lastlog formats and prints the contents of the last login log /var/log/lastlog file. The login-name, port, and last login time will be printed. The default (no flags) causes lastlog entries to be printed, sorted by their order in /etc/passwd.
The options which apply to the lastlog command are:
|-b, --before DAYS|
|Print only lastlog records older than DAYS.|
|Display help message and exit.|
|-t, --time DAYS|
|Print the lastlog records more recent than DAYS.|
|-u, --user LOGIN|
|Print the lastlog record for user with specified LOGIN only.|
|The -t flag overrides the use of -u.|
The lastlog file is a database which contains info on the last login of each user. You should not rotate it. It is a sparse file, so its size on the disk is usually much smaller than the one shown by "ls -l" (which can indicate a really big file if you have in passwd users with a high UID). You can display its real size with "ls -s".
|Database times of previous user logins.|
Large gaps in UID numbers will cause the lastlog program to run longer with no output to the screen (i.e. if in lastlog database there is no entries for users with UID between 170 and 800 lastlog will appear to hang as it processes entries with UIDs 171-799).