Using printf output as a variable in sed

Question

I've got a script with a sed command in it that i use to preprend some text to the top of a file.

sed '1s|^|'"$name"' \
'"$(usageblock "$name" "$desc")"' \
|' "$dest" > "${dest}.new" && \
mv "$dest" "${dest}.bak" && \
mv "${dest}.new" "$dest" && \
rm "${dest}.bak"

As you can see, that script is calling a usageblock() function with arguments (function which is defined in another file) that prints out the actual text needed, and for that i use printf and its nice formatting capabilities.

local format
format="%-15s %-4s %-7s\n"
printf "$format" "Description:" "" "$desc"

The problem is in the format variable, the \n, more precisely. Once I call that function within the sed command, it throws out:

unescaped newline inside substitute pattern

And as soon as I delete that \n, it prints out fine (except that there is no new line at the end of every printf statement, which defeats the purpose of using printf in the first place).

So for the moment, I modified my function to print out using echo instead of printf but I don't like it. I have to use the echo command for every line that I need to print. Ugly.

So the question is, how can I use printf formatting capabilities for printing out to sed without causing problems with unescaped new lines? I've tried:

format="%-15b %-4b %-7b\n \\" OR
format="%-15b %-4b %-7b \\" OR
printf "$format" "Description:" "" "$desc \\"

and i've tried different combinations but simply nothing works. It seems like sed needs a single thing and is for me to take the \n part out of the format altogether.

Anybody knows about a sed or a printf flag to use in that case?

Why bother using `sed` to insert a literal block of text? You could just use `cat` (and perhaps a "here document"). — John Zwinck, Sep 23 '15 at 13:46
Well if you could show me an example of how to prepend (not append) text to a file with cat, I would be glad to learn something new... — Jeanmichel Cote, Sep 23 '15 at 13:49
@chepner It does work. Now I would need to use it like so: cat - file > file2 < — Jeanmichel Cote, Sep 23 '15 at 14:31
Of course, sed copies the whole file to the backup. It's incredibly difficult to do what you want without temporarily duplicating the file either as another file (sed, perl, gawk, etc.) or in a buffer (ed). See http://stackoverflow.com/a/17331179/1745001 if you REALLY want to attempt that. — Ed Morton, Sep 23 '15 at 19:23

Using printf output as a variable in sed

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