description.
...
CONFIGURATION The following configuration variables in /etc/login.defs change the behavior of this tool:
... SYS_UID_MAX (number), SYS_UID_MIN (number) Range of user IDs used for the creation of system users by useradd or newusers. The default value for SYS_UID_MIN (resp. SYS_UID_MAX) is 101 (resp. UID_MIN-1). UID_MAX (number), UID_MIN (number) Range of user IDs used for the creation of regular users by useradd or newusers. The default value for UID_MIN (resp. UID_MAX) is 1000 (resp. 60000).

One reason that I use adduser, rather than useradd is the --encrypt-home option to adduser. Either one, however, could be replaced by editing a bunch of files, copying others, creating directories, etc using any UID one picks (Why, in the old days, I ...). There is nothing magic about adduser or useradd.

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