This program is run by individual users to inspect or modify their crontab files. If a change is made to the file, then the root daemon process will be given a kick, and will immediately read the new configuration. A warning will be issued to standard output if it appears that a cron daemon is not running.
The command is used as
crontab [-u user] file
crontab [-u user] ( -l | -e | -r )
Only the root user can use the -u option, to specify the manipulation of another user's crontab file. In the first instance, the entire crontab file of the user is replaced with the contents of the specified file, or standard input if the file is “-”.
In the latter case, the program behaves according to which of the (mutually exclusive) options was given (note that the long options are an mcron extension).