Allow to watch for port being LISTENING

[SheGe]

At the time of creating this request, the watch: true feature does not support using anything other than sources: []. I would like to configure a task that works with the watch: true feature to check for a LISTENING port.

Ideas:

1. Allow the watch setting to accept a boolean or a nested cmd/cmds. The result of custom shell command execution could determine when a task needs to be executed.
2. Upgrade the watch implementation to respect the existing status setting, allowing runs of custom commands without a need to change the schema.
3. Implement watch: true with port: 8080, where the listening check would be a part of the watch task implementation.

[trulede]

You may consider #1951, here the bash command nc (netcat) and a while loop will solve your problem (any many others of a similar nature).

Having watch evaluate a tasks preconditions might be interesting. That makes some sense, since a waiting Task should not run unless a watched source file has changed AND its preconditions are satisfied. It also make things more complicated and difficult to use.

But again, the same thing can be achieved with a bash while loop, that that mechanism is well understood, and easy to adapt ... so perhaps it still a better strategy ; a precondition cmd with a bash while loop, using nc.