Maintains crontab files
crontab [-u user-name] filename
crontab [-u user-name] options
A crontab file associates periodic times (such as 14:00 on Wednesdays) with commands. The cron utility executes each command at the specified time. The crontab utility installs, removes, lists, and edits your crontab file. Superuser can process any user's crontab file.
 A point of confusion here: There is a utility named crontab. There are files referred to as crontab files. But the crontab files are not named crontab; they are named using users' login names. So the file named alex could be a crontab file that is edited using crontab.
The first format copies the contents of filename (which contains crontab commands) into the crontab ...