process-streams

1.0.1 • Public • Published

Motivation

The goal of this small package is to provide a interface for streaming data to and from child-processes. It may be possible to stream data to a process via stdout and read the result from stdin, but it may also be necessary to store the data in a temporary file and provide the filename to the process as argument.

A concrete example is ffmpeg: It is possible to encode webm-videos to a stream, but h264 must always be stored in a temporary file first. On the other hand in some cases it is not possible to stream data into ffmpeg, so data must be stored in a temporary file prior to calling ffmpeg.

Overview

ProcessStreams provides the methods exec, execFile and spawn from the child_process with the same arguments. The return value however is always a through-stream. The command line arguments are examined for occurences of the strings <INPUT> and <OUTPUT>.

  • If <INPUT> is present, the stream input is piped into a temporary file and <INPUT> is replaced by its filename.
  • If <OUTPUT> is present, it is replaced by the name of a temporary file and the contents of this file is used as stream output for the resulting stream.
  • If <INPUT> or <OUTPUT> are not present, the stream input is directly piped to the child processes stdin (or the child processes stdout is piped to the stream output).

Temporary files are always deleted when no longer needed.

Functions

ps.spawn(command, [args], [options])

For details about function arguments please refer to the api documentation of child_process.spawn(command, [args], [options])

ps.exec(command, [options], callback)

For details about function arguments please refer to the api documentation of child_process.exec(command, [options], callback)

ps.execFile(file, [args], [options], [callback])

For details about function arguments please refer to the api documentation of child_process.execFile(file, [args], [options], [callback])

ps.factory(useTmpIn, useTmpOut, callback)

This function uses the provided callback to connect input and output of the resulting stream. useTmpIn and useTmpOut are booleans that define which parts of the stream temp should use temp files. callback has the signature function(input, output, callback). "input" and "output" are either streams of paths of temporary files. The callback must be called when data is available for output. If "tmpUseOut" is false, this can be called immediately. It "tmpUseOut" is true it must be called, when the output tempfile has completely been written to.

Simple Examples

The following examples actually only pipe data from stdin to stdout, but via child processes with different temp-file options.

   var ProcessStream = require("process-streams");
   var ps = new ProcessStream();
   // Temporary files for input and output
   process.stdin.pipe(ps.exec("cp <INPUT> <OUTPUT>")).pipe(process.stdout);
   process.stdin.pipe(ps.spawn("cp",["<INPUT>","<OUTPUT>"])).pipe(process.stdout);
   process.stdin.pipe(ps.execFile("cp",["<INPUT>","<OUTPUT>"])).pipe(process.stdout);
 
   // Stream input, use temp-file for output
   process.stdin.pipe(ps.spawn("tee",["<OUTPUT>"])).pipe(process.stdout);
 
   // Temp-file for input, Stream for output
   process.stdin.pipe(ps.spawn("cat ",["<INPUT>"])).pipe(process.stdout);
 
   // Pipe both sides
   process.stdin.pipe(ps.spawn("cat")).pipe(process.stdout);

Changing the placeholder tokens

The tokens <INPUT> and <OUTPUT> can be changed:

   var ProcessStream = require("process-streams");
   var ps = new ProcessStream('[IN]','[OUT]');
   process.stdin.pipe(ps.exec("cp [IN] [OUT]")).pipe(process.stdout);

Events

Process errors (such as not finding the executable file) are emitted on the resulting stream as 'error' event. The 'started' event is emitted when the is started. Its first argument is the child-process object, second and third arguments are the command and args passed to ps.exec, ps.spawn or ps.execFile), but with the placeholders resolved to the their actual temporary files.

    var ProcessStream = require("process-streams");
    var ps = new ProcessStream('[IN]','[OUT]');
    process.stdin.pipe(ps.spawn("cp", ["[IN]","[OUT]"])).on("error", function(err) {
        // Handle errors
    }).on("input-closed", function(err) {
       // Handle ECONNRESET and EPIPE processe's stdin
    }).on("started", function(process, command, args) {
       // If "ps.exec" is called, 'command' contains the whole resolved command and 'args' is undefined.
    }).on("exit", function(code, signel) {
      // see the 'child_process' documentation for the 'exit'-event.
    }).pipe(process.stdout);

As of version 1.0.0 the API will only be changed in accordance to semver. Feedback is welcome, although I cannot guarantee any response times at the moment.

Changes

1.0.1

  • Testcase-fixes for iojs 1.2 and node 0.12
  • Changed some dependencies to stable versions of other packages

1.0.0

  • There are no API changes in this version, but I have decided that the API should be stable now. Thus, version 1.0.0

0.4.5

  • All testcases should now run after npm install. All test-data is provided in dependencies (even for testECONRESET.js). package.json is now complete, the README updated.

0.4.4

  • Added license information to package.json

0.4.3

  • Fixed error handling for exec and execFile
  • Callback for exec and execFile is now forwarded to child_process at the correct location, so that callbacks actually get called.

0.4.2

  • When using no in-tempfile, it may happen that the command (e.g. 'head -2') close the input stream before it is completely read. This may result in a EPIPE or ECONNRESET but is not an actual error, since the output is still correct. This error does not cause an error-event anymore, but an input-closed event.

Readme

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Install

npm i process-streams

Weekly Downloads

32,165

Version

1.0.1

License

MIT

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Collaborators

  • knappi