Error when porting a MongoDB aggregation query to a bash script

Question

I am trying to make a bash script that finds the maximum value in the following collection.

{ "_id" : ObjectId("5eaf83d1f803806404796611"), "livres" : "poeme", "ts" : ISODate("2020-05-04T02:54:09.668Z"), "val" : { "pages" : 30 } }
{ "_id" : ObjectId("5eaf83eff803806404796612"), "livres" : "poeme", "ts" : ISODate("2020-05-04T02:54:39.670Z"), "val" : { "pages" : 50 } }
{ "_id" : ObjectId("5eaf840df803806404796613"), "livres" : "histoire", "ts" : ISODate("2020-05-04T02:55:09.673Z"), "val" : { "pages" : 70 } }
{ "_id" : ObjectId("5eaf842bf803806404796614"), "livres" : "poeme", "ts" : ISODate("2020-05-04T02:55:39.675Z"), "val" : { "pages" : 0 } }
{ "_id" : ObjectId("5eaf8449f803806404796615"), "livres" : "histoire", "ts" : ISODate("2020-05-04T02:56:09.673Z"), "val" : { "pages" : 10 } }
{ "_id" : ObjectId("5eaf8467f803806404796616"), "livres" : "poeme", "ts" : ISODate("2020-05-04T02:56:39.675Z"), "val" : { "pages" : 50 } }
{ "_id" : ObjectId("5eaf8485f803806404796617"), "livres" : "poeme", "ts" : ISODate("2020-05-04T02:57:09.675Z"), "val" : { "pages" : 20 } }

The following query works fine in mongo cli:

db.livres_data.aggregate([{"$match": {"livres": "poeme"}},{"$group": {"_id": null,"MA": {"$max": "$val"}}}])

it returns:

  { "_id" : null, "MA" : { "pages" : 50 } }

When I ported this query to a bash script as below:

\#!/bin/bash

mongo myDB_name --eval "printjson(db.livres_data.aggregate([{'$match': {'livres': '$1'}},{'$group': {'_id': null,'MA': {'$max': 'val'}}}]))"

and execute it as: script_name.sh poeme

I got the following error:

MongoDB shell version v4.2.3
connecting to: mongodb://127.0.0.1:27017/myDB_name?compressors=disabled&gssapiServiceName=mongodb
Implicit session: session { "id" : UUID("d7fa1320-6f51-4836-829d-50b823f5f007") }
MongoDB server version: 4.2.0
2020-05-04T08:38:50.399+0000 E  QUERY    [js] uncaught exception: Error: command failed: {
    "ok" : 0,
    "errmsg" : **"Unrecognized pipeline stage name: ''",**
    "code" : 40324,
    "codeName" : "Location40324"
} : aggregate failed :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:18:14
_assertCommandWorked@src/mongo/shell/assert.js:583:17
assert.commandWorked@src/mongo/shell/assert.js:673:16
DB.prototype._runAggregate@src/mongo/shell/db.js:266:5
DBCollection.prototype.aggregate@src/mongo/shell /collection.js:1012:12
@(shell eval):1:11
2020-05-04T08:38:50.399+0000 E  -        [main] exiting with code -4

Any help is appreciated. Thank you very much.

there's a typo in your script, you should have $val instead of val in your $group stage — Ayoub
– Ayoub, Commented May 4, 2020 at 9:12
you need to be careful with which $variables you want expanded in your script and which should be kept as literals — Mat
– Mat, Commented May 4, 2020 at 11:55
Thank you for your help. I changed as you advised: ` mongo myDB_name --eval "printjson(db.livres_data.aggregate([{'$match': {'livres': '$1'}},{'$group': {'_id': null,'MA': {'$max': '$val'}}}]))" ` but still the same error :( — Sam Carli
– Sam Carli, Commented May 4, 2020 at 13:18
@SamCarli : Why do you've {'$match': {'livres': '$1'}} in bash script ? — whoami - fakeFaceTrueSoul
– whoami - fakeFaceTrueSoul, Commented May 4, 2020 at 16:31
@whoami: in fact I am trying to execute something like this sql query: ** select max(val) from livres_data where livres="poeme". I suppose that **"where livres="poeme" in the sql query is ported to mongodb as {'$match': {'livres': '$1'}} — Sam Carli
– Sam Carli, Commented May 5, 2020 at 2:01

Omkar Kulkarni · Accepted Answer · 2021-01-08 21:48:04Z

1

The issue was solved by changing the " by ' in the query as below:

mongo myDB_name --eval 'printjson(db.livres_data.aggregate([{"$match": {"livres": "'$1'"}},{"$group": {"_id": null,"MA": {"$max": "val"}}}]))'

Also '$1' was changed to "'$1'"

edited Jan 8, 2021 at 21:48

Omkar Kulkarni

1,22512 silver badges26 bronze badges

answered May 14, 2020 at 5:13

Sam Carli

254 bronze badges

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