utmp, wtmp - login records

#include <utmp.h>

The utmp file allows one to discover information about who is currently using UNIX. The file is a sequence of entries with the following structure declared in the include file:

struct utmp {
	char	ut_line[8];		/* tty name */
	char	ut_name[8];		/* user id */
	long	ut_time;		/* time on */
};

This structure gives the name of the special file associated with the user's terminal, the user's login name, and the time of the login in the form of time (2).

The wtmp file records all logins and logouts. Its format is exactly like utmp except that a null user name indicates a logout on the associated terminal. Furthermore, the terminal name `~' indicates that the system was rebooted at the indicated time; the adjacent pair of entries with terminal names `\^|\^' and `}' indicate the system-maintained time just before and just after a date command has changed the system's idea of the time.

Wtmp is maintained by login (1) and init (8). Neither of these programs creates the file, so if it is removed record-keeping is turned off. It is summarized by ac (1).

/etc/utmp
/usr/adm/wtmp

login(1), init(8), who(1), ac(1)