bash run script from here doc [duplicate]

Answer 1

You can see what happens when you remove the |bash

X=oldvalue
cat <<EOF 
X=world
echo "hello $X"
EOF

The $X is replaced before piping it to bash.
You can check the following

X=oldvalue
cat <<"EOF"
X=world
echo "hello $X"
EOF

This is what you want to execute:

cat <<"EOF" | bash
X=world
echo "hello $X"
EOF