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, e.g. in a single -e type of command I can run;

$ echo "Today is XX" | sed -e "s/XX/$(date +%F)/"
Today is 2012-10-11

However I can only specify the sed aspects in a file (and then point the process at the file), E.g. a file called replacements.sed might contain;

s/XX/Thursday/

So obviously;

$ echo "Today is XX" | sed -f replacements.sed
Today is Thursday

If I want to use an environment variable or shell value, though, I can’t find a way to make it expand, e.g. if replacements.txt contains;

s/XX/$(date +%F)/

Then;

$ echo "Today is XX" | sed -f replacements.sed
Today is $(date +%F)

Including double quotes in the text of the file just prints the double quotes.

Does anyone know a way to be able to use variables in a sed file?

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