$ mongoexport -c test -d test --fields x
connected to: 127.0.0.1
{ "_id" : { "$oid" : "5237218d6b8872341646d85f" }, "x" : 1 }
{ "_id" : { "$oid" : "5237218e6b8872341646d860" }, "x" : 1 }
{ "_id" : { "$oid" : "523721916b8872341646d861" }, "x" : 1 }
{ "_id" : { "$oid" : "523721966b8872341646d862" }, "x" : 2 }
{ "_id" : { "$oid" : "523721976b8872341646d863" }, "x" : 2 }
exported 5 records
$ cat fields.txt
x
$ mongoexport -c test -d test --fieldFile fields.txt
connected to: 127.0.0.1
{ "_id" : { "$oid" : "5237218d6b8872341646d85f" }, "x" : 1, "y" : 1 }
{ "_id" : { "$oid" : "5237218e6b8872341646d860" }, "x" : 1, "y" : 1 }
{ "_id" : { "$oid" : "523721916b8872341646d861" }, "x" : 1, "y" : 2 }
{ "_id" : { "$oid" : "523721966b8872341646d862" }, "x" : 2, "y" : 2 }
{ "_id" : { "$oid" : "523721976b8872341646d863" }, "x" : 2, "y" : 1 }
exported 5 records
$