child_process exec command is getting executed multiple times

Question

I am using exec() to run a terminal command to show dialog in electron mac application.

The code I am using:

var exec = require('child_process').exec;
var request = require('request');

request('https://server_url', function (error, response, data) {

    console.log("inside request");

    exec(`osascript -e 'with timeout of 86400 seconds
                                    tell app "System Events"
                                        display dialog "` + data.pop_up_message + `" buttons {"OK", "Cancel"} 
                                    end tell 
                                end timeout'
    `, function(error, stdout, stderr){

        console.log("inside exec");

    });

});

Its showing multiple dialogs in a single request.

Console output:

inside request
inside exec
inside exec
inside exec

Here 'inside request' is getting printed only once. But the 'inside exec' is getting printed multiple times. What is the reason for this issue. How can I solve this.

*inside exec* is printed every time the callback for `exec` is called ... not whenever exec is called .... hmmm, which should only happen once anyway - how odd — Jaromanda X, Aug 29 '16 at 04:57
Yes. The dialog is shown multple times. When I am clicking the dialog buttons the 'inside exec' is printed — Arun D, Aug 29 '16 at 04:59
would be more interesting to log the values of error, stdout and stderr each time to see what is happening — Jaromanda X, Aug 29 '16 at 05:00
I have logged error values too. But its showing empty string — Arun D, Aug 29 '16 at 05:01
error and stderr are null. stdout returns error 'button returned:Ok' — Arun D, Aug 29 '16 at 05:09
I was referring to the question ... do you always get inside request x 1 followed by inside exec x 3 .... that **exact** pattern? — Jaromanda X, Aug 29 '16 at 05:27
inside request only once. inside exec is not exactly 3 times. sometimes it shows twice, sometimes 3 times or multiple times — Arun D, Aug 29 '16 at 05:29
Which version of nodejs, and are you using the "builtin" child_process or is that an external library? — Jaromanda X, Aug 29 '16 at 05:32
I am using the built in child_process library and Node version 4.4.5 — Arun D, Aug 29 '16 at 05:34

score 0 · Answer 1 · edited May 23 '17 at 10:33

0

The inner function is a callback that is connected to 2 streams: stdout and stderr. The third parameter is an error object that typically contains something if the child process failed or timed out (see: documentation).

Whenever you exec something, his output will arrive after some time by sending messages through one of these. It can happen quite shortly after, or a very long time after. Also, it can be a rather small or big output, so this will come into chunks of data depending on what the execution does. The "under the hood" of stdout and stderr can be easier to visualize in the C / C++ perspective.

Let's say that you write a C program that prints to console every 5 seconds by writing to stdout and then you call it with your NodeJS module.

Your console.log here will print something as soon as it detects a message being sent through one of these streams. In that specific case, it will print every 5 seconds.

Just a small picture, since there could be a lot more to say about what streams are and how they work. This answer adds some interesting details about them.

Update - 29/18/2016:

Updated the streams part. Like Jaromanda X suggested, error isn't a buffer but rather and Error object.

edited May 23 '17 at 10:33

Community

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answered Aug 29 '16 at 05:11

Frederik.L

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error isn't a stream, is it? I know what stdout is, I know what stderr is ... but error? [documentation](https://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback) doesn't say anything about being called for each stream - in fact, there are no streams involved ... just an Error object and two Buffers – Jaromanda X Aug 29 '16 at 05:27
You might be right. Im still looking for an evidence on what it is exactly. I think this is a message that NodeJS sends himself, which is (I think) a stream. It would be interesting to clarify this. – Frederik.L Aug 29 '16 at 05:31
I won't take a chance on this, an error object is safer for now. Thanks for pointing this out! Will update. – Frederik.L Aug 29 '16 at 05:33
But not only the 'inside exec' is running multiple times. The dialog is also shown multiple times. So the exec command is also running multiple times. – Arun D Aug 29 '16 at 05:36

child_process exec command is getting executed multiple times

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