How do I replace ALL text in a file after a certain line with the content of another text file, using sed/awk?

In GNU/awk:

Name the two files as arguments.

awk '{ print; } /^### REPLACE EVERYTHING AFTER THIS LINE ###$/ { nextfile; }' fn1 fn2

Print all lines until "replace after" line is found, then read from other file & quit:

sed '/^### REPLACE EVERYTHING AFTER THIS LINE ###$/{r replacement_file.txt
q;}' original_file.txt

---

• use sed -i to save changes
• or ... > tmp && mv tmp original

Using sed:

sed -n -e '1,/^### REPLACE EVERYTHING AFTER THIS LINE ###$/{ p; d; }' \
       -e 'r replacement_file.txt' \
       -e 'q' original_file.txt

The three sed blocks do this:

1. The first block prints all lines from line 1 to the line with the special contents. I print these lines explicitly with p and then invoke d to force a new cycle to start ("print; next" in awk).
2. After the initial lines have been outputted by the first block, the second block outputs the contents of the extra file.
3. The editing script is then terminated.

Ordinarily, q in the third block would output the current line before quitting (this would be the line in the example data reading text_that_will), but since sed is invoked with -n, this default outputting of a line at the end of a cycle is inhibited.

The result of the above command, given your data, is

common_text
### REPLACE EVERYTHING AFTER THIS LINE ###
testing123
this_is_the_replacement_text

To update the original file, you could use sed -i ..., or redirect the output to a new file that you then replace the original with:

sed ... original_file.txt >original_file.txt.new &&
mv original_file.txt.new original_file.txt