Now let us ask parallel to execute all the commands in that file simultaneously. How can I count all the lines of code in a directory recursively? Following would start downloading in 3 process: so 4 downloadable lines of URLs are generated and sent to xargs. How to send a header using a HTTP request through a curl call? For this purpose, there are tools like clustershell and pdsh (agreed that GNU parallel has parameters like sshlogin and sshloginfile), but i did not find that straight forward. Simple, but effective for a first step. How can I make a long wall perfectly level? You can use GNU Parallel for some of the below use cases. It only takes a minute to sign up. Stack Exchange network consists of 176 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. ab is a very simple yet effective tool of this type, which works for any web server … Again, from a computer/CPU standpoint it mainly deals with one task at a time, but keeps on switching between tasks, which happens too fast, and we are perfectly fine as far as multiple tasks are progressing simultaneously. Once all files have been successfully downloaded, make will report that there is nothing more to be done, so there is no harm in running it one extra time to be "safe". DNS failure crashing web site - CURL requests. For example, you can have a file with a list of URLs to download. Without specifying the RCMD environment variable, you can also run commands like the one shown below. In the above example, a maximum of 10 compression will happen together. You might be running multiple things at the same time on a computer, but the computer will always execute one single task from your list of tasks at a time. Really the best solution since it allows resuming failed downloads and uses 'make' which is both robust and available on any unix system. If you see the above command, parallel is using -j option (very similar to xargs, to specify the number of processes to run in parallel), and -I option (to remove the default space character). It's possible to do something similar in a for loop in a shell, but end up doing process management, which starts to seem pretty pointless once you realize how insanely great this use of xargs is. Am going to start this with clustershell and then pdsh. seq command will simply give 1, 2, and 3 as output in three lines. Its called GNU Parallel. Who is the "young student" André Weil is referring to in his letter from the prison? => Use wait built-in … Without -I{} and the last junkfile{}, xargs would construct our command with the file name of "junkfile 1, junkfile 2 etc" (we needed to remove the space), rather than junkfile1, junkfile2 and so on. A typical computer will do 100s of switching between tasks in a single second. How Does SSL/TLS Chain Certificates and Its Validation work? You can now imagine why we are under the illusion that multiple tasks are being executed at the same time.