Using variables in sed -f (where sed script is in a file rather than inline)

Question

We have a process which can use a file containing sed commands to alter piped input.

I need to replace a placeholder in the input with a variable value,

Answer 1

This might work for you (GNU sed):

cat <<\! > replacements.sed
/XX/{s//'"$(date +%F)"'/;s/.*/echo '&'/e}
!
echo "Today is XX" | sed -f replacements.sed

If you don't have GNU sed, try:

cat <<\! > replacements.sed
/XX/{
    s//'"$(date +%F)"'/
    s/.*/echo '&'/
}
!
echo "Today is XX" | sed -f replacements.sed | sh