验证 jq 和 bash 是否存在 json 字段？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/29563145/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
verify that a json field exists with jq and bash?
提问by sop
I have a script that uses jq for parsing a json string MESSAGE(that is read from another application). Meanwhile the json has changed and a field is split in 2 fields: file_pathis now split into folderand file. The script was reading the file_path, now the foldermay not be present, so for creating the path of the file I have to verify if the field is there. I have search for a while on the internet, and manage to do:
我有一个使用 jq 解析 json 字符串MESSAGE（从另一个应用程序读取）的脚本。同时 json 发生了变化，一个字段被拆分为 2 个字段：file_path现在拆分为folder和file。脚本正在读取file_path，现在folder可能不存在，因此为了创建文件的路径，我必须验证该字段是否存在。我在互联网上搜索了一段时间，并设法做到了：
echo $(echo $MESSAGE | jq .folder -r)$'/'$(echo $MESSAGE | jq .file -r)
if [ $MESSAGE | jq 'has(".folder")' -r ]
then
echo $(echo $MESSAGE | jq .folder -r)$'/'$(echo $MESSAGE | jq .file -r)
else
echo $(echo $MESSAGE | jq .file -r)
fi
where MESSAGE='{"folder":"FLDR","file":"fl"}'or MESSAGE='{"file":"fl"}'
哪里MESSAGE='{"folder":"FLDR","file":"fl"}'或MESSAGE='{"file":"fl"}'
The first line is printing FLDR/flor null/flif the folder field is not present. So I have thought to create an if that is verifying if the folder field is present or not, but it seems that I am doing it wrong and cannot figure out what is wrong. The output is
第一行正在打印FLDR/fl或者null/fl文件夹字段不存在。所以我想创建一个 if 来验证文件夹字段是否存在，但似乎我做错了，无法弄清楚出了什么问题。输出是
bash: [: missing `]'
jq: ]: No such file or directory
null/fl
回答by Wintermute
I'd do the whole thing in a jqfilter:
我会在jq过滤器中完成整个事情：
echo "$MESSAGE" | jq -r '[ .folder, .file ] | join("/")'
In the event that you want to do it with bash (or to learn how to do this sort of thing in bash), two points:
如果你想用 bash 来做（或学习如何用 bash 做这种事情），两点：
- Shell variables should almost always be quoted when they are used (i.e.,
"$MESSAGE"instead of$MESSAGE). You will run into funny problems if one of the strings in your JSON ever contains a shell metacharacter (such as*) and you forgot to do that; the string will be subject to shell expansion (and that*will be expanded into a list of files in the current working directory). - A shell
ifaccepts as condition a command, and the decision where to branch is made depending on the exit status of that command (true if the exit status is0, false otherwise). The[you attempted to use is just a command (an alias fortest, seeman test) and not special in any way.
- Shell 变量在使用时几乎总是被引用（即，
"$MESSAGE"而不是$MESSAGE）。如果您的 JSON 中的一个字符串包含 shell 元字符（例如*）而您忘记这样做，您将遇到有趣的问题；该字符串将受到 shell 扩展（并且*将扩展为当前工作目录中的文件列表）。 - shell
if接受一个命令作为条件，并根据该命令的退出状态决定分支到哪里（如果退出状态为0，则为真，否则为假）。在[你试图使用只是一个命令（一个别名test，见man test），并以任何方式不特别。
So, the goal is to construct a command that exits with 0if the JSON object has a folderproperty, non-zero otherwise. jqhas a -eoption that makes it return 0if the last output value was not falseor nulland non-zero otherwise, so we can write
因此，目标是构建一个命令，0如果 JSON 对象具有folder属性，则该命令退出，否则为非零。jq有一个-e选项，0如果最后一个输出值不是false或null非零则返回，否则我们可以写
if echo "$MESSAGE" | jq -e 'has("folder")' > /dev/null; then
echo "$MESSAGE" | jq -r '.folder + "/" + .file'
else
echo "$MESSAGE" | jq -r .file
fi
The > /dev/nullbit redirects the output from jqto /dev/null(where it is ignored) so that we don't see it on the console.
该> /dev/null位将输出重定向jq到/dev/null（忽略它的地方），以便我们在控制台上看不到它。
