My Favorite bash Tips and Tricks | Linux Journal: "Command Substitution
Another bash trick I like to use is command substitution. To use command substitution, enclose any command that generates output to standard output inside parentheses and precede the opening parenthesis with a dollar sign, $(command). Command substitution is useful when assigning a value to a variable. This is typical in shell scripts, where a common operation is to assign the date or time to a variable. It also is handy for using the output of one command as an argument to another command. If you want to assign the date to a variable, you can do this:"
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