Replace a string in each occurence of string in a file, add additional line at first line in that file - awk

Question

I did search and found how to replace each occurrence of a string in files. Besides that I want to add one line to a file only at the first occurrence of the string.
I know this
grep -rl 'windows' ./ | xargs sed -i 's/windows/linux/g'
will replace each occurrence of string. So how do I add a line to that file at first match of the string? Can any have an idea how to do that? Appreciate your time.
Edited :
Exaple : replace xxx with TTT in file, add a line at starting of file for first match.
Input : file1, file2.
file1
abc xxx pp
xxxy rrr
aaaaaaaaaaaddddd
file2
aaaaaaaaaaaddddd
Output
file1
#ADD LINE HERE FOR FIRST MATCH DONT ADD FOR REST OF MATCHES
abc TTT pp
TTTy rrr
aaaaaaaaaaaddddd
file2
aaaaaaaaaaaddddd

Answer 1

Cribbing from the answers to this question.
Something like this would seem to work:
sed -e '0,/windows/{s/windows/linux/; p; T e; a \new line
;:e;d}; s/windows/linux/g'
From start of the file to the first match of /windows/ do:
replace windows with linux
print the line
if s/windows/linux/ did not replace anything jump to label e
add the line new line
create label e
delete the current pattern space, read the next line and start processing again
Alternatively:
awk '{s=$0; gsub(/windows/, "linux")} 7; (s ~ /windows/) && !w {w=1; print "new line"}' file
save the line in s
replace windows with linux
print the line (7 is true and any true pattern runs the default action of {print})
if the original line contained windows and w is false (variables are empty strings by default and empty strings are false-y in awk)
set w to 1 (truth-y value)
add the new line

Answer 2

If I understand you correctly, all you need is:
find . -type f -print |
while IFS= read -r file; do
awk 'gsub(/windows/,"unix"){if (!f) $0 = $0 ORS "an added line"; f=1} 1' "$file" > tmp &&
mv tmp "$file"
done
Note that the above, like sed and grep would, is working with REs, not strings. To use strings would require the use of index() and substr() in awk, is not possible with sed, and with grep requires an extra flag.
To add a leading line to the file if a change is made using gNU awk for multi-char RS (and we may as well do sed-like inplace editing since we're using gawk):
find . -type f -print |
while IFS= read -r file; do
gawk -i inplace -v RS='^$' -v ORS= 'gsub(/windows/,"unix"){print "an added line"} 1' "$file"
done