NIFI-8215: --wait-for-init feature in nifi.sh refactored

[pgyori]

https://issues.apache.org/jira/browse/NIFI-8215

This makes the --wait-for-init feature in the nifi.sh script runnable in sh on Ubuntu.

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[pgyori]

@jfrazee , may I ask you to have a look at this PR?
Thank you in advance!

[jfrazee]

@pgyori I gave this a very quick test on a system using dash as /bin/sh and it works as expected. Reading the code, the changes look correct too. I'm going to do a few other tests.

[pgyori]

Thank you, @jfrazee !

[kevdoran]

+1, ran shellcheck and tested an assembly based on this in an openjdk:8-jre-alpine, which does not use bash for its POSIX shell

[jfrazee]

@pgyori I should have mentioned this since I saw it yesterday, but this doesn't apply the --wait-for-init to restart since the $@ isn't provided to the start. The naive change would be this:

restart)
    init                                                                                           
    shift
    run "stop"             
    run "start" "$@"
    ;;

I'm not 100% sure whether that's correct though because in run() there's some use of $1 and the shift changes $@. Should we change this now, or do it later?

[pgyori]

@jfrazee I would recommend to postpone this modification until after the release so that we have more time to test it.