finding which files are causing disk to get full from linux command line

One of our servers ran out of disk space, so in order to diagnose the issue from the terminal, used the technique mentioned in this askubuntu post, that is,
du -cha --max-depth=1 / | grep -E "M|G"

Then, to reduce log file usage by journald, as this thread says,

edit /etc/systemd/journald.conf
Activate the SystemMaxUse= option there, e.g. as SystemMaxUse=100M to only use 100 MB.
service systemd-journald restart to activate the changed configuration.

Then, to clear up emails for root from /var/mail - they are mostly cron emails - various techniques discussed here, easiest would be to just empty it - but the safest way to do it might be using the mail command, 
sudo su -
mail
delete *
q
which immediately cleared up all the root emails.
(By default, if you press enter and read a mail, it gets saved to an mbox file in that user's home directory, so in this case, /root/mbox )

Then, redirecting the cron job emails to /dev/null to prevent this from happening again, by appending
> /dev/null 2>&1
to all those cron jobs.