Hello all,
I had a bit of fun recently tracking down quoting issues with the
"system" command in Windows. For the examples below, I'll consistently
use "Windows> " as the beginning of some text sent to the Windows
command prompt cmd.exe, and use "GHC> " as the beginning of some text
sent to a ghci session running in cmd.exe with System.Cmd imported.
The situation is this: I want to hand off a command line which has
both a possibly-quoted command name and a (definitely) quoted
argument. For concreteness, let's use "more" as the command and
"foo.txt" as the argument, so that you can follow along at home on
your favorite Windows system.
Windows> echo foo > foo.txt
Windows> "more" "foo.txt"
foo
All good so far. But:
GHC> system "\"more\" \"foo.txt\""
'more" "foo.txt' is not recognized as an internal or external command,
operable program or batch file.
ExitFailure 1
After some digging, I discovered that system is shipping out to cmd
/c, and indeed:
Windows> cmd /c "more" "foo.txt"
'more" "foo.txt' is not recognized as an internal or external command,
operable program or batch file.
I don't know what the *right* fix is. However, after a bit of playing
around, I discovered the following:
Windows> cmd /c ""more" "foo.txt""
foo
GHC> system "\"\"more\" \"foo.txt\"\""
foo
ExitSuccess
Wrapping commands with an extra pair of double-quotes this way seemed
to give behavior matching the bare cmd.exe for all the examples I
could think of, even ones I thought it would break. For example:
GHC> system "\"more foo.txt\""
foo
ExitSuccess
If this turns out to be the right thing to do, it's pretty easy to
implement. In the commandToProcess function, at
libraries/process/System/Process/Internals.hs:455, the change is just
- return (cmd, translate cmd ++ "/c " ++ string)
+ return (cmd, translate cmd ++ "/c \"" ++ string ++ "\"")
(And in any case, the examples above should answer this amusing
comment, immediately following those lines:
-- We don't want to put the cmd into a single
-- argument, because cmd.exe will not try to split it up. Instead,
-- we just tack the command on the end of the cmd.exe command line,
-- which partly works. There seem to be some quoting issues, but
-- I don't have the energy to find+fix them right now (ToDo). --SDM
-- (later) Now I don't know what the above comment means. sigh.
=)
~d
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