I recently burned some time because I didn't realize that
System.Process.createProcess doesn't report when the binary isn't
found. I guess I was used to that behaviour from python's subprocess
module, which goes to some effort to report the error from the
fork()ed child when exec() fails. After a failed solution, I did some
poking around in the source to see exactly what haskell's process
does.
In the interests of saving someone else that time, I submit a patch to
process to add some documentation. I'm not sure how to do a 'darcs
send' with git, but here's the patch:
% git log | head
commit 5a46937961b3400fa58ccffc785d0de8c1c01d94
Author: Evan Laforge
(_, Just hout, _, _) <- -> createProcess (proc "ls" []){ cwd = Just "\home\bob", +> createProcess (proc "ls" []){ cwd = Just "/home/bob", std_out = CreatePipe }
+Note that this does /not/ throw an error if the binary is not found, +or not executable. Instead, the process will fail with the exit code 127. +But you can't rely on it failing quickly, because it may e.g. hang up +searching the PATH on a slow NFS mount. Since there's no way to tell when +the exec() has succeeded, the only way to reliably know if the createProcess +failed is to 'waitForProcess'. + +If the CreateProcess record's 'cwd' directory doesn't exist, the exit code +will be 126. -} createProcess :: CreateProcess diff --git a/System/Process/Internals.hs b/System/Process/Internals.hs index 11de1f4..2a6c047 100644 --- a/System/Process/Internals.hs +++ b/System/Process/Internals.hs @@ -199,7 +199,8 @@ data CmdSpec = ShellCommand String -- ^ a command line to execute using the shell | RawCommand FilePath [String] - -- ^ the filename of an executable with a list of arguments + -- ^ The filename of an executable with a list of arguments. + -- The PATH is searched for the executable. data StdStream = Inherit -- ^ Inherit Handle from parent