What is the difference between spawn and exec methodd of child_process?#

The Node.js child_process module has two methods spawn() and exec(), using which we can start a child process to execute other programs on the system. Those new to child_process may wonder why there are two functions to do the same thing, and which one they should use.

I'll explain the differences between spawn() and exec() to help you decide when to use what.

The most significant difference between child_process.spawn() and child_process.exec() is in what they return:

  • spawn() returns a stream
  • exec() returns a buffer.

child_process.spawn() returns an object with stdout and stderr streams. You can tap on the stdout stream to read data that the child process sends back to Node. stdout being a stream has the "data", "end", and other events that streams have. spawn is best used to when you want the child process to return a large amount of data to Node - image processing, reading binary data etc.

child_process.spawn() is "asynchronously asynchronous", meaning it starts sending back data from the child process in a stream as soon as the child process starts executing.

You can see an example here where I used spawn to read the results of a curl request to Node.

child_process.exec() returns the whole buffer output from the child process. By default the buffer size is set at 200k. If the child process returns anything more than that, you program will crash with the error message "Error: maxBuffer exceeded". You can fix that problem by setting a bigger buffer size in the exec options. But you should not do it because exec is not meant for processes that return HUGE buffers to Node. You should use spawn for that. So what do you use exec for? Use it to run programs that return result statuses, instead of data.

child_process.exec() is "synchronously asynchronous", meaning although the .exec is asynchronous, it waits for the child process to end and tries to return all the buffered data at once. If the buffer size of exec is not set big enough, it fails with a "maxBuffer exceeded" error.

See an example here where I used exec() to execute wget to download files and update Node with the status the execution.

So there it is - the differences between spawn() and exec() of Node's child_process. Use spawn() when you want the child process to return huge binary data to Node, use exec() when you want the child process to return simple status messages.

References#

  1. Node.js child_process