UTMP(5)

NAME
utmp, wtmp, lastlog - login records

SYNOPSIS
#include <utmp.h>

DESCRIPTION
The file <utmp.h> declares the structures used to record information about current users in the file utmp, logins and logouts in the file wtmp, and last logins in the file lastlog. The time stamps of date changes, shutdowns and reboots are also logged in the wtmp file.

These files can grow rapidly on busy systems, daily or weekly rotation is recommended. If any of these files do not exist, it is not created. These files must be created manually and are normally maintained in either the script /etc/daily or the script /etc/weekly. (See cron(8).)

    #define _PATH_UTMP      "/var/run/utmp"
    #define _PATH_WTMP      "/var/log/wtmp"
    #define _PATH_LASTLOG   "/var/log/lastlog"
    #define UT_NAMESIZE     8
    #define UT_LINESIZE     8
    #define UT_HOSTSIZE     16
    struct lastlog {
            time_t  ll_time;
            char    ll_line[UT_LINESIZE];
            char    ll_host[UT_HOSTSIZE];
    };
    struct utmp {
            char    ut_line[UT_LINESIZE];
            char    ut_name[UT_NAMESIZE];
            char    ut_host[UT_HOSTSIZE];
            long    ut_time;
    };

Each time a user logs in, the login program looks up the user's UID in the file lastlog. If it is found, the timestamp of the last time the user logged in, the terminal line and the hostname are written to the standard output. (Providing the login is not quiet, see login(1).) The login program then records the new login time in the file lastlog.

After the new lastlog record is written , the file utmp is opened and the utmp record for the user inserted. This record remains there until the user logs out at which time it is deleted. The utmp file is used by the programs rwho(1), users(1), w(1), and who(1).

Next, the login program opens the file wtmp, and appends the user's utmp record. The same utmp record, with an updated time stamp is later appended to the file when the user logs out. (See init(8).) The wtmp file is used by the programs last(1) and ac(8).

In the event of a date change, a shutdown or reboot, the following items are logged in the wtmp file.

reboot
shutdown
    A system reboot or shutdown has been initiated. The character ‘~’ is placed in the field ut_line, and reboot or shutdown in the field ut_name. (See shutdown(8) and reboot(8).)

date
    The system time has been manually or automatically updated. (See date(1).) The command name date is recorded in the field ut_name. In the field ut_line, the character ‘\*(Ba’ indicates the time prior to the change, and the character ‘{’ indicates the new time.

FILES
/var/run/utmp
    The utmp file.
/var/log/wtmp
    The wtmp file.
/var/log/lastlog
    The lastlog file.

SEE ALSO
last(1), login(1), who(1), ac(8), init(8)

HISTORY
A utmp and wtmp file format appeared in AT&T UNIX. The lastlog file format appeared in 3.0BSD.