Add access to HOSTNAME enviroment variable

[coolacid]

$HOSTNAME is a bash specific variable, and not exported by default.

This PR will check to see if the BASH $HOSTNAME is available, otherwise set it using the hostname command.

Then, EXPORT this variable for access using the ENV filter.

[jordansissel]

Curious, what does this fix? I don't think we use $HOSTNAME anywhere, do we? Plugins should be using Socket#gethostname if they want to know the name of the machine.

[coolacid]

It allows users to reference the logstash host name in the ENV filter. This can be set to say a field. This then can be referenced in future filters or outputs. For example, I'd like to reference the logstash indexer host name when defining outputs to graphite, or statsd.

It's also useful should you want to log/trace the path of an event by setting fields at each processing point.

I'm sure there are other uses, just can't think of them off the top of my head.

[coolacid]

@jameshueston We talked about this on IRC - if it works can you give a 👍

[elasticsearch-release]

Can one of the admins verify this patch?

[elasticsearch-release]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run; then say 'jenkins, test it'.

[elasticsearch-release]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run; then say 'jenkins, test it'.