bash script - unable to set variable with double quotes in value - bash

Question

Need help in fixing this bash script to set a variable with a value including double quotes. Somehow I am defining this incorrectly as my values foo and bar are not enclosed in double quotes as needed.
My script thus far:
#!/usr/local/bin/bash
set -e
set -x
host='127.0.0.1'
db='mydev'
_account="foo"
_profile="bar"
_version=$1
_mongo=$(which mongo);
exp="db.profile_versions_20170420.find({account:${_account}, profile:${_profile}, version:${_version}}).pretty();";
${_mongo} ${host}/${db} --eval "$exp"
set +x
Output shows:
+ host=127.0.0.1
+ db=mydev
+ _account=foo
+ _profile=bar
+ _version=201704112004
++ which mongo
+ _mongo=/usr/local/bin/mongo
+ exp='db.profile_versions_20170420.find({account:foo, profile:bar, version:201704112004}).pretty();'
+ /usr/local/bin/mongo 127.0.0.1/mydev --eval 'db.profile_versions_20170420.find({account:foo, profile:bar, version:201704112004}).pretty();'
MongoDB shell version: 3.2.4
connecting to: 127.0.0.1/mydev
2017-04-22T15:32:55.012-0700 E QUERY [thread1] ReferenceError: foo is not defined :
#(shell eval):1:36
What i need is account:"foo", profile:"bar" to be enclosed in double quotes.

Answer 1

In bash (and other POSIX shells), the following 2 states are equivalent:
_account=foo
_account="foo"
What you want to do is to preserve the quotations, therefore you can do the following:
_account='"foo"'

Answer 2

Since part of what you're doing here is forming JSON, consider using jq -- which will guarantee that it's well-formed, no matter what the values are.
host='127.0.0.1'
db='mydev'
_account="foo"
_profile="bar"
_version=$1
json=$(jq -n --arg account "$_account" --arg profile "$_profile" --arg version "$_version" \
'{$account, $profile, version: $version | tonumber}')
exp="db.profile_versions_20170420.find($json).pretty();"
mongo "${host}/${db}" --eval "$exp"
This makes jq responsible for adding literal quotes where appropriate, and will avoid attempted injection attacks (for instance, via a version passed in $1 containing something like 1, "other_argument": "malicious_value"), by replacing any literal " in a string with \"; a literal newline with \n, etc -- or, with the | tonumber conversion, failing outright with any non-numeric value.
Note that some of the syntax above requires jq 1.5 -- if you have 1.4 or prior, you'll want to write {account: $account, profile: $profile} instead of being able to write {$account, $profile} with the key names inferred from the variable names.

Answer 3

When you need to use double quotes inside a double quoted string, escape them with backslashes:
$ foo="acount:\"foo\"" sh -c 'echo $foo'
acount:"foo"

Answer 4

I needed to enquote something already in a variable and stick that in a variable. Expanding on Robert Seaman's answer, I found this worked:
VAR='"'$1'"'
(single quote, double quote, single quote, variable,single quote, double quote, single quote)