I'm using GNU Parallel for scanning a list of urls (from different hosts) for vulnerability, like this:

cat urls.txt | parallel --gnu -j 50 ./scan {}

The 'scan' program works for one url in one thread. And I need to hard-limit the number of simultaneous requests (jobs) to each host, for example to 5 connections. How can I achieve this?

  • Just to clarify, are you saying you want 5 jobs to each host or 5 urls to be processed at a time?
    – Bratchley
    Commented Apr 5, 2015 at 23:11
  • I want total 50 urls to be processed at a time and 5 or less jobs to each host. (to not overload that servers) Commented Apr 6, 2015 at 20:24

1 Answer 1


Split urls into one file per host. Then run 'parallel -j5' on each file.

Or sort urls and insert a delimiter '\0' when a new host is met, then split on '\0' and remove '\0' while passing that as a block to a new instance of parallel:

sort urls.txt | 
  perl -pe '(not m://$last:) and print "\0";m://([^/]+): and $last=$1' |
  parallel -j10 --pipe --rrs -N1 --recend '\0' parallel -j5 ./scan


I think this will work:

cat urls.txt | parallel -q -j50 sem --fg --id '{= m://([^/]+):; $_=$1 =}' -j5 ./scan {}

sem is part of GNU Parallel (it is a shorthand for parallel --semaphore). {= m://([^/]+):; $_=$1 =} grabs the hostname. -j5 tells sem to make a counting semaphore with 5 slots. --fg forces sem to not spawn the job in the background. By using the hostname as ID you will get a counting semaphore for each hostname.

-q is needed for parallel if some of your URLs contain special shell chars (such as &). They need to be protected from shell expansion because sem will also shell expand them.

  • Thank you for the idea. But in my input file there are lots of hosts having 4 or less links. And also in your solution, each of the 'parallel -j5' will run as long as the slowest scan of these 5. Maybe there is a way to make a total number of running jobs equal to 50? Commented Apr 6, 2015 at 20:18

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .