Node js spawn child process and get terminal output live

Question

I have a script that outputs  hi   sleeps for a second  outputs  hi   sleeps for 1 second  and so on and so forth  Now I thought I would be able to tackle this problem with this model   var spawn   require  child process   spawn  temp      spawn  PATH TO SCRIPT WITH THE ABOVE BEHAVIOUR     temp stdout pipe process stdout     Now the problem is that the task needs to be finished in order for the output to be displayed  As I am understanding it  this is due to the fact that the newly spawned process takes execution control  Obviously node js does not support threads so any solutions  My idea was to possibly run two instances  first one for the specific purpose of creating the task and have it pipe the output to process of the second instance  considering this can be achieved

User · Answer

I had a little trouble getting logging output from the "npm install" command when I spawned npm in a child process. The realtime logging of dependencies did not show in the parent console.

The simplest way to do what the original poster wants seems to be this (spawn npm on windows and log everything to parent console):

var args = ['install'];

var options = {
    stdio: 'inherit' //feed all child process logging into parent process
};

var childProcess = spawn('npm.cmd', args, options);
childProcess.on('close', function(code) {
    process.stdout.write('"npm install" finished with code ' + code + '\n');
});

User · Answer

Adding an answer related to child process exec as I too had needed live feedback and wasn t getting any until after the script finished  This also supplements my comment to the accepted answer  but as it s formatted it will a bit more understandable and easier to read   Basically  I have a npm script that calls Gulp  invoking a task which subsequently uses child process exec to execute a bash or batch script depending on the OS  Either script runs a build process via Gulp and then makes some calls to some binaries that work with the Gulp output    It s exactly like the others  spawn  etc    but for the sake of completion  here s exactly how to do it      INCLUDES import   as childProcess from  child process      ES6 Syntax      GLOBALS let exec   childProcess exec     Or use  var  for more proper                                   semantics  though  let  is                                   true-to-scope      Assign exec to a variable  or chain stdout at the end of the call    to exec - the choice  yours  i e  exec        stdout on           let childProcess   exec          binary command -- --argument argumentValue         error  stdout  stderr     gt                if  error                            This won t show up until the process completes              console log    ERROR        error name      -     error message                console log    STACK       error stack                 console log  stdout                console log  stderr                callback                  Gulp stuff             return                        Neither will this          console log  stdout            console log  stderr            callback                      Gulp stuff            Now its as simple as adding an event listener  For stdout   childProcess stdout on        data         data     gt                   This will render  live           console log    STDOUT       data               And for stderr   childProcess stderr on        data         data     gt                   This will render  live  too          console log    STDERR       data               Not too bad at all - HTH

User · Answer

I found myself requiring this functionality often enough that I packaged it into a library called std-pour   It should let you execute a command and view the output in real time  To install simply   npm install std-pour   Then it s simple enough to execute a command and see the output in realtime   const   pour     require  std-pour    pour  ping     8 8 8 8    -c    4    then code   gt  console log  Error Code    code        It s promised based so you can chain multiple commands  It s even function signature-compatible with child process spawn so it should be a drop in replacement anywhere you re using it

User · Answer

I m still getting my feet wet with Node js  but I have a few ideas  first  I believe you need to use execFile instead of spawn  execFile is for when you have the path to a script  whereas spawn is for executing a well-known command that Node js can resolve against your system path  1  Provide a callback to process the buffered output  var child   require  child process   execFile  path to script           arg1    arg2    arg3       function err  stdout  stderr            Node js will invoke this callback when process terminates      console log stdout           2  Add a listener to the child process  stdout stream  9thport net  var child   require  child process   execFile  path to script           arg1    arg2    arg3          use event hooks to provide a callback to execute when data are available   child stdout on  data   function data        console log data toString           Further  there appear to be options whereby you can detach the spawned process from Node s controlling terminal  which would allow it to run asynchronously  I haven t tested this yet  but there are examples in the API docs that go something like this  child   require  child process   execFile  path to script           arg1    arg2    arg3                 detachment and ignored stdin are the key here       detached  true       stdio     ignore   1  2           and unref   somehow disentangles the child s event loop from the parent s   child unref     child stdout on  data   function data        console log data toString

User · Answer

PHP-like passthru import   spawn   from  child process    export default async function passthru exe  args  options        return new Promise  resolve  reject    gt            const env   Object create process env           const child   spawn exe  args                   options              env                       env                     options env                                     child stdout setEncoding  utf8            child stderr setEncoding  utf8            child stdout on  data   data   gt  console log data            child stderr on  data   data   gt  console log data            child on  error   error   gt  reject error            child on  close   exitCode   gt                console log  Exit code    exitCode               resolve exitCode                          Usage const exitCode   await passthru  ls     -al      cwd    var www html

User · Answer

It s much easier now  6 years later    Spawn returns a childObject  which you can then listen for events with  The events are    Class  ChildProcess   Event   error  Event   exit  Event   close  Event   disconnect  Event   message     There are also a bunch of objects from childObject  they are    Class  ChildProcess   child stdin child stdout child stderr child stdio child pid child connected child kill  signal   child send message   sendHandle    callback   child disconnect      See more information here about childObject  https   nodejs org api child process html  Asynchronous  If you want to run your process in the background while node is still able to continue to execute  use the asynchronous method  You can still choose to perform actions after your process completes  and when the process has any output  for example if you want to send a script s output to the client    child process spawn        Node v0 1 90   var spawn   require  child process   spawn  var child   spawn  node   commands server js        You can also use a variable to save the output     for when the script closes later var scriptOutput        child stdout setEncoding  utf8    child stdout on  data   function data          Here is where the output goes      console log  stdout      data        data data toString        scriptOutput  data       child stderr setEncoding  utf8    child stderr on  data   function data          Here is where the error output goes      console log  stderr      data        data data toString        scriptOutput  data       child on  close   function code          Here you can get the exit code of the script      console log  closing code      code        console log  Full output of script    scriptOutput         Here s how you would use a callback   asynchronous method   var child process   require  child process     console log  Node Version     process version    run script  ls     -l     home    function output  exit code        console log  Process Finished         console log  closing code      exit code       console log  Full output of script    output        console log   Continuing to do node things while the process runs at the same time           This function will output the lines from the script     AS is runs  AND will return the full combined output    as well as exit code when it s done  using the callback   function run script command  args  callback        console log  Starting Process         var child   child process spawn command  args        var scriptOutput            child stdout setEncoding  utf8        child stdout on  data   function data            console log  stdout      data            data data toString            scriptOutput  data               child stderr setEncoding  utf8        child stderr on  data   function data            console log  stderr      data            data data toString            scriptOutput  data               child on  close   function code            callback scriptOutput code               Using the method above  you can send every line of output from the script to the client  for example using Socket io to send each line when you receive events on stdout or stderr    Synchronous  If you want node to stop what it s doing and wait until the script completes  you can use the synchronous version   child process spawnSync        Node v0 11 12    Issues with this method    If the script takes a while to complete  your server will hang for that amount of time   The stdout will only be returned once the script has finished running  Because it s synchronous  it cannot continue until the current line has finished  Therefore it s unable to capture the  stdout  event until the spawn line has finished    How to use it   var child process   require  child process     var child   child process spawnSync  ls     -l     home      encoding    utf8      console log  Process finished     if child error        console log  ERROR    child error     console log  stdout    child stdout   console log  stderr    child stderr   console log  exist code    child status

User · Answer

child   setInterval function         process stdout write  hi       1000      or however else you want to run a timer   parent   require  child process   fork    childfile js       fork d children use the parent s stdio

User · Answer

Here is the cleanest approach I ve found   require  child process   spawn  bash       script sh        cwd  process cwd      detached  true    stdio   inherit

[node.js] Node.js spawn child process and get terminal output live

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