[GHC] #8838: Allow running bash shell commands

[GHC]

#8838: Allow running bash shell commands
--------------------------+------------------------------------------------
       Reporter:          |             Owner:
  jstolarek               |            Status:  new
           Type:          |         Milestone:  7.10.1
  feature request         |           Version:  7.8.1-rc2
       Priority:  normal  |  Operating System:  Linux
      Component:          |   Type of failure:  Incorrect result at runtime
  libraries/process       |         Test Case:
       Keywords:          |          Blocking:
   Architecture:          |
  Unknown/Multiple        |
     Difficulty:          |
  Unknown                 |
     Blocked By:          |
Related Tickets:  #8802   |
--------------------------+------------------------------------------------
 Current implementation of `process` library has limited usability.
 `CreateProcess` record stores `CmdSpec` field, which is either a
 `RawCommand` or `ShellCommand`. The problem is that:

   * `RawCommand` command quotes and escapes the command parameters
   * `ShellCommand` does no escaping but it runs command in `sh` shell

 Corollary: there is no way to run `bash` command with unescaped
 parameters. As a result there is no way to run this command (and many
 others):

 {{{
 diff <(echo $ENV_FOO) <(echo $ENV_BAR)
 }}}

 Running it as a `RawCommand` (using `proc` function) fails because command
 line parameters are escaped and become incorrect. Running it as
 `ShellCommand` (using `shell` function) fails because this is not valid
 `sh` syntax. I propose to create function that allows user to run `bash`
 commands without escaping the parameters (or even better, run any shell
 the user wants). In other words this program:

 {{{
 import System.Exit
 import System.Process

 main :: IO ()
 main = do
   (_, _, _, pid) <- createProcess (SOME_NEW_FUNCTION "diff" ["<(echo
 $FOO)", "<(echo $BAR)"] )
                                         { env = Just
 [("FOO","Foo"),("BAR","Bar")] }
   ecode <- waitForProcess pid
   case ecode of
     ExitSuccess   -> putStrLn "All’s right with the world!"
     ExitFailure _ -> putStrLn ":-("
 }}}

 should produce:

 {{{
 1c1
 < Foo
 ---
 > Bar
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8838>
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