don't break when stderr is used

[jomo]

df printing to stderr does not mean the command failed.

for example, on heroku, using df fails:

$ df; echo $?
df: cannot read table of mounted file systems: No such file or directory
1

but using df /app works with a warning:

$ df /app; echo $?
df: Warning: cannot read table of mounted file systems: No such file or directory
Filesystem           1K-blocks Used Available Use% Mounted on
-                        12345  123 1234      123% /app
0

This change first checks if there is an error, and then sets the error's message to whatever stderr is.
It will always call back with the error or stderr only, and with the parsed info from stdout.
However, it will always try to parse, even if stderr is used or an error occured

That makes it possible to use node-df on systems like heroku where df prints warnings to stderr, but works anyway

[jomo]

I think I forgot something, don't merge this yet :P 💩

[jomo]

nvm, works exactly as intended 😎

df({}, function(err, result) {
  console.error("Error", err);
  console.log(result);
});
//=> error { [Error: df: cannot read table of mounted file systems: No such file or directory
//   ] killed: false, code: 1, signal: null, cmd: '/bin/sh -c df -kP' }
//=> undefined

df({file: "/app"/}, function(err, result) {
  console.error("Error", err);
  console.log(result);
});
//=> error df: Warning: cannot read table of mounted file systems: No such file or directory
//=> [ { filesystem: '-',
//   size: 12345,
//   used: 123,
//   available: 1234,
//   capacity: 123,
//   mount: '/app' } ]

[adriano-di-giovanni]

Hi @jomo,
and thanks for your contribution.

Can you tell me about line 21

if (error) { error.message = stderr; }

I think that error and stderr aren't interrelated. Are they?
I mean, you can have stderr without error and maybe error without stderr.

[jomo]

You're right.
The error has no message by default, so if there is something in stderr it might provide more information about the error.

[adriano-di-giovanni]

I mean, you can have error === null and stderr !== null and viceversa.

[jomo]

Yes, that's where

callback(error || stderr, response);

kicks in.
If there is an error, it will include stderr, otherwise it's just stderr.