#!/bin/bash
s="This is the content of the textfile
version=0.4
foo=true
bar=/path/to/whatever
date=20180101
number=1
imgpath=/path/to/whatever
name=foobar"
sed '/^\(version\|date\|number\)=/!d' <<< "$s"
echo "-----"
sed -E '/^(version|date|number)=/!d' <<< "$s"
IyEvYmluL2Jhc2gKcz0iVGhpcyBpcyB0aGUgY29udGVudCBvZiB0aGUgdGV4dGZpbGUKdmVyc2lvbj0wLjQKZm9vPXRydWUKYmFyPS9wYXRoL3RvL3doYXRldmVyCmRhdGU9MjAxODAxMDEKbnVtYmVyPTEKaW1ncGF0aD0vcGF0aC90by93aGF0ZXZlcgpuYW1lPWZvb2JhciIKc2VkICcvXlwodmVyc2lvblx8ZGF0ZVx8bnVtYmVyXCk9LyFkJyA8PDwgIiRzIgplY2hvICItLS0tLSIKc2VkIC1FICcvXih2ZXJzaW9ufGRhdGV8bnVtYmVyKT0vIWQnIDw8PCAiJHMi