Store JSON directly in bash script with variables?

Question

I'm going to preface by saying that "no, find a different way to do it" is an acceptable answer here.

Is there a reliable way to store a short bit of JSON in a bash variable for use in a AWS CLI command running from the same script?

I'll be running a job from Jenkins that's updating an AWS Route53 record, which requires UPSERTing a JSON file with the change in records. Because it's running from Jenkins, there's no local storage where I can keep this file, and I'd really like to avoid needing to do a git checkout every time this project will run (which will be once an hour).

Ideally, storing the data in a variable ($foo) and calling it as part of the change-resource-record-sets command would be most convenient given the Jenkins setup, but I'm unfamiliar with exactly how to quote/store JSON inside bash - can it be done safely?

The specific JSON in this case is the following;

{"Comment":"Update DNSName.","Changes":[{"Action":"UPSERT","ResourceRecordSet":{"Name":"alex.","Type":"A","AliasTarget":{"HostedZoneId":"######","DNSName":"$bar","EvaluateTargetHealth":false}}}]} 

As an added complication the DNSName value - $bar - needs to be expanded.

Answer 1

You could use a here-doc:

foo=$(cat <<EOF {"Comment":"Update DNSName.","Changes":[{"Action":"UPSERT","ResourceRecordSet":{"Name":"alex.","Type":"A","AliasTarget":{"HostedZoneId":"######","DNSName":"$bar","EvaluateTargetHealth":false}}}]} EOF ) 

By leaving EOF in the first line unquoted, the contents of the here-doc will be subject to parameter expansion, so your $bar expands to whatever you put in there.

If you can have linebreaks in your JSON, you can make it a little more readable:

foo=$(cat <<EOF {   "Comment": "Update DNSName.",   "Changes": [     {       "Action": "UPSERT",       "ResourceRecordSet": {         "Name": "alex.",         "Type": "A",         "AliasTarget": {           "HostedZoneId": "######",           "DNSName": "$bar",           "EvaluateTargetHealth": false         }       }     }   ] } EOF ) 

or even (first indent on each line must be a tab)

foo=$(cat <<-EOF     {       "Comment": "Update DNSName.",       "Changes": [         {           "Action": "UPSERT",           "ResourceRecordSet": {             "Name": "alex.",             "Type": "A",             "AliasTarget": {               "HostedZoneId": "######",               "DNSName": "$bar",               "EvaluateTargetHealth": false             }           }         }       ]     }     EOF ) 

and to show how that is stored, including quoting (assuming that bar=baz):

$ declare -p foo declare -- foo="{   \"Comment\": \"Update DNSName.\",   \"Changes\": [     {       \"Action\": \"UPSERT\",       \"ResourceRecordSet\": {         \"Name\": \"alex.\",         \"Type\": \"A\",         \"AliasTarget\": {           \"HostedZoneId\": \"######\",           \"DNSName\": \"baz\",           \"EvaluateTargetHealth\": false         }       }     }   ] }" 

Because this expands some shell metacharacters, you could run into trouble if your JSON contains something like `, so alternatively, you could assign directly, but be careful about quoting around $bar:

foo='{"Comment":"Update DNSName.","Changes":[{"Action":"UPSERT","ResourceRecordSet":{"Name":"alex.","Type":"A","AliasTarget":{"HostedZoneId":"######","DNSName":"'"$bar"'","EvaluateTargetHealth":false}}}]}' 

Notice the quoting for $bar: it's

"'"$bar"'" │││    │││ │││    ││└ literal double quote │││    │└ opening syntactical single quote │││    └ closing syntactical double quote ││└ opening syntactical double quote │└ closing syntactical single quote └ literal double quote