doc: cover stdio option in child_process

sam-github · chrisdickinson · commit 3a08b7c3e0bc · 2014-11-18T19:28:46.000-08:00
- Add hyperlinks from spawn options to subsections detailing what those options do. - Clarify some verbiage around ChildProcess.prototype.std{in,out,err}. - Remove second-person pronoun. PR-URL: #8639 Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>
diff --git a/doc/api/child_process.markdown b/doc/api/child_process.markdown
@@ -90,29 +90,71 @@ Messages send by `.send(message, [sendHandle])` are obtained using the
 * {Stream object}
 
 A `Writable Stream` that represents the child process's `stdin`.
-Closing this stream via `end()` often causes the child process to terminate.
+If the child is waiting to read all its input, it will not continue until this
+stream has been closed via `end()`.
 
-If the child stdio streams are shared with the parent, then this will
+If the child was not spawned with `stdio[0]` set to `'pipe'`, then this will
 not be set.
 
+`child.stdin` is shorthand for `child.stdio[0]`. Both properties will refer
+to the same object, or null.
+
 ### child.stdout
 
 * {Stream object}
 
 A `Readable Stream` that represents the child process's `stdout`.
 
-If the child stdio streams are shared with the parent, then this will
+If the child was not spawned with `stdio[1]` set to `'pipe'`, then this will
 not be set.
 
+`child.stdout` is shorthand for `child.stdio[1]`. Both properties will refer
+to the same object, or null.
+
 ### child.stderr
 
 * {Stream object}
 
 A `Readable Stream` that represents the child process's `stderr`.
 
-If the child stdio streams are shared with the parent, then this will
+If the child was not spawned with `stdio[2]` set to `'pipe'`, then this will
 not be set.
 
+`child.stderr` is shorthand for `child.stdio[2]`. Both properties will refer
+to the same object, or null.
+
+### child.stdio
+
+* {Array}
+
+A sparse array of pipes to the child process, corresponding with positions in
+the [stdio](#child_process_options_stdio) option to
+[spawn](#child_process_child_process_spawn_command_args_options) that have been
+set to `'pipe'`.
+Note that streams 0-2 are also available as ChildProcess.stdin,
+ChildProcess.stdout, and ChildProcess.stderr, respectively.
+
+In the following example, only the child's fd `1` is setup as a pipe, so only
+the parent's `child.stdio[1]` is a stream, all other values in the array are
+`null`.
+
+    child = child_process.spawn("ls", {
+        stdio: [
+          0, // use parents stdin for child
+          'pipe', // pipe child's stdout to parent
+          fs.openSync("err.out", "w") // direct child's stderr to a file
+        ]
+    });
+
+    assert.equal(child.stdio[0], null);
+    assert.equal(child.stdio[0], child.stdin);
+
+    assert(child.stdout);
+    assert.equal(child.stdio[1], child.stdout);
+
+    assert.equal(child.stdio[2], null);
+    assert.equal(child.stdio[2], child.stderr);
+
 ### child.pid
 
 * {Integer}
@@ -299,11 +341,13 @@ child process has any open IPC channels with the parent (i.e `fork()`).
 * `args` {Array} List of string arguments
 * `options` {Object}
   * `cwd` {String} Current working directory of the child process
-  * `stdio` {Array|String} Child's stdio configuration. (See below)
-  * `customFds` {Array} **Deprecated** File descriptors for the child to use
-    for stdio.  (See below)
   * `env` {Object} Environment key-value pairs
-  * `detached` {Boolean} The child will be a process group leader.  (See below)
+  * `stdio` {Array|String} Child's stdio configuration. (See
+    [below](#child_process_options_stdio))
+  * `customFds` {Array} **Deprecated** File descriptors for the child to use
+    for stdio.  (See [below](#child_process_options_customFds))
+  * `detached` {Boolean} The child will be a process group leader.  (See
+    [below](#child_process_options_detached))
   * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
   * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
 * return: {ChildProcess object}
@@ -317,8 +361,11 @@ The third argument is used to specify additional options, which defaults to:
       env: process.env
     }
 
-`cwd` allows you to specify the working directory from which the process is spawned.
-Use `env` to specify environment variables that will be visible to the new process.
+Use `cwd` to specify the working directory from which the process is spawned.
+If not given, the default is to inherit the current working directory.
+
+Use `env` to specify environment variables that will be visible to the new
+process, the default is `process.env`.
 
 Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
 
@@ -374,24 +421,16 @@ Example: A very elaborate way to run 'ps ax | grep ssh'
     });
 
 
-Example of checking for failed exec:
-
-    var spawn = require('child_process').spawn,
-        child = spawn('bad_command');
+### options.stdio
 
-    child.stderr.setEncoding('utf8');
-    child.stderr.on('data', function (data) {
-      if (/^execvp\(\)/.test(data)) {
-        console.log('Failed to start child process.');
-      }
-    });
+As a shorthand, the `stdio` argument may also be one of the following
+strings:
 
-Note that if spawn receives an empty options object, it will result in
-spawning the process with an empty environment rather than using
-`process.env`. This due to backwards compatibility issues with a deprecated
-API.
+* `'pipe'` - `['pipe', 'pipe', 'pipe']`, this is the default value
+* `'ignore'` - `['ignore', 'ignore', 'ignore']`
+* `'inherit'` - `[process.stdin, process.stdout, process.stderr]` or `[0,1,2]`
 
-The 'stdio' option to `child_process.spawn()` is an array where each
+Otherwise, the 'stdio' option to `child_process.spawn()` is an array where each
 index corresponds to a fd in the child.  The value is one of the following:
 
 1. `'pipe'` - Create a pipe between the child process and the parent process.
@@ -422,13 +461,6 @@ index corresponds to a fd in the child.  The value is one of the following:
    words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the
    default is `'ignore'`.
 
-As a shorthand, the `stdio` argument may also be one of the following
-strings, rather than an array:
-
-* `ignore` - `['ignore', 'ignore', 'ignore']`
-* `pipe` - `['pipe', 'pipe', 'pipe']`
-* `inherit` - `[process.stdin, process.stdout, process.stderr]` or `[0,1,2]`
-
 Example:
 
     var spawn = require('child_process').spawn;
@@ -443,6 +475,8 @@ Example:
     // startd-style interface.
     spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
 
+### options.detached
+
 If the `detached` option is set, the child process will be made the leader of a
 new process group.  This makes it possible for the child to continue running 
 after the parent exits.
@@ -471,6 +505,8 @@ will not stay running in the background unless it is provided with a `stdio`
 configuration that is not connected to the parent.  If the parent's `stdio` is
 inherited, the child will remain attached to the controlling terminal.
 
+### options.customFds
+
 There is a deprecated option called `customFds` which allows one to specify
 specific file descriptors for the stdio of the child process. This API was
 not portable to all platforms and therefore removed.
@@ -561,7 +597,6 @@ leaner than `child_process.exec`. It has the same options.
 * `options` {Object}
   * `cwd` {String} Current working directory of the child process
   * `env` {Object} Environment key-value pairs
-  * `encoding` {String} (Default: 'utf8')
   * `execPath` {String} Executable used to create the child process
   * `execArgv` {Array} List of string arguments passed to the executable
     (Default: `process.execArgv`)
