how to reference nested payload value

[iscaswang]

my gitlab webhook receives merge request payload like this:

{
"id": 59259,
"target_branch": "master",
"source_branch": "feature/ccc/ci_test",
"source_project_id": 14904,
"author_id": 6205,
"assignee_id": 6205,
"title": "Feature/ccc/ci test",
"created_at": "2017-09-11T08:41:57+0000",
"updated_at": "2017-09-11T08:41:57+0000",
"st_commits": null,
"st_diffs": null,
"milestone_id": null,
"state": "opened",
"merge_status": "unchecked",
"target_project_id": 14904,
"iid": 693,
"description": "test",
"source": {
"name": "backend",
"ssh_url": "git@git.code.oa.com:mmad_delivery/backend.git",
"http_url": "http://git.code.oa.com/mmad_delivery/backend.git",
"web_url": "http://git.code.oa.com/mmad_delivery/backend",
"namespace": "mmad_delivery",
"visibility_level": 0
},
"target": {
"name": "backend",
"ssh_url": "git@git.code.oa.com:mmad_delivery/backend.git",
"http_url": "http://git.code.oa.com/mmad_delivery/backend.git",
"web_url": "http://git.code.oa.com/mmad_delivery/backend",
"namespace": "mmad_delivery",
"visibility_level": 0
},
"last_commit": {
"id": "1c791ccdcdc22c8f070d6ad5e3b3afb79a6d7757",
"message": "commit 4\n",
"timestamp": "2017-09-11T11:33:46+0000",
"url": "http://git.code.oa.com/mmad_delivery/backend/commits/1c791ccdcdc22c8f070d6ad5e3b3afb79a6d7757",
"author": {
"name": "ccc",
"email": "ccc@163.com"
}
},
"url": "http://git.code.oa.com/mmad_delivery/backend/merge_requests/693",
"action": "open"
}

now i want to get the last message via environment variable last_commit.message in my shell, and I already parameterized this variable, but in fact i can only get the default value for the variable, so what is the corrent method to get the last commit message value from payload? many thanks