Execute curl requests in parallel in bash

Use '&' after a command to background a process, and 'wait' to wait for them to finish. Use '()' around the commands if you need to create a sub-shell.

#!/bin/bash

curl -s -o foo http://example.com/file1 && echo "done1" &
curl -s -o bar http://example.com/file2 && echo "done2" & 
curl -s -o baz http://example.com/file3 && echo "done3" &

wait

xargs has a "-P" parameter to run processes in parallel. For example:

wget -nv http://en.wikipedia.org/wiki/Linux -O- | egrep -o "http://[^[:space:]]*.jpg" | xargs -P 10 -r -n 1 wget -nv

Reference: http://www.commandlinefu.com/commands/view/3269/parallel-file-downloading-with-wget

I use gnu parallel for tasks like this.

Here's a curl example with xargs:

$ cat URLS.txt | xargs -P 10 -n 1 curl

The above example should curl each of the URLs in parallel, 10 at a time. The -n 1 is there so that xargs only uses 1 line from the URLS.txt file per curl execution.

What each of the xargs parameters do:

$ man xargs

-P maxprocs
             Parallel mode: run at most maxprocs invocations of utility at once.
-n number
             Set the maximum number of arguments taken from standard input for 
             each invocation of utility.  An invocation of utility will use less 
             than number standard input arguments if the number of bytes 
             accumulated (see the -s option) exceeds the specified size or there 
             are fewer than number arguments remaining for the last invocation of 
             utility.  The current default value for number is 5000.