Differences between spawn and exec of child_process Back

The Node.js Child Processes module (child_process) has two functions 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 and 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 span 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.