dist/tools/*/start_network.sh: ensure TUN/TAP interface

[miri64]

Contribution description

Previously, when the creation of the TUN/TAP interface failed in one of the start_network.sh scripts, the script will fail with a cryptic error like

dist/tools/ethos/start_network.sh: 68: [: -eq: unexpected operator

This fix ensures, that the value of this variable checked is always set such that in the error case, ethos/sliptty won't start.

Testing procedure

Ask @benpicco. I was not able to reproduce it.

Issues/PRs references

See #13734 (comment)

[benpicco]

I don't see how that would help - START_ETHOS will be overwritten in any case

[leandrolanzieri]

I don't see how that would help - START_ETHOS will be overwritten in any case

Then, this does not fix your problem? Shall we close?

[miri64]

I don't see how that would help - START_ETHOS will be overwritten in any case

Then, this does not fix your problem? Shall we close?

I don't understand @benpicco's argument to be honest. The error comes because if create_tap fails, START_ETHOS/START_SLIP won't be initialized, having an empty string for numerical comparison. This fix initializes both with a default value, that says "don't start ethos/slip".

[benpicco]

The error comes because if create_tap fails

Ah! I overlooked the && after create_tap, so the if only gets evaluated if that succeeds.

In that case, the patch is the proper thing to do.

[benpicco]

But shouldn't it be START_SLIP=0 so slip does not get started when the interface can not be created?

[miri64]

But shouldn't it be START_SLIP=0 so slip does not get started when the interface can not be created?

START_SLIP=0 would mean it gets started... the value check is for a return code.