insert text allocated in a variable before the first empty line - bash

Question

I have some text files $f resembling the following
function
%blah
%blah
%blah
code here
I want to append the following text before the first empty line:
%
%This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike
%3.0 Unported License. See notes at the end of this file for more information.
I tried the following:
top=$(cat ./PATH/text.txt)
top="${top//$'\n'/\\n}"
sed -i.bak 's#^$#'"$top"'\\n#' $f
where the second line (I think) preserves the new line in the text and the third line (I think) substitutes the first empty line with the text plus a new empty line.
Two problems:
1- My code appends the following text:
%n%This work is licensed under the Creative Commons
Attribution-NonCommercial-ShareAlike n%3.0 Unported License. See notes
at the end of this file for more information.\n
2- It appends it at end of the file.
Can someone please help me understand the problems with my code?

Answer 1

If you are using GNU sed, following would work.
Use ^$ to find the empty line and then use sed to replace/put the text that you want.
# Define your replacement text in a variable
a="%\n%This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike\n%3.0 Unported License. See notes at the end of this file for more information."
Note, $a should include those \n that will be directly interpreted by sed as newlines.
$ sed "0,/^$/s//$a/" inputfile.txt
In the above syntax, 0 represents the first occurrence.
Output:
function
%blah
%blah
%
%This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike
%3.0 Unported License. See notes at the end of this file for more information.
%blah
code here
test

Answer 2

You've included bash and sed tags in your question. Since I can't seem to come up with a way of doing this in sed, here's a bash-only solution. It's likely to perform the worst of all working solutions you might find.
The following works with your sample input:
$ while read -r x; do [[ -z "$x" ]] && cat boilerplate; printf '%s\n' "$x"; done < src
This will however insert the boilerplate before EVERY blank line, which is probably not what you're after. Instead, we should probably make this more than a one-liner:
#!/usr/bin/env bash
y=true
while read -r x; do
if [[ -z "$x" ]] && $y; then
cat boilerplate
y=false
fi
printf '%s\n' "$x"
done < src
Note that unlike the code in your question, this doesn't store your boilerplate in a variable, it just cats it "at the right time".
Note that this sends the combined output to stdout. If your goal is to modify the original file, you'll need to wrap this in something that moves around temporary files. (Note that sed's -i option also doesn't really edit files in place, it only hides the moving-around-temp-files from you.)
The following alternatives are probably a better idea.
A similar solution to the bash one might be achieved with better performance using awk:
awk 'NR==FNR{b=b $0 ORS;next} /^$/&&!y{printf "%s",b;y++} 1' boilerplate src
This awk solution obviously reads your boilerplate into a variable, though it's not a shell variable.
Notwithstanding non-standard platform-specific extensions, awk does not have any facility for editing files "in place" either. A portable solution using awk would still need to push temp files around.
And of course, the following old standard of ed is great to keep in your back pocket:
printf 'H\n/^$/\n-\n.r boilerplate\nw\nq\n' | ed src
In bash, of course, you could always use heretext, which might be clearer:
$ ed src <<< $'H\n/^$/\n-\n.r boilerplate\nw\nq\n'
The ed command is non-stream version of sed. Or rather, sed is the stream version of ed, which has been around since before the dinosaurs and is still going strong.
The commands we're using are separated by newlines and fed to ed's standard input. You can discard stdout if you feel the urge. The commands shown here are:
H - instruct ed to print more useful errors, if it gets any.
/^$/ - search for the first occurrence of a newline.
- - GO BACK ONE LINE. Awesome, right?
.r boilerplate - Read your boilerplate at the current line,
w - and write the file.
q - Quit.
Note that this does not keep a .bak file. You'll need to do that yourself if you really want one.
And if, as you suggested in comments, the filename you're reading is to be constructed from a variable, note that variable expansion does not happen inside format quoting ($' .. '). You can either switch quoting mechanisms mid-script:
ed "$file" <<< $'H\n/^$/\n-\n.r ./TATTOO_'"$currn"$'/top.txt\nw\nq\n'
Or you could put ed script in a variable constructed by printf
printf -v scr 'H\n/^$/\n-\n.r ./TATTOO_%s/top.txt\nw\nq\n' "$currn"
ed "$file" <<< "$scr"`

Answer 3

Adding the text to a variable so you can interpolate the variable is wasteful and an unnecessary complication. sed can easily read the contents of a file by itself.
sed -i.bak '1r./PATH/text.txt' "$f"
Unfortunately, this part of sed is poorly standardized, so you may have to experiment a little bit. Some dialects require a newline (perhaps, or perhaps not, preceded by a backslash) before the filename.
sed -i.bak '1r\
./PATH/text.txt' "$f"
(Notice also the double quotes around the file name. You generally always want double quotes around variables which contain file names. More here.)
Adapting the recipe from here we can extend this to apply to the first empty line instead of the first line.
sed -i.bak -e '/^$/!b' -e 'r./PATH/text.txt' -e :a -e '$!{' -e n -e ba -e } "$f"
This adds the boilerplate after the first empty line but perhaps that's acceptable. Refactoring it to replace it or add an empty line after should not be too challenging anyway. (Maybe use sed -n and instead explicitly print everything except the empty line.)
In brief terms, this skips to the end (simply prints) up until we find the first empty line. Then, we read and print the file, and go into a loop which prints the remainder of the file without returning to the beginning of the script.

Answer 4

sed that I think works. Uses files for the extra bit to be inserted.
b='##\n## comment piece\n##'
sed --posix -ne '
1,/^$/ {
/^$/ {
x;
/^true$/ !{
x
s/^$/true/
i\
'"$b"'
};
x;
s/^.*$//
}
}
p
' file1
with the examples using ranges of 1,/^$/, an empty first line would result in the disclaimer being printed twice. To avoid this, I've set it up to put a flag in the hold space ( x; s/^$/true/ ) that I can swap to the pattern space to check whether its the first blank. Once theres a match for blank line, i\ inserts the comment ($b) in front of the pattern space.
Thanks to ghoti for the initial plan.