Difference between spawn and exec functions of child_process
The Node.js Child Processes module (
child_process) has two functions
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
exec to help you decide when to use what.
The most significant difference between
child_process.exec is in what they return - spawn returns a stream and exec returns a buffer.
child_process.spawn returns an object with
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
exec of Node's
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.