how can i append a file into another file with a suitable format using script shell

I have this json file template.json

{
    "PTF_INSTALL_DATE": " 2020-03-31 09:12:10",
    "PTF_CONTENT": [
      {
        "NAME": "api_batch_API",
        "CHECKED": "api_batch_NOT_SELECTED",
        "VERSION": "G02R00C13_c14"
      }
    ]
  }

And this file historique.json,It created by script bash , I call the script in Jenkinsfile

  [
   {
        "PTF_INSTALL_DATE": " 2020-03-31 09:12:10",
        "PTF_CONTENT": [
          {
            "NAME": "api_batch_API",
            "CHECKED": "api_batch_NOT_SELECTED",
            "VERSION": "G02R00C13_c14"
          }
        ]
     }
   ]

So when I do a build in jenkins an element has added in the last of file historique.json like below:

[
    {
           "PTF_INSTALL_DATE": " 2020-03-31 09:12:10",
           "PTF_CONTENT": [
             {
                "NAME": "api_batch_API",
                "CHECKED": "api_batch_NOT_SELECTED",
                "VERSION": "G02R00C13_c14"
              }
            ]
         }
]
{
        "PTF_INSTALL_DATE": " 2020-03-31 09:12:10",
        "PTF_CONTENT": [
          {
            "NAME": "api_batch_API",
            "CHECKED": "api_batch_NOT_SELECTED",
            "VERSION": "G02R00C13_c14"
          }
        ]
  }

And ect. So my question is how can I formatted the historique file to be like below:

 [
      {
           "PTF_INSTALL_DATE": " 2020-03-31 09:12:10",
           "PTF_CONTENT": [
                  {
                    "NAME": "api_batch_API",
                    "CHECKED": "api_batch_NOT_SELECTED",
                    "VERSION": "G02R00C13_c14"
                  }
        ]
     },

    {
            "PTF_INSTALL_DATE": " 2020-03-31 09:12:10",
            "PTF_CONTENT": [
              {
                "NAME": "api_batch_API",
                "CHECKED": "api_batch_NOT_SELECTED",
                "VERSION": "G02R00C13_c14"
              }
       ]
    }
]

This my script script.sh

#!/bin/bash

a=$(cat template.json)

if [ -e $WORKSPACE/project/historique.json ];
then
  cat template.json >> historique.json
else
  cat <<-EOF > historique.json
   [
    $a
   ]
EOF
fi

PS: The file template.json change at each build in jenkins

The problem is the command after "then". Any suggestion please??(And sorry for my English)

After oguz ismail and shawn response the script file become like that:

#!/bin/bash

a=$(cat template.json)

if [ -e $WORKSPACE/project-CRMGP/historique.json ];
then
  jq '. + [input]' historique.json template.json > historique1.json 
else
  cat <<-EOF > historique.json
   [
    $a
   ]
EOF
fi

mv historique1.json historique.json