How did I wipe out all my cron jobs?

Question

I have accidentally wiped out all my cron jobs. I'm not sure what I have done. I don't remember deleting entries from it by issuing crontab -e. What are the possible ways that the cron jobs can be completely wiped out?

Answer 1

If you type crontab with no arguments, it reads a crontab from standard input. If you then type Control-D, it will create an empty crontab, overwriting your previous crontab. (Control-C aborts the command and leaves your crontab alone.)

jpmath's answer correctly points out that crontab -r will also wipe out your cron jobs (that's what it's for).

I avoid this by never using crontab -e (edit crontab) or crontab with no arguments (which reads from stdin). Instead, I keep my crontab entries in a separate file, which I maintain in a source control system, and run the crontab command with that file name as an argument.

(I temporarily clobbered my own crontab while writing this answer, knowing that I could recover it.)

Answer 2

If you type crontab -r instead of crontab -e by mistake (e and r are next to each other), your crontab will be removed as well.

Answer 3

Here's how I confirmed why I lost my crontab file:

[me@myUbuntuPC ~]$ history | grep crontab
...
197  crontab
198  man crontab
198  crontab -e
...

And there's the evidence. My employer used to call that "Learn After Doing"!

I can confirm that under Ubuntu 12.04 and 18.04 (so I assume versions in between), if you type crontab by itself, and hit ctrl-C it also wipes your crontab.

I don't use crontab very often, and while setting up a new machine, my faulty memory told me to type crontab on the old one to view the contents (having forgotten it's crontab -e), I then said "Oh, that's not right. Abort!" and hit Ctrl-C. No more crontab! D'oh!

Then I reviewed the man pages, my notes and/or Google... (L.A.D. as we used to say.)

Not sure if other distros act like this with ctrl-C.

So now I added notes inside my crontab file reminding me to "crontab -l > crontab.backup" to save, and "crontab crontab.backup" to restore. Now, when it's time to build a new Ubuntu 22.04LTS, I'll look at crontab.backup, hopefully read the notes, and remember enough to do it right!

I just tested with a LinHES system (Arch based MythTV) and crontab 4.5 gives you the following:

[me@myLinHESpc ~]$ crontab
crontab 4.5
crontab file [-u user]  replace crontab from file
crontab -  [-u user]    replace crontab from stdin
crontab -l [-u user]    list crontab
crontab -e [-u user]    edit crontab
crontab -d [-u user]    delete crontab
crontab -c dir <opts>   specify crontab directory

I like this much better.

Arch (and LinHES) are rolling upgrade distros, so this would reflect the latest versions in the repos as of mid-2018.

So, chances are, you wiped your crontab by typing crontab by itself, and you don't have crontab 4.5!

I've had years of experience running Linux, but Arch has taught me I know very little about it!