Insert the same line after several line numbers in one pass with awk (no renumbering drift)

Inserting text after lines 237, 253, 267 and so on with repeated sed -i "Na ..." shifts every later line number after the first insert, so the later inserts land in the wrong place unless you go bottom-up. A single awk pass avoids this, because NR is always the original line number:

awk -v L="$LABEL" 'BEGIN{split("237 253 267 286",a," ");for(i in a)s[a[i]]=1} {print} s[NR]{print L}' f.md > /tmp/f.new && mv /tmp/f.new f.md

Notes: pass the text with -v so there are no quoting fights. -v does interpret backslash escapes, so use ENVIRON["LABEL"] if the text has backslashes. Re-check the targets first with sed -n "Np" (line numbers from someone else's grep may be off by one; in my case a header was at 515, not the reported 514). Afterwards grep -c the label to confirm the insert count. BSD and GNU awk both handle this.