Historically the shell has accepted a negative value for batchSize and passed this through to the server as a negative "ntoreturn" value. The query

db.collection.find().batchSize(-8);

means return up to 8 documents in the initial response batch, and then close the cursor.

When the shell is configured with --readMode commands, however, it currently passes the negative batchSize through to the server as a find command parameter. The server's find command implementation rejects negative batchSize:

> db.c.drop()
true
> db.c.insert({a: 1})
WriteResult({ "nInserted" : 1 })
> db.c.find().batchSize(-1)
Error: error: {
	"waitedMS" : NumberLong(0),
	"ok" : 0,
	"errmsg" : "batchSize value must be non-negative",
	"code" : 2
}

This behavior is inconsistent with the drivers find command specification for the behavior of limit, skip, and batchSize. Instead, the shell should translate a batchSize of -n to a batchSize of +n with the singleBatch option set to true.