os/exec: add convenient, portable way to execute "./foo" (foo in working directory)

[alercah]

exec.Command documentation states: "If name contains no path separators, Command uses LookPath to resolve name to a complete path if possible. Otherwise it uses name directly as Path."

filepath.Join documentation states: "Join calls Clean on the result"

filepath.Clean documentation states: "2. Eliminate each . path name element (the current directory)."

The result is that the most obvious way to execute a command in the working directory, exec.Command(filepath.Join(".", cmd)) is buggy. The result of Join is just cmd, and therefore exec.Command will do a PATH lookup before running the command.

There are four reasonable workarounds. The first is to specify the "/" explicitly and just write exec.Command("./" + cmd) and hope it works. The second is to use filepath.Separator manually, writing exec.Command("." + string(filepath.Separator) + cmd) which is hideous. The third is to create the Command directly. The fourth is to use an absolute path via filepath.Abs, but it requires adding an extra error check.

There should be some facility somewhere to let you do this with less of a headache. Whether this is a new method in the filepath package which allows for creation of paths with a leading "./" or a function in exec to create a Command that does not do PATH resolution, I don't think it is important.

[vcabbage]

What about filepath.FromSlash("./"+cmd)?

[alercah]

Ooh, missed that one. Still feels a little hacky, but it's better than the others!

[vcabbage]

Another option, which I just now noticed, is to create the *exec.Cmd without the the use of exec.Command(), since the purpose of that function is to resolve the path.

&exec.Cmd{Path: cmd}

From https://golang.org/pkg/os/exec/#Cmd:

    // Path is the path of the command to run.
    //
    // This is the only field that must be set to a non-zero
    // value. If Path is relative, it is evaluated relative
    // to Dir.
    Path string

   ...

    // Dir specifies the working directory of the command.
    // If Dir is the empty string, Run runs the command in the
    // calling process's current directory.
    Dir string

Edit: exec.Command() properly sets Cmd.Args to []string{cmd, arg1, arg2, ...} as well, so there is one extra step if you have arguments to pass.

[tejasmanohar]

Personally, I don't feel like we need a helper function in os/exec that depends on os.Getwd

[mvdan]

I've only needed to do this a few times and I just used a full path with a directory like os.Getwd. That's at most a few extra lines, and in my opinion it's self-explanatory and simple.

I feel like adding extra API or magic to os/exec to avoid a bit of typing is unnecessary.

[rsc]

This seems like a non-problem. People can write "./foo", which works everywhere already, or they can use a full path with os.Getwd, as @mvdan suggests. If this really happens a lot, then instead of

"." + string(filepath.Separator) + cmd

define

const dotSlash = "." + string(filepath.Separator)

and write dotSlash + cmd.