Today, something unexpected happened while I was working on one of our Linux
machines. I issued the crontab -e command to add a cron job to my user’s
crontab file, modified and saved it, only to the following error:
crontab: installing new crontab
crontab: crontabs/<user>: rename: Operation not permitted
crontab: edits left in /tmp/crontab.hgmsOH/crontab
Puzzled, I checked whether my user permissions were all right, if the disk was full, and several other things. Long story short, the fix was this one:
sudo chattr -i /var/spool/cron/crontabs/<user>
sudo chown <user>:crontab /var/spool/cron/crontabs/<user>
Somehow, the user’s crontab file had become immutable.