How can I know the size of all tmp folders for all users. I tried du -h /home/*/tmpBut it did not add them all together. 
Find out how much all tmp directy occupy
disk usage
If you have GNU du, the -c flag should do what you want.$ du -hcs /home/*/tmp[...]3.9M totalYou can trim to the final total by using tail:$ du -hcs /home/*/tmp | tail -1