After upgrading to 5.26 I enabled Bleve and set it to index. After this it seems as though shell commands via mattermost no longer function.
For example, if I execute a simple:
$ /opt/mattermost/bin/mattermost channel list TEAM_NAME
with Bleve enabled, I just get:
{"level":"info","msg":"Server is initializing..."}
2020-08-16T20:30:41.846Z info app/web_hub.go:83 Starting websocket hubs {"number_of_hubs": 2}
2020-08-16T20:30:41.847Z info bleveengine/bleve.go:157 EXPERIMENTAL: Starting Bleve
^C
After I disable Bleve again within the System Console shell commands work again just fine.
I also believe Bleve is not actually indexing when turned on because it stays at 0% indexed on a relatively small Mattermost installation after even two hours of it just sitting there.
Enabling Bleve search engine makes the Command Line Interface (CLI) mutually exclusive with the running server. This issue does not apply when using mmctl Command Line Tool.