Insert the first file into the second file after the first empty line with sed.

insert-a-file-into-file-with-sed-after-first-empty-line.sh

# basefile.txt - The file where we want to put the content of the
#                injectedfile.txt.
# injectedfile.txt - The file we want to inject into the basefile.txt.

# -n disables automatic line printing.
sed -n \ 
  # Set up a label to jump to. 
  # It's called 'head' because we use it to print lines before the first empty
  # line.
  -e ':head' \ 
  # Print the line and if the current line is not empty load the next one and
  # jump to 'head'.
  -e 'p;/./n;/./b head' \
  # We get here if we didn't jump to head (the current line is empty). Print
  # the empty line and inject the content of injectedfile.txt.
  -e 'p;r injectedfile.txt' \
  # Set up a label called 'tail' to handle the remaining lines of basefile.txt.
  -e ':tail' \
  # Load the next line, print it and jump to 'tail'.
  -e 'n;p;b tail' \
  basefile.txt