BASH command output to the variable has great examples of how to do this. Basically, it boils down to something like this:

variable=$(command)
variable=$(command [opotion...] arg1 arg2 ...)
variable=$(/path/to/command)

You can also use the "`" as well, with all the caveats of using it in bash entails.

Code or Examples

START_TIME=$(date)

#### ... stuff happens here

printf "started:   ${START_TIME}\n"
printf "finished:  $(date)\n\n"

The sample above is from one of my scripts and it works quite nicely.