Question

I am a newbie as far as node.js is concerned.I wrote the following code to pipeline two linux commands.
This is my nodejs code:

var spawn = require('child_process').spawn,
    ls = spawn('ls',['-lh','/usr']),
    grep = spawn('grep',['bin']);

/*
ls.stdout.on('data',function(data){
    console.log('stdout: '+data);
});
*/

ls.stdout.on('data',function(data){
    console.log(""+data);
    grep.stdin.write(data);
});

ls.stderr.on('data',function(data){
    console.log('stderr: '+data);
});


ls.on('exit',function(code){
    console.log('Exit code '+code);
    grep.stdin.end();
})

// ------------------------------------

grep.stdout.on('data',function(data){
    console.log('stdout: '+data);
});

grep.stderr.on('data',function(data){
    console.log('stderr: '+data);
});

Now this code fails sometimes and runs sometimes.I’m getting confused now.
When it fails,it says:

Exit code 0
total 160K
drwxr-xr-x   2 root root  68K Oct 12 12:54 bin
drwxr-xr-x   2 root root 4.0K Jun 20 19:58 games
drwxr-xr-x  54 root root 4.0K Sep 24 17:52 include
drwxr-xr-x 252 root root  44K Oct  2 21:53 lib
drwxr-xr-x  10 root root 4.0K Apr 28 19:16 local
drwxr-xr-x   2 root root  12K Sep 18 15:51 sbin
drwxr-xr-x 362 root root  12K Sep 28 17:58 share
drwxr-xr-x   5 root root 4.0K Jul  7 23:39 src


events.js:66
        throw arguments[1]; // Unhandled 'error' event
                       ^
Error: This socket is closed.
    at Socket._write (net.js:517:19)
    at Socket.write (net.js:509:15)
    at Socket.<anonymous> (/home/rajat/nodexperiments/full-spawn.js:13:13)
    at Socket.EventEmitter.emit (events.js:88:17)
    at Pipe.onread (net.js:395:14)

And when it runs,it says:

total 160K
drwxr-xr-x   2 root root  68K Oct 12 12:54 bin
drwxr-xr-x   2 root root 4.0K Jun 20 19:58 games
drwxr-xr-x  54 root root 4.0K Sep 24 17:52 include
drwxr-xr-x 252 root root  44K Oct  2 21:53 lib
drwxr-xr-x  10 root root 4.0K Apr 28 19:16 local
drwxr-xr-x   2 root root  12K Sep 18 15:51 sbin
drwxr-xr-x 362 root root  12K Sep 28 17:58 share
drwxr-xr-x   5 root root 4.0K Jul  7 23:39 src

Exit code 0
stdout: drwxr-xr-x   2 root root  68K Oct 12 12:54 bin
drwxr-xr-x   2 root root  12K Sep 18 15:51 sbin

Any Ideas?

Answer 1

This is quite clearly a race condition, because node.js is a highly parallel environment you have found a really nice example to demonstrate this.

ls.on('exit',function(code){
    console.log('Exit code '+code);
    grep.stdin.end();
})

ls fires the above event, before it has finished writing to grep, and then closes the socket that’s being used to communicate. The hint your output gives you is that the exit(0) message coming from ls appears once at the top of the output, and once just above the error message.

You shouldn’t be closing grep’s stdin channel here.

But what about using .exec() instead of spawn on a bash script that does something like

#/bin/bash mybashscript
ls $1 | grep bin

and then using the JavaScript closure, such as

child = exec('mybashscript',['-lh','/usr']
  function (error, stdout, stderr) {
    console.log('stdout: ' + stdout);
    console.log('stderr: ' + stderr);
    if (error !== null) {
      console.log('exec error: ' + error);
    }
});

With the events your code ordering is more difficult to read.