xargs in parallel with -P

xargs -P runs N invocations in parallel. -n1 ensures one item per call:

cat urls.txt | xargs -P 8 -n 1 curl -sS -o /dev/null -w '%{http_code}\n'

-P 8 = 8 concurrent processes. Pairs well with -I {} to position the argument in the middle of the command.