How to read a file into a variable in shell?

In cross-platform, lowest-common-denominator sh you use:

#!/bin/sh
value=`cat config.txt`
echo "$value"

In bash or zsh, to read a whole file into a variable without invoking cat:

#!/bin/bash
value=$(<config.txt)
echo "$value"

from : https://stackoverflow.com/questions/7427262/how-to-read-a-file-into-a-variable-in-shell