exec - is used to execute a command from the bash itself

set hostname with

$ hostnamectl set-hostname ilugc.hashlabs.in

to make effect the hostname without rebooting

$ exec bash

exec with a command as an argument

$ bash

$ exec df -Th

To change the permission of files
$ sudo find /home/ilugc/Downloads -type f -exec chmod 755 {} \;

regards,

T.Dhanasekar