$ find $(brew --cellar elasticsearch) -type f -name plugin
/usr/local/Cellar/elasticsearch/2.2.0_1/libexec/bin/plugin
from the command line (kegs are symlinked from /usr/local/opt/, so you can get to it from either of these).
The scripts in bin off a keg are symlinked from /usr/local/bin which is usually included in your PATH. However, plugin is too generic of a name to be included on the path. Lastly, the script will be renamed to elasticsearch-plugin in a future version of Elasticsearch; when that happens we will revisit this.
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