Command-line script to run commands in parallel at a fixed rate

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0.1.3 2022-03-18 22:34 UTC

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Last update: 2024-04-19 03:03:21 UTC


A command-line script to run commands in parallel at a fixed rate.

Latest Stable Version License

What is it?

Ever needed to execute a command n times per second, allowing these commands to run simultaneously if the first command has not finished executing when the second is started? This tool does just that.

As a safeguard, you must specify the maximum number of processes allowed to run simultaneously. If the commands take too long to execute, and the maximum number of processes is reached, the next execution(s) will be skipped until a slot becomes available.

When is it useful?

Let's say you have a script that queries a remote API. Your request limit on this API is 5 requests per second.

You can't just keep 5 instances of the script running in parallel:

  • if the API response time is lower than 1s, you would need to pause each script to ensure that it lasts at least 1s
  • if the API response time is greater than 1s, you would not fully use your request allowance

Parallel.phar can start 5 instances of your script every second, regardless of how many are already running, up to a limit specified by you. This limit is necessary to avoid server & network congestion in case of sudden network issue, slowdown or lock.

How to use it?

Ensure that you have PHP installed, and download parallel.phar:

chmod +x parallel.phar

Alternatively, you can install it with Composer.

Then run:

./parallel.phar Rate Concurrency Command [...]


  • Rate is the number of executions per second
  • Concurrency is the maximum number of concurrent processes
  • Command is the command to execute, with its optional arguments


./parallel.phar 10 20 date "+%X %N"

This would output 10 lines per second, such as:

04:27:12 PM 031273903
04:27:12 PM 121497348
04:27:12 PM 221800224
04:27:12 PM 322584008
04:27:12 PM 423034796

In this example, there cannot be more than 20 processes running in parallel; this is irrelevant for our date example as the execution is really fast, but would be important for long-running scripts, potentially invoking a remote API.

To stop, just press Ctrl + C.