mjhjmtu

I need to replace the first line of a file with the complete content from another file. Does on see what I'm missing?

$ cat old.txt
---
foo
bar

$ cat append.txt
---
123
<321>

$ cat process.sh
old=old.txt
append=append.txt
sed -i "1s/.*/$append/" $old
cat $old

I'd expect

$ bash process.sh
---
123
<321>
foo
bar

Though instead I got

sed: -e expression #1, char 9: unterminated `s' command

when trying to sed with a variable inside my script

while this one works (without variable)
sed -i "1s/.*/loremipsum/" $old

To replace the first line of old.txt with the content of append.txt:

$ sed -e '1 r append.txt' -e 'd;' old.txt
---
123
<321>
foo
bar

Add -i before the first -e to do the edit in-place in old.txt.

This runs the following short sed script on the given file:

1 # we're on the first line
 r append.txt # read in the whole of the append.txt file
 d; # delete the current line

 # (implicit print)

On the command line, this is divided into two separate -e expression string since the filename used with the r command must be terminated by a newline (or the end of the current expression string).