Using sticky bit you can restrict other users from deleting files or directories except the owner who created them and the root user In symbolic : chmod o+t /backup (or) chmod +t /backup

In numeric : chmod 1755 /backup

before sticky bit set :

drwxr-xr-x. 42 user1 abc  4096 Feb 14 17:30 backup

After sticky set :

drwxr-xr-t. 42 user1 abc 4096 Feb 14 17:30 backup

Now the stickybit 't' added in the above list