如何在Bash中使用jq从JSON文件提取EventId？

问题场景 #

我有一个包含多个JSON对象的文件（内容如下），希望仅提取其中的EventId：

{
    "DocumentIncarnation":  1,
    "Events":  [
               ]
}

{
    "DocumentIncarnation":  2,
    "Events":  [
                   {
                       "EventId":  "C7061BAC-AFDC-4513-B24B-AA5F13A16123",
                       "EventStatus":  "Scheduled",
                       "EventType":  "Freeze",
                       "ResourceType":  "VirtualMachine",
                       "Resources":  [
                                         "WestNO_0",
                                         "WestNO_1"
                                     ],
                       "NotBefore":  "Mon, 11 Apr 2022 22:26:58 GMT",
                       "Description":  "Virtual machine is being paused because of a memory-preserving Live Migration operation.",
                       "EventSource":  "Platform",
                       "DurationInSeconds":  5
                   }
               ]
}

{
    "DocumentIncarnation":  3,
    "Events":  [
                   {
                       "EventId":  "C7061BAC-AFDC-4513-B24B-AA5F13A16123",
                       "EventStatus":  "Started",
                       "EventType":  "Freeze",
                       "ResourceType":  "VirtualMachine",
                       "Resources":  [
                                         "WestNO_0",
                                         "WestNO_1"
                                     ],
                       "NotBefore":  "",
                       "Description":  "Virtual machine is being paused because of a memory-preserving Live Migration operation.",
                       "EventSource":  "Platform",
                       "DurationInSeconds":  5
                   }
               ]
}

{
    "DocumentIncarnation":  4,
    "Events":  [
               ]
}

尝试执行以下命令时失败：

cat test.json | jq -c '.[] | [.EventId]'

报错信息：

jq: error (at <stdin>:5): Cannot index number with string "EventId"
jq: error (at <stdin>:25): Cannot index number with string "EventId"
jq: error (at <stdin>:45): Cannot index number with string "EventId"
jq: error (at <stdin>:51): Cannot index number with string "EventId"

问题原因 #

命令里的.[]会遍历顶级对象的所有属性，包括数值类型的DocumentIncarnation，而数值无法用字符串EventId索引，因此触发报错。另外EventId并非在顶级对象中，而是嵌套在Events数组的元素内，路径定位错误。

解决方法 #

使用以下命令可正确提取所有非空的EventId：

cat test.json | jq -r '.Events[]?.EventId'

参数说明：

• .Events[]：遍历每个JSON对象中的Events数组
• ?：跳过空的Events数组，避免无元素时触发报错
• .EventId：提取数组元素中的EventId字段
• -r：输出原始字符串格式，不带JSON引号

如果需要对结果去重，可追加unique处理：

cat test.json | jq -r '.Events[]?.EventId' | unique

内容的提问来源于stack exchange，提问作者gotothesky