Where is the crontab in Ubuntu?

  If you’re looking to make a copy of a user’s crontab as a backup or just to view it without using the crontab editor, you can locate it at:  
/var/spool/cron/crontabs
  Each user’s crontab will be in this directory in a file named as their username (e.g. root). If you’re working with a lengthy crontab it’s a good idea to back it up – can save you a lot of work if everything goes pear-shaped.

How to easily find the full path of a command in Ubuntu

  If you’re writing scripts or making cron jobs you will need to know the full path of the commands you’re using; rather than just being able to use “ls” you would have to use “/bin/ls” instead. You could use the find command here but there’s a quicker and more elegant way: which. Use it like so:  
which ls
  will return:  
/bin/ls
  Not everything will be in /bin, e.g.:  
which timeout
  will likely return:  
/usr/bin/timeout
  Simple but it will make your life quite a bit easier when writing scripts, particularly with new commands or command which you don’t use often.