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lastlog file

One of our regular reader asks:

I’d like to discover information about who is currently using the system. When a user logs in what files are updated in UNIX / Linux?

Linux / UNIX have utmp and wtmp files to keep login records. Following three files keeps track of all logins and logouts to the system.

=> /var/run/utmp : List of current login sessions.
=> /var/log/wtmp : Database of past user logins / previous login sessions.
=> /var/log/lastlog : Last logins information about users

How do I access login records files?

These are a binary log files, and grows linearly at its end. So you cannot view records using cat or other text based utilities. The file <utmp.h> declares the structures used to record information about current users in the file. This can be accessed using C programs or other specialized utilities:

Suggested readings:

  • Man pages - ac(1), date(1), last(1), login(1), who(1), getutent(3), updwtmp(3), init(8), wtmp(5)
  • Header file /usr/include/utmp.h