V4/usr/man/man5/wtmp.5

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.th WTMP V 9/10/73
.sh NAME
wtmp \*- user login history
.sh DESCRIPTION
This
file records all logins and logouts.
Its format is exactly like utmp(V) except that
a null user name indicates a logout on the associated
typewriter, and the typewriter name `x' indicates
that UNIX was rebooted at that point.
.s3
Wtmp is maintained by login(I) and init(VII).
Neither of these programs creates the file,
so if it is removed record-keeping is turned off.
.s3
This file resides in directory /tmp.
.sh "SEE ALSO"
init(VII), login(I)