Build a JSON string with Bash variables

JsonBashQuoting

Json Problem Overview

I need to read these bash variables into my JSON string and I am not familiar with bash. any help is appreciated.

#!/bin/sh

BUCKET_NAME=testbucket
OBJECT_NAME=testworkflow-2.0.1.jar
TARGET_LOCATION=/opt/test/testworkflow-2.0.1.jar

JSON_STRING='{"bucketname":"$BUCKET_NAME"","objectname":"$OBJECT_NAME","targetlocation":"$TARGET_LOCATION"}'


echo $JSON_STRING

Json Solutions

Solution 1 - Json

You are better off using a program like jq to generate the JSON, if you don't know ahead of time if the contents of the variables are properly escaped for inclusion in JSON. Otherwise, you will just end up with invalid JSON for your trouble.

BUCKET_NAME=testbucket
OBJECT_NAME=testworkflow-2.0.1.jar
TARGET_LOCATION=/opt/test/testworkflow-2.0.1.jar

JSON_STRING=$( jq -n \
                  --arg bn "$BUCKET_NAME" \
                  --arg on "$OBJECT_NAME" \
                  --arg tl "$TARGET_LOCATION" \
                  '{bucketname: $bn, objectname: $on, targetlocation: $tl}' )

Solution 2 - Json

You can use printf:

JSON_FMT='{"bucketname":"%s","objectname":"%s","targetlocation":"%s"}\n'
printf "$JSON_FMT" "$BUCKET_NAME" "$OBJECT_NAME" "$TARGET_LOCATION"

much clear and simpler

Solution 3 - Json

A possibility:

JSON_STRING='{"bucketname":"'"$BUCKET_NAME"'","objectname":"'"$OBJECT_NAME"'","targetlocation":"'"$TARGET_LOCATION"'"}'

Solution 4 - Json

In addition to chepner's answer, it's also possible to construct the object completely from args with this simple recipe:

BUCKET_NAME=testbucket
OBJECT_NAME=testworkflow-2.0.1.jar
TARGET_LOCATION=/opt/test/testworkflow-2.0.1.jar

JSON_STRING=$(jq -n \
                  --arg bucketname "$BUCKET_NAME" \
                  --arg objectname "$OBJECT_NAME" \
                  --arg targetlocation "$TARGET_LOCATION" \
                   '$ARGS.named')

Explanation:

  • --null-input | -n disabled reading input. From the man page: Don't read any input at all! Instead, the filter is run once using null as the input. This is useful when using jq as a simple calculator or to construct JSON data from scratch.
  • --arg name value passes values to the program as predefined variables: value is available as $name. All named arguments are also available as $ARGS.named

Because the format of $ARGS.named is already an object, jq can output it as is.

Solution 5 - Json

First, don't use ALL_CAPS_VARNAMES: it's too easy to accidentally overwrite a crucial shell variable (like PATH)

Mixing single and double quotes in shell strings can be a hassle. In this case, I'd use printf:

bucket_name=testbucket
object_name=testworkflow-2.0.1.jar
target_location=/opt/test/testworkflow-2.0.1.jar
template='{"bucketname":"%s","objectname":"%s","targetlocation":"%s"}'

json_string=$(printf "$template" "$BUCKET_NAME" "$OBJECT_NAME" "$TARGET_LOCATION")

echo "$json_string"

For homework, read this page carefully: Security implications of forgetting to quote a variable in bash/POSIX shells

A note on creating JSON with string concatenation: there are edge cases. For example, if any of your strings contain double quotes, you can broken JSON:

$ bucket_name='a "string with quotes"'
$ printf '{"bucket":"%s"}\n' "$bucket_name"
{"bucket":"a "string with quotes""}

Do do this more safely with bash, we need to escape that string's double quotes:

$ printf '{"bucket":"%s"}\n' "${bucket_name//\"/\\\"}"
{"bucket":"a \"string with quotes\""}

Solution 6 - Json

I had to work out all possible ways to deal json strings in a command request, Please look at the following code to see why using single quotes can fail if used incorrectly.

# Create Release and Tag commit in Github repository

# returns string with in-place substituted variables 

json=$(cat <<-END
	{
		"tag_name": "${version}", 
		"target_commitish": "${branch}", 
		"name": "${title}", 
		"body": "${notes}", 
		"draft": ${is_draft}, 
		"prerelease": ${is_prerelease} 
	}
END
)

# returns raw string without any substitutions
# single or double quoted delimiter - check HEREDOC specs

json=$(cat <<-!"END"   # or 'END' 
	{
		"tag_name": "${version}", 
		"target_commitish": "${branch}", 
		"name": "${title}", 
		"body": "${notes}", 
		"draft": ${is_draft}, 
		"prerelease": ${is_prerelease} 
	}
END
)

# prints fully formatted string with substituted variables as follows:

echo "${json}"  
{ 
    "tag_name" : "My_tag", 
    "target_commitish":"My_branch"
    ....
}

Note 1: Use of single vs double quotes

# enclosing in single quotes means no variable substitution 
# (treats everything as raw char literals)

echo '${json}'   
${json} 

echo '"${json}"'   
"${json}" 

# enclosing in single quotes and outer double quotes causes
# variable expansion surrounded by single quotes(treated as raw char literals).

echo "'${json}'" 
'{ 
    "tag_name" : "My_tag", 
    "target_commitish":"My_branch"
    ....
}'

Note 2: Caution with Line terminators

  • Note the json string is formatted with line terminators such as LF \n
  • or carriage return \r(if its encoded on windows it contains CRLF \r\n)
  • using (translate) tr utility from shell we can remove the line terminators if any

# following code serializes json and removes any line terminators 
# in substituted value/object variables too

json=$(echo "$json" | tr -d '\n' | tr -d '\r' )

# string enclosed in single quotes are still raw literals

echo '${json}'   
${json} 

echo '"${json}"'   
"${json}" 

# After CRLF/LF are removed

echo "'${json}'" 
'{ "tag_name" : "My_tag", "target_commitish":"My_branch" .... }'

Note 3: Formatting

  • while manipulating json string with variables, we can use combination of ' and " such as following, if we want to protect some raw literals using outer double quotes to have in place substirution/string interpolation:
# mixing ' and " 

username=admin
password=pass

echo "$username:$password"
admin:pass

echo "$username"':'"$password"
admin:pass

echo "$username"'[${delimiter}]'"$password"
admin[${delimiter}]pass

Note 4: Using in a command

  • Following curl request already removes existing \n (ie serializes json)
response=$(curl -i \
            --user ${username}:${api_token} \
            -X POST \
            -H 'Accept: application/vnd.github.v3+json' \
            -d "$json" \
            "https://api.github.com/repos/${username}/${repository}/releases" \
            --output /dev/null \
            --write-out "%{http_code}" \
            --silent
          )

So when using it for command variables, validate if it is properly formatted before using it :)

Solution 7 - Json

If you need to build a JSON representation where members mapped to undefined or empty variables should be ommited, then jo can help.

#!/bin/bash

BUCKET_NAME=testbucket
OBJECT_NAME=""

JO_OPTS=()

if [[ ! "${BUCKET_NAME}x" = "x" ]] ; then
		JO_OPTS+=("bucketname=${BUCKET_NAME}")
fi

if [[ ! "${OBJECT_NAME}x" = "x" ]] ; then
		JO_OPTS+=("objectname=${OBJECT_NAME}")
fi

if [[ ! "${TARGET_LOCATION}x" = "x" ]] ; then
		JO_OPTS+=("targetlocation=${TARGET_LOCATION}")
fi

jo "${JO_OPTS[@]}"

Solution 8 - Json

can be done following way:

JSON_STRING='{"bucketname":"'$BUCKET_NAME'","objectname":"'$OBJECT_NAME'","targetlocation":"'$TARGET_LOCATION'"}'

Solution 9 - Json

For Node.js Developer, or if you have node environment installed, you can try this:

JSON_STRING=$(node -e "console.log(JSON.stringify({bucketname: $BUCKET_NAME, objectname: $OBJECT_NAME, targetlocation: $TARGET_LOCATION}))")

Advantage of this method is you can easily convert very complicated JSON Object (like object contains array, or if you need int value instead of string) to JSON String without worrying about invalid json error.

Disadvantage is it's relying on Node.js environment.

Solution 10 - Json

To build upon Hao's answer using NodeJS: you can split up the lines, and use the -p option which saves having to use console.log.

JSON_STRING=$(node -pe "
  JSON.stringify({
    bucketname: process.env.BUCKET_NAME,
    objectname: process.env.OBJECT_NAME,
    targetlocation: process.env.TARGET_LOCATION
  });
")

An inconvenience is that you need to export the variables beforehand, i.e.

export BUCKET_NAME=testbucket
# etc.

Note: You might be thinking, why use process.env? Why not just use single quotes and have bucketname: '$BUCKET_NAME', etc so bash inserts the variables? The reason is that using process.env is safer - if you don't have control over the contents of $TARGET_LOCATION it could inject JavaScript into your node command and do malicious things (by closing the single quote, e.g. the $TARGET_LOCATION string contents could be '}); /* Here I can run commands to delete files! */; console.log({'a': 'b. On the other hand, process.env takes care of sanitising the input.

Solution 11 - Json

These solutions come a little late but I think they are inherently simpler that previous suggestions (avoiding the complications of quoting and escaping).

    BUCKET_NAME=testbucket
    OBJECT_NAME=testworkflow-2.0.1.jar
    TARGET_LOCATION=/opt/test/testworkflow-2.0.1.jar
    
    # Initial unsuccessful solution
    JSON_STRING='{"bucketname":"$BUCKET_NAME","objectname":"$OBJECT_NAME","targetlocation":"$TARGET_LOCATION"}'
    echo $JSON_STRING 
    
    # If your substitution variables have NO whitespace this is sufficient
    JSON_STRING=$(tr -d [:space:] <<JSON
    {"bucketname":"$BUCKET_NAME","objectname":"$OBJECT_NAME","targetlocation":"$TARGET_LOCATION"}
    JSON
    )
    echo $JSON_STRING 
    
    # If your substitution variables are more general and maybe have whitespace this works
    JSON_STRING=$(jq -c . <<JSON
    {"bucketname":"$BUCKET_NAME","objectname":"$OBJECT_NAME","targetlocation":"$TARGET_LOCATION"}
    JSON
    )
    echo $JSON_STRING 
    
    #... A change in layout could also make it more maintainable
    JSON_STRING=$(jq -c . <<JSON
    {
       "bucketname" : "$BUCKET_NAME",
       "objectname" : "$OBJECT_NAME",
       "targetlocation" : "$TARGET_LOCATION"
    }
    JSON
    )
    echo $JSON_STRING

Solution 12 - Json

You could use envsubst:

  export VAR="some_value_here"
  echo '{"test":"$VAR"}' | envsubst > json.json

also it might be a "template" file:

//json.template
{"var": "$VALUE", "another_var":"$ANOTHER_VALUE"}

So after you could do:

export VALUE="some_value_here"
export ANOTHER_VALUE="something_else"
cat  json.template | envsubst > misha.json

Solution 13 - Json

Bash will not insert variables into a single-quote string. In order to get the variables bash needs a double-quote string. You need to use double-quote string for the JSON and just escape double-quote characters inside JSON string. Example:

#!/bin/sh

BUCKET_NAME=testbucket
OBJECT_NAME=testworkflow-2.0.1.jar
TARGET_LOCATION=/opt/test/testworkflow-2.0.1.jar

JSON_STRING="{\"bucketname\":\"$BUCKET_NAME\",\"objectname\":\"$OBJECT_NAME\",\"targetlocation\":\"$TARGET_LOCATION\"}"


echo $JSON_STRING

Solution 14 - Json

if you have node.js and get minimist installed in global:

jc() {
    node -p "JSON.stringify(require('minimist')(process.argv), (k,v) => k=='_'?undefined:v)" -- "$@"
}
jc --key1 foo --number 12 --boolean \
    --under_score 'abc def' --'white space' '   '
# {"key1":"foo","number":12,"boolean":true,"under_score":"abc def","white space":"   "}

you can post it with curl or what:

curl --data "$(jc --type message --value 'hello world!')" \
    --header 'content-type: application/json' \
    http://server.ip/api/endpoint

be careful that minimist will parse dot:

jc --m.room.member @gholk:ccns.io
# {"m":{"room":{"member":"@gholk:ccns.io"}}}
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Content TypeOriginal AuthorOriginal Content on Stackoverflow
Questionnad87563View Question on Stackoverflow
Solution 1 - JsonchepnerView Answer on Stackoverflow
Solution 2 - JsonDiego Torres MilanoView Answer on Stackoverflow
Solution 3 - JsonCyrusView Answer on Stackoverflow
Solution 4 - JsonEvy BongersView Answer on Stackoverflow
Solution 5 - Jsonglenn jackmanView Answer on Stackoverflow
Solution 6 - Jsonmahee96View Answer on Stackoverflow
Solution 7 - JsonGuillermo López AlejosView Answer on Stackoverflow
Solution 8 - JsonPrasad WargadView Answer on Stackoverflow
Solution 9 - JsonHaoView Answer on Stackoverflow
Solution 10 - JsonbinaryfuntView Answer on Stackoverflow
Solution 11 - JsonSteve ElliottView Answer on Stackoverflow
Solution 12 - JsontamerlahaView Answer on Stackoverflow
Solution 13 - Jsonkiko283View Answer on Stackoverflow
Solution 14 - JsongholkView Answer on Stackoverflow