Exec display stdout live

Question

I have this simple script    var exec   require  child process   exec   exec  coffee -cw my file coffee   function error  stdout  stderr        console log stdout         where I simply execute a command to compile a coffee-script file  But stdout never get displayed in the console  because the command never ends  because of the -w option of coffee   If I execute the command directly from the console I get message like this    18 05 59 - compiled my file coffee   My question is   is it possible to display these messages with the node js exec   If yes how      Thanks

User · Answer

Here is an async helper function written in typescript that seems to do the trick for me  I guess this will not work for long-lived processes but still might be handy for someone   import   as child process from  child process    private async spawn command  string  args  string     Promise lt  code  number   null  result  string  gt        return new Promise  resolve  reject    gt            const spawn   child process spawn command  args          let result  string         spawn stdout on  data    data  any    gt                if  result                    reject Error  Helper function does not work for long lived proccess                              result   data toString                      spawn stderr on  data    error  any    gt                reject Error error toString                        spawn on  exit   code   gt                resolve  code  result

User · Answer

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   so you can solve your problem using child process spawn as used below   var spawn   require  child process   spawn  ls   spawn  coffee -cw my file coffee     ls stdout on  data   function  data      console log  stdout      data toString          ls stderr on  data   function  data      console log  stderr      data toString          ls on  exit   function  code      console log  code     code toString

User · Answer

There are already several answers however none of them mention the best  and easiest  way to do this  which is using spawn and the   stdio   inherit    option  It seems to produce the most accurate output  for example when displaying the progress information from a git clone   Simply do this   var spawn   require  child process   spawn   spawn  coffee     -cw    my file coffee      stdio   inherit        Credit to  MorganTouvereyQuilling for pointing this out in this comment

User · Answer

exec will also return a ChildProcess object that is an EventEmitter    var exec   require  child process   exec  var coffeeProcess   exec  coffee -cw my file coffee     coffeeProcess stdout on  data   function data        console log data          OR pipe the child process s stdout to the main stdout    coffeeProcess stdout pipe process stdout     OR inherit stdio using spawn  spawn  coffee -cw my file coffee     stdio   inherit

User · Answer

Don t use exec  Use spawn which is an EventEmmiter object  Then you can listen to stdout stderr events  spawn stdout on  data  callback     as they happen    From NodeJS documentation     var spawn   require  child process   spawn      ls      spawn  ls     -lh     usr      ls stdout on  data   function  data      console log  stdout      data toString          ls stderr on  data   function  data      console log  stderr      data toString          ls on  exit   function  code      console log  child process exited with code     code toString           exec buffers the output and usually returns it when the command has finished executing

User · Answer

After reviewing all the other answers  I ended up with this   function oldSchoolMakeBuild cb        var makeProcess   exec  make -C   oldSchoolMakeBuild            function  error  stdout  stderr                 stderr  amp  amp  console error stderr                cb error                   makeProcess stdout on  data   function data            process stdout write  oldSchoolMakeBuild     data               Sometimes data will be multiple lines  so the oldSchoolMakeBuild header will appear once for multiple lines   But this didn t bother me enough to change it

User · Answer

I d just like to add that one small issue with outputting the buffer strings from a spawned process with console log   is that it adds newlines  which can spread your spawned process output over additional lines  If you output stdout or stderr with process stdout write   instead of console log    then you ll get the console output from the spawned process  as is    I saw that solution here  Node js  printing to console without a trailing newline   Hope that helps someone using the solution above  which is a great one for live output  even if it is from the documentation

User · Answer

Inspired by Nathanael Smith s answer and Eric Freese s comment  it could be as simple as   var exec   require  child process   exec  exec  coffee -cw my file coffee   stdout pipe process stdout

User · Answer

I have found it helpful to add a custom exec script to my utilities that do this   utilities js  const   exec     require  child process    module exports exec    command    gt      const process   exec command     process stdout on  data    data    gt        console log  stdout      data toString            process stderr on  data    data    gt        console log  stderr      data toString            process on  exit    code    gt        console log  child process exited with code     code toString             app js  const   exec     require    utilities js    exec  coffee -cw my file coffee

[node.js] Exec : display stdout "live"

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