为什么jq有时需要过滤器表达式，而有时则不需要过滤器表达式? [英] Why does jq sometimes require a filter expression, and sometimes not?

问题描述

我有一些未格式化的JSON文件.我想用jq格式化它们，然后通过less看到格式化的输出:

I have some unformatted JSON files. I want to format them with jq and see the formatted output via less:

cat *.json | jq | less

但是它不起作用.只是显示jq的帮助.

But it doesn't work. Just shows the jq's help.

好吧，jq需要一个过滤器，最少需要一个.，但是...

O.K., jq needs a filter, minimum a ., but...

cat *.json | jq

为我工作.

有什么区别?看到解析命令行是Shell的任务，这是Bash的错误吗?为什么只有jq会发生这种情况?

What is the difference? Is this a Bash bug, seeing that parsing the command line is the shell's task? Why does this only happen with jq?

推荐答案

jq 在检测到其输出流是终端的情况下会做一些不同的事情.一方面，它对输出进行颜色编码(除非您显式关闭该选项).但是与这个问题更相关的是，对于1.5版，如果它检测到其输出流是一个终端并且其输入流不是一个终端，并且根本没有任何非选项命令行参数，则假定您打算使用过滤器..

jq does some things differently if it detects that its output stream is a terminal. For one thing, it colour-codes its output (unless you explicitly turn that option off). But more relevantly to this question, for version 1.5, if it detects that its output stream is a terminal and its input stream is not a terminal and there are no non-option command-line arguments at all, it assumes that you meant to use the filter ..

因此，当您键入 cat * .json |时会发生这种情况.jq .在这种情况下，jq的输入流是管道(来自 cat 命令)，但其输出流是终端.因此，它假定默认过滤器为..

So that's what happens when you type cat *.json | jq. In that case, jq's input stream is a pipe (from the cat command) but its output stream is the terminal. So it assumes a default filter of ..

但是，当通过 less 通过 jq 的输出传递管道时， jq 的输出流不再是终端.现在是一个管道(连接到一个 less 进程).因此，没有插入自动过滤器.，并且 jq 抱怨没有提供过滤器.

But when you pipe the output of jq through less, jq's output stream is no longer a terminal. Now it's a pipe (connected to a less process). So the automatic filter . is not inserted, and jq complains that no filter was provided.

但是，如果升级到v1.6，您会发现行为已更改.在v1.6中，没有任何过滤器的调用将被调整为.除非两者都是输入流和输出流都是终端.因此，如果升级，您会发现 cat * .json |jq |less 的工作方式与 cat * .json |大致相同jq ，除了输出是彩色的，当然不分页.

However, if you upgrade to v1.6, you'll find that the behaviour has changed. In v1.6, an invocation without any filter will be adjusted to . unless both the input stream and the output stream are a terminal. So if you upgrade, you'll find that cat *.json | jq | less works roughly the same way as cat *.json | jq, except that the output is coloured and, of course, not paged.

顺便说一句，如果您想使用 less (并且正在使用Gnu less )查看彩色输出，则可以使用以下命令:

By the way, if you wanted to see coloured output with less (and you're using Gnu less), then you could use the command:

cat *.json | jq . -C | less -R   # The . is not necessary with v1.6

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