Re: [Haskell-beginners] "system" call uses a different shell, or does not pick up the whole environment

28 Aug 2012


      Ooh, neat, I didn't even know about that option. =)

On Tue, Aug 28, 2012 at 08:40:09AM -0700, Matthew wrote:
...
Not to further discourage you from experimenting, but xargs can also
run commands in parallel. Check out the -P argument. :)
On Tue, Aug 28, 2012 at 8:19 AM, Hong Yang  wrote:
...
Hi Brent,
Thanks for the xargs command info. I did not know it before.
The other reason I want to play with my mapm version is eventually I want to
make it concurrent.
Thanks again,
Hong
On Tue, Aug 28, 2012 at 10:08 AM, Brent Yorgey 
wrote:
...
I do not know the solution to your problem -- dealing with shells,
environments, etc. can be tricky.
However, do you know about the 'xargs' command?  E.g. your example
could be accomplished with
ls | xargs -L 1 -I{} cp -pr {} destination_dir/{}
-Brent
On Tue, Aug 28, 2012 at 09:58:16AM -0500, Hong Yang wrote:
...
Hi,
I am trying to mimic mapM() at shell command line. I define the
interface
as "mapm cmd2 cmd1," so cmd2 will be run for each of the cmd1 results.
"$_"
can be used inside cmd2 to represent the current cmd1 result.
For example, the command
        mapm    'cp -pr $_ destination_dir/$_'    ls
copies everything under the current directory to the destination
directory.
The code is as follows:
--
module Main where
import System.Environment ( getArgs )
import System.Exit
import System.IO
import System.Process
import Text.Regex
import Text.Regex.Posix
main = do
    hs_argv <- getArgs
    if length hs_argv /= 2
      then
        putStrLn "wrong arguments!" >> exitFailure
      else do
        let [cmd2, cmd1] = hs_argv
        (_, hOut, hErr, _) <- runInteractiveCommand cmd1
        err <- hGetContents hErr
        hClose hErr
        if null err
          then do
            out <- hGetContents hOut
            mapM (f cmd2) (lines out)
          else
            putStr err >> exitFailure
f :: String -> String -> IO ExitCode
f cmd2 item = system cmd2'
  where cmd2' = if cmd2 =~ "\\$\\_"::Bool
                then subRegex (mkRegex "\\$\\_") cmd2 item
                else cmd2
--
It works, except one issue that is bothering me.
If I issue
        mapm    'lt $_'    ls,
I get a bunch of
        /bin/sh: lt: command not found,
while I expect it act the same as
        mapm    'ls -Alrt --color=auto $_'    ls,
because "lt" is aliased to "ls -Alrt --color=auto."
Notice "/bin/sh" above. My shell is actually tcsh. All the aliases are
in
~/.cshrc.
I tried replacing "system cmd2'" with
        system ("source ~/.cshrc; " ++ cmd2')
    and
        system ("tcsh -c " ++ "'source ~/.cshrc; " ++ cmd2' ++ "'"),
but they did not solve the problem.
Can someone please help me?
Thanks,
Hong
...
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