readPipe util

1 files changed, 38 insertions, 1 deletions

diff --git a/lib/ProcessUtils.hs b/lib/ProcessUtils.hs
index 4e3ac38..49483b5 100644
--- a/lib/ProcessUtils.hs
+++ b/lib/ProcessUtils.hs
@@ -42,4 +42,41 @@ systemEnv vars cmd = do
       _ <- installHandler sigQUIT old_quit Nothing
       return r
 
-
+-- | readPipe
+--
+-- This is like System.Process.readProcess but instead of a single
+-- command with arguments, you pass it a list which it then executes
+-- piping the the output from the prior command into the input for
+-- the next command. Like readProcess the next parameter is a string
+-- which is sent as input into the initial standard in.
+--
+--  > readPipe [("grep",["HostConfig"]),("wc",["-l"]) =<< readFile "input"
+--
+-- is equivalent to shell code:
+--
+--  > grep HostConfig input | wc -l
+--
+readPipe :: [(FilePath,[String])] -> String -> IO String
+readPipe [(cmd,args)] input = readProcess cmd args input
+readPipe ((cmd,args):xs) stdin0 = do
+    let p = (shell cmd) { std_in = CreatePipe
+                        , std_out = CreatePipe
+                        , std_err = CreatePipe
+                        , cmdspec = RawCommand cmd args
+                        }
+    (Just sinh,Just sout,Just serr, ph) <- createProcess p
+    hPutStr sinh stdin0
+    hClose sinh
+    readPipe0 xs sout
+    where
+        readPipe0 :: [(FilePath,[String])] -> Handle -> IO String
+        readPipe0 ((cmd,args):xs) stdin0 = do
+            let p = (shell cmd)
+                        { std_in = UseHandle stdin0
+                        , std_out = CreatePipe
+                        , std_err = CreatePipe
+                        , cmdspec = RawCommand cmd args
+                        }
+            (Nothing,Just sout,Just serr, ph) <- createProcess p
+            readPipe0 xs sout
+        readPipe0 [] h = hGetContents h