var fs = require('fs')
, childProcess = require('child_process')
, stream = fs.createWriteStream('/tmp/output.log')
, child = childProcess.spawn('bash', ['-c', 'echo STDOUT; echo STDERR
>&2; echo DONE;'])
child.stderr.pipe(stream)
child.stdout.pipe(stream)
On Sunday, October 27, 2013 5:19:50 AM UTC+4, Tim Cuthbertson wrote:
>
> I want to spawn a child process and have *all* output go to the same file.
>
> I tried this:
>
> var fs = require('fs'), childProcess = require('child_process');
> var output = fs.openSync('/tmp/output.log', 'w');
> childProcess.spawn('bash', ['-c', 'echo STOUT; echo STDERR >&2; echo
> DONE;'], {stdio: ['ignore', output, output]});
>
> Which presumably provides the same FD for both stdout and stderr in the
> child process. Looking at the file produced, I get:
>
> STOUT
> DONE
>
> stderr is apparently being ignored. Looking at the child_process docs,
> there's an example that opens *two* versions of the same file (in append
> mode), and uses this for stdout / stderr. Since I want to truncate the file
> (not append), I tried:
>
> var fs = require('fs'), childProcess = require('child_process');
> var output = fs.openSync('/tmp/output.log', 'w');
> var output2 = fs.openSync('/tmp/output.log', 'a');
> childProcess.spawn('bash', ['-c', 'echo STOUT; echo STDERR >&2; echo
> DONE;'], {stdio: ['ignore', output, output2]});
>
> But it looks like the write stream is not playing nice and is overwriting
> the other stream's output, as I get:
>
> STOUT
> DONE
> R
>
> So I guess I have to explicitly truncate the file first, then open two
> append streams to it?
>
> fs.closeSync(fs.openSync('/tmp/output.log', 'w'));
> var output = fs.openSync('/tmp/output.log', 'a');
> var output2 = fs.openSync('/tmp/output.log', 'a');
> childProcess.spawn('bash', ['-c', 'echo STOUT; echo STDERR >&2; echo
> DONE;'], {stdio: ['ignore', output, output2]});
>
> Which finally gives me the desired output:
>
> STOUT
> STDERR
> DONE
>
> For kicks, I also tried reusing a single append stream:
>
> fs.closeSync(fs.openSync('/tmp/output.log', 'w'));
> var output = fs.openSync('/tmp/output.log', 'a');
> childProcess.spawn('bash', ['-c', 'echo STOUT; echo STDERR >&2; echo
> DONE;'], {stdio: ['ignore', output, output]});
>
> But that gave me the same results as my initial attempt (no stderr at all).
>
>
> I guess I've discovered *how* to do this (my third attempt), but that
> seems pretty ugly. Can anyone suggest a better way? In python, we can
> explicitly redirect the child's stderr to stdout:
>
> output = open('/tmp/output.log', 'w');
> subprocess.Popen(['bash', '-c', 'echo STOUT; echo STDERR >&2; echo
> DONE;'], stderr = subprocess.STDOUT)
>
> Is there anything similar in nodejs, or am I stuck with explicitly
> truncating the file, followed by opening two append-mode descriptors?
>
> Cheers,
> - Tim.
>
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