Insert block of text at beginning of a file

Question

I've seen a number of posts about injecting single lines at the beginning of a file. However, I haven't really any that answer to injecting a block of text into the beginning of an existing file. While I have a solution:

install -bDm 0660 -o root -g root <(
    echo "# Set external URL"
    printf "external_url = '%s'\n\n" "${EXTERNAL_URL}"
    cat <PATH_TO_EXISTING_FILE>
  ) <PATH_TO_EXISTING_FILE>

This feels really ugly. So, I was wondering if there was something less kludgey (and that didn't rely on encapsulating python/perl/etc. within a bash script).

Note: I had ultimately settled on this method because when you try something more like:

(
  echo "# Set external URL"
  printf "external_url = '%s'\n\n" "${EXTERNAL_URL}"
  cat <PATH_TO_EXISTING_FILE>
) <PATH_TO_EXISTING_FILE>

You get an error like cat: <PATH_TO_EXISTING_FILE>: input file is output file.

Answer 1

I would solve this with an in-place sed command:

sed -i -e "1i # Set external URL\nexternal_url = '${EXTERNAL_URL}'\n\n" <PATH_TO_EXISTING_FILE>

Explanation:

• -i means edit the file in place
• -e to provide the expression on the command line
• 1i <contents> says insert <contents> at the first line
• and \n gets replaced by a newline by sed itself, not the shell, so this should work in any shell.