Cabal Setup.hs to copy scripts during postInst:

Setup.hs

import Control.Monad ( unless )
import Distribution.Simple
import Distribution.Simple.Program.Types ( Program(..), simpleProgram )
import Distribution.PackageDescription ( PackageDescription(..), executables,
                                         hsSourceDirs, exeName, buildInfo )
import Distribution.Simple.LocalBuildInfo ( LocalBuildInfo(..), buildDir, bindir
                                          , substPathTemplate, absoluteInstallDirs )
import Distribution.Simple.Setup ( InstallFlags(..), BuildFlags, Flag, fromFlag
                                 , buildVerbose, CopyFlags(..), defaultCopyFlags )
import Distribution.Simple.Utils ( rawSystemExit, findProgramVersion)
import Distribution.Verbosity ( Verbosity )

import System.Directory ( doesDirectoryExist, copyFile )
import System.FilePath ( (</>) )

main = defaultMainWithHooks $
       simpleUserHooks { hookedPrograms = [cabalInstallProgram]
                       , postInst = postInstCp (postInst simpleUserHooks)
                       }

type PostInstHook = Args -> InstallFlags -> PackageDescription -> LocalBuildInfo -> IO ()

postInstCp :: PostInstHook -> Args -> InstallFlags -> PackageDescription -> LocalBuildInfo -> IO ()
postInstCp oldHook args iflags pDesc lbi = do
  let -- The filename to copy from.  This assumes 'srcFileName.sh' is adjacent to Setup.hs:
      inFile :: FilePath
      inFile = "srcFileName.sh"

      -- The filename to copy to:
      outFile :: FilePath
      outFile = "destFileName.sh"

      prefix = fromFlag $ installDistPref iflags

      -- Make a concrete binDir from the LocalBuildInfo & PackageDescription:
      instBinDir :: FilePath
      instBinDir = bindir $ absoluteInstallDirs pDesc lbi
                                           (fromFlag $ copyDest defaultCopyFlags)

      -- layer of indirection, in case we wanted to get a specific 
      -- src directory from the cabal file:
      src :: FilePath
      src = inFile

      -- qualify the destination.
      dest :: FilePath
      dest = instBinDir </> outFile

  -- Do the copy, creating outFile in the bin dir:
  copyFile src dest

  -- now invoke the old hook:
  oldHook args iflags pDesc lbi


cabalInstallProgram :: Program
cabalInstallProgram = (simpleProgram "cabal") {
  programFindVersion = findProgramVersion "--numeric-version" id
  }