|Enabling Process Accounting on Linux HOWTO|
ac prints out statistics about users' connection times in hours based on the logins and logouts in the current /var/log/wtmp file. ac is also capable of printing out time totals for each day (-d option), and for each user (-p option).
accton is used to turn on or turn off process accounting. The file is normally executed at system bootup or shutdown via system init scripts.
last goes through the /var/log/wtmp file and prints out information about users' connection times.
sa summarizes accounting information from previously executed commands, software I/O operation times, and CPU times, as recorded in the accounting record file /var/account/pacct.
lastcomm prints out the information about all previously executed commands, recorded in /var/account/pacct.