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sed -i '1s/^/<!DOCTYPE html> n/' ${file_name.html}

sed -i '' '1s/^/<!DOCTYPE html> n/' ${file_name.html}

sed -i.bak -e '…' SOMEFILE

case $(sed --help 2>&1) in
  *GNU*) set sed -i;;
  *) set sed -i '';;
esac
"$@" -e '…' "$file"

case $(sed --help 2>&1) in
  *GNU*) sed_i () { sed -i "$@"; };;
  *) sed_i () { sed -i '' "$@"; };;
esac
sed_i -e '…' "$file"

perl -i -pe '…' "$file"

sed -e '…' "$file" >"$file.new"
mv -- "$file.new" "$file"

sed '1s/^/<!DOCTYPE html> n/' "${file_name.html}" > "${file_name.html}.tmp" &&
  mv "${file_name.html}.tmp" "${file_name.html}"

perl -i -pe 'print "<!DOCTYPE html> n" if $.==1;' "${file_name.html}"

$ printf '0an<!DOCTYPE html>n.nwn' | ed my.html

$ sed -i '1i<!DOCTYPE html>n` testfile.csv

{ rm -f file && { echo '<!DOCTYPE html>'; cat; } > file;} < file

sed '1i
<!DOCTYPE html>' file 1<> file

echo a > testfile.txt
ls -li testfile.txt
#gsed -i -e 's/a/A/' testfile.txt
#bsdsed -i 's/a/A/' testfile.txt  # does not work
bsdsed -i -e 's/a/A/' testfile.txt
ls -li testfile.txt
cat testfile.txt

printf '%sn' 1i '<!DOCTYPE html>' . x | ex file

sed 'script' <<FILE >file
$(cat file)
FILE

sed 'script' <<FILE >file
$(tee file.old <file)
FILE