反引号中管道的使用 [英] Use of pipes in backticks

问题描述

我正在尝试使用管道运行命令，但收到错误:

I'm trying to run a command with a pipe but receive errors:

echo abc | `echo "grep a | grep b"`
grep: |: No such file or directory
grep: grep: No such file or directory
grep: b: No such file or directory

代码有什么问题?

预期的结果是要执行以下命令:

Expected results is for the following command to be executed:

echo abc | grep a | grep b

结果

abc

推荐答案

目前尚不清楚您要做什么，但这是您在做什么:

It's not clear what you are trying to do, but here is what you are doing:

echo "grep a | grep b"

输出字符串 grep a |grep b .

这是反引号的输出.您在外壳需要命令的位置使用反引号，因此"grep'a''|'尝试将'grep''b'作为命令行使用，所有标记均按字面解释(我希望添加单引号使它更清晰一些)，因此shell会忽略 echo 的输入，并且而是尝试在命名文件中查找正则表达式 a .您显然没有名为"|"，"grep"或"b"的文件，因此会收到错误消息.

This is the output from the backticks. You are using the backticks in a position where the shell wants a command, so "grep 'a' '|' 'grep' 'b'" is attempted as the command line, with all the tokens interpreted literally (I added single quotes to make this a bit clearer, hopefully) so the shell ignores the input from echo and instead attempts to look for the regular expression a in the named files. You apparently have no files named "|", "grep", or "b", so you get an error message.

您可能想要的是

echo abc | grep a | grep b

会在"echo"的输出中搜索"a"，然后在第一个grep的输出中搜索"b".因为 abc 匹配正则表达式 a ，所以第一个 grep 成功，因此将打印匹配的行；并且因为它也与正则表达式 b 匹配，所以它也会由最终的 grep 打印.

which searches for "a" in the output from "echo", then searches for "b" in the output from the first grep. Because abc matches the regular expression a, the first grep succeeds, so the matching line is printed; and because it also matches the regular expression b it is printed by the final grep as well.

要对此进行扩展，请尝试以下操作:

To expand a bit on this, try the following:

sh$ echo abc; echo bcd; echo xya
abc
bcd
xya

sh$ ( echo abc; echo bcd; echo xya ) | grep a  # print lines matching a
abc
xya

sh$ (echo abc; echo bcd; echo xya ) | grep a | grep b  # print lines matching a and b
abc

不清楚为什么要使用反引号；如果这不是您要实现的目标，请更详细地说明您要实现的目标.

It is not clear why you are using backticks; if this is not what you are trying to achieve, please explain in more detail what you want to accomplish.

如果要查找与a或b匹配的行，请尝试以下操作:

If you want to find lines matching either a or b, try this:

sh$ ( echo abc; echo bcd; ) | grep '[ab]'

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