Bash Assign Output of Shell Command To Variable

Originally published at: https://www.cyberciti.biz/faq/unix-linux-bsd-appleosx-bash-assign-variable-command-output/

How do I assign the output of a shell command to a shell variable under Unix like operating system? For example, I want to store the date command output to a variable called $now. How do you do that? How can I set a variable to the output of a command in Bash?