Invoking juju with no verb but a path results in confusing error messages

[David Lawson]

So, running juju with an invalid verb gives you a reasonable error message:

$ juju foo
ERROR unrecognized command: juju foo

Running it with an invalid verb that can be interpreted as a path gives you an error message that's less obvious:

$ juju /foo
ERROR fork/exec juju-/foo: no such file or directory

Even when the path does in fact exist:

$ ls
collect manifest services
$ juju ./services
ERROR fork/exec juju-./services: no such file or directory

I'm assuming this is because it tries to interpret unknown verbs as plugins but I typoed a command and spent quite some time trying to figure out why it claimed it couldn't access a file that I could clearly see on the file system, had no permissions problems, etc. Even updating the error message to refer to no such plugin or something along those lines would be helpful in pointing people in the right direction in this situation.

[David Lawson (deej)]

Ah, this is with 2.3.2, forgot to add that.

[Nam Nguyen (nammn)]

I suspect that this related more to the cmd package from juju than juju itself. But I need to take a further look into it.

[Nam Nguyen (nammn)]

The place where this can be changed in juju would be here:
https://github.com/juju/juju/blob/develop/cmd/juju/commands/plugin.go#L77

This usually should trigger if we don't have the file in the path, so that should be fine.
https://github.com/juju/juju/blob/develop/cmd/juju/commands/plugin.go#L73

The "problem" is that the "command" package normally returns if the file exists or not. But if the name contains a path, it will be cut and we still try to run it.
E.g. "./foobar" command cuts it to be "foobar" and thinks because we gave a path this has to exist.
Thus leading to the error: "fork/exec"

With this we can change it ~~around here https://github.com/juju/juju/blob/develop/cmd/juju/commands/plugin.go#L73
with one of the following solutions:

If this error "fork/exec" returns it means that we tried to interpret a non existing file. We can either:

`if err is kind of "fork/exec"`
1. report it as an error : `return error.Errorf("....")`
2. try to find a similiar command `return callback(...)`

[Nam Nguyen (nammn)]

PR for discussion open: https://github.com/juju/juju/pull/10683