startup script supports reading environment variable for some param

[zhipcui]

[szaboferee]

Hi @zhipcui why do you need this?

These are internal variables and they can be set with different parameters.

What are you trying to achieve?

[zhipcui]

All these variables can be set by flume-env.sh file on conf dir. Take JAVA_OPTS as example, if I want to change it, I need flume-env.sh file.
Setting this variables from environment variable would be a more clean way especially run the flume using Kubernetes.

[asfgit]

Can one of the admins verify this patch?

[majorendre]

LGTM

[turcsanyip]

We can add these environment variables, but I do not think it is really necessary.

Regarding JAVA_OPTS: yes, it is common the pass parameters and system properties to java, but FLUME_JAVA_OPTS can be used for this and can be defined as an environment variable, FLUME_JAVA_OPTS is appended after JAVA_OPTS so it has higher precedence

Regarding classpath: the best practice for using third party libraries is through plugins (by default the plugins.d directory), so adjusting the classpath should be needed in quite rare/special cases only