Creating new bash var from value of json bash var : My environment created a variable that looks like this:

    SM_TRAINING_ENV={"additional_framework_parameters":{},"channel_input_dirs":{"training":"/opt/ml/input/data/training"},"current_host":"algo-1","framework_module":"sagemaker_tensorflow_container.training:main","hosts":["algo-1"],"hyperparameters":{"bool_param":true,"float_param":1.25,"int_param":5,"model_dir":"s3://bucket/detection/prefix/testing-2019-04-06-02-24-20-194/model","str_param":"bla"},"input_config_dir":"/opt/ml/input/config","input_data_config":{"training":{"RecordWrapperType":"None","S3DistributionType":"FullyReplicated","TrainingInputMode":"File"}},"input_dir":"/opt/ml/input","is_master":true,"job_name":"testing-2019-04-06-02-24-20-194","log_level":20,"master_hostname":"algo-1","model_dir":"/opt/ml/model","module_dir":"s3://bucket/prefix/testing-2019-04-06-02-24-20-194/source/sourcedir.tar.gz","module_name":"launcher.sh","network_interface_name":"ethwe","num_cpus":8,"num_gpus":1,"output_data_dir":"/opt/ml/output/data","output_dir":"/opt/ml/output","output_intermediate_dir":"/opt/ml/output/intermediate","resource_config":{"current_host":"algo-1","hosts":["algo-1"],"network_interface_name":"ethwe"},"user_entry_point":"launcher.sh"}

How can I create a new bash variable that is equal to the value of `SM_TRAINING_ENV["hyperparameters"]["model_dir"]`?