为什么Open3.popen3在缺少可执行文件时返回错误的错误？

Question

我正在围绕CLI创建一个Ruby包装器.我发现了一个简洁的方法Open3.capture3(内部使用Open3.popen3),它允许我执行命令并捕获stdout,stderr和退出代码.

我想要检测的一件事是,是否找不到CLI可执行文件(并为此引发特殊错误).我知道UNIX shell 127在找不到命令时会提供退出代码.当我$ foo在bash中执行时,我得到了-bash: foo: command not found,这正是我想要显示的错误消息.

考虑到所有这些,我编写了这样的代码:

require "open3"

stdout, stderr, status = Open3.capture3(command)
case status.exitstatus
when 0
  return stdout
when 1, 127
  raise MyError, stderr
end

但是,当我运行它时command = "foo",我收到一个错误:

Errno::ENOENT: No such file or directory - foo
  /Users/janko/.rbenv/versions/2.1.3/lib/ruby/2.1.0/open3.rb:193:in `spawn'
  /Users/janko/.rbenv/versions/2.1.3/lib/ruby/2.1.0/open3.rb:193:in `popen_run'
  /Users/janko/.rbenv/versions/2.1.3/lib/ruby/2.1.0/open3.rb:93:in `popen3'
  /Users/janko/.rbenv/versions/2.1.3/lib/ruby/2.1.0/open3.rb:252:in `capture3'

为什么会出现此错误？我以为Open3.capture3应该直接在shell中执行该命令,为什么我不能得到正常的STDERR并退出代码127？

Answer 1

Open3.popen3委托给Kernel.spawn,取决于传递命令的方式,将命令提供给shell或直接给OS.

commandline                 : command line string which is passed to the standard shell
cmdname, arg1, ...          : command name and one or more arguments (This form does not use the shell. See below for caveats.)
[cmdname, argv0], arg1, ... : command name, argv[0] and zero or more arguments (no shell)

我们可能期望,如果我们调用Kernel.spawn("foo")它,它将被传递给shell(而不是OS).但它没有,文档Kernel.exec解释了原因:

If the string from the first form (exec("command")) follows these simple rules:

* no meta characters
* no shell reserved word and no special built-in
* Ruby invokes the command directly without shell

You can force shell invocation by adding ";" to the string (because ";" is a meta character).

最后一段揭示了解决方案.

require "open3"

stdout, stderr, status = Open3.capture3(command + ";")
case status.exitstatus
when 0
  return stdout
when 1, 127
  raise MyError, stderr
end