bash 使用命令管道作为 diff 的参数
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Use pipe of commands as argument for diff
提问by rluks
I am having trouble with this simple task:
我在完成这个简单的任务时遇到了麻烦：
cat file | grep -E ^[0-9]+$ > file_grep
diff file file_grep
Problem is, I want to do this without file_grep
问题是，我想在没有 file_grep
I have tried:
我试过了：
diff file `cat file | grep -E ^[0-9]+$`
and
和
diff file "`cat file | grep -E ^[0-9]+$`"
and a few other combinations :-) but I can't get it to work.
I always get an error, when the diffgets extra argument which is content of file filtered by grep.
和其他一些组合:-) 但我无法让它工作。我总是收到一个错误，当diff获取额外的参数是由grep.
Something similar always worked for me, when I wanted to echocommand outputs from within a script like this (using backtick escapes):
类似的东西总是对我有用，当我想echo从这样的脚本中命令输出时（使用反引号转义）：
echo `ls`
Thanks
谢谢
回答by Keith Thompson
If you're using bash:
如果您使用 bash：
diff file <(grep -E '^[0-9]+$' file)
The <(COMMAND)sequence expands to the name of a pseudo-file (such as /dev/fd/63) from which you can read the output of the command.
该<(COMMAND)序列扩展为伪文件的名称（例如/dev/fd/63），您可以从中读取命令的输出。
But for this particular case, ruakh's solution is simpler. It takes advantage of the fact that -as an argument to diffcauses it to read its standard input. The <(COMMAND)syntax becomes more useful when both arguments to diffare command output, such as:
但是对于这种特殊情况，ruakh 的解决方案更简单。它利用了这样-一个事实，即作为参数diff使其读取其标准输入。的<(COMMAND)语法变得更加有用当两个参数diff是命令输出，如：
diff <(this_command) <(that_command)
回答by ruakh
The simplest approach is:
最简单的方法是：
grep -E '^[0-9]+$' file | diff file -
The hyphen -as the filename is a specific notation that tells diff"use standard input"; it's documented in the diffman-page. (Most of the common utilities support the same notation.)
-作为文件名的连字符是一个特定的符号，它告诉diff“使用标准输入”；它记录在diff手册页中。（大多数常用实用程序都支持相同的表示法。）
The reason that backticks don't work is that they capture the output of a command and pass it as an argument. For example, this:
反引号不起作用的原因是它们捕获命令的输出并将其作为参数传递。例如，这个：
cat `echo file`
is equivalent to this:
相当于：
cat file
and this:
和这个：
diff file "`cat file | grep -E ^[0-9]+$`"
is equivalent to something like this:
相当于这样的事情：
diff file "123
234
456"
That is, it actually tries to pass 123234345(plus newlines) as a filename, rather than as the contentsof a file. Technically, you could achieve the latter by using Bash's "process substitution" feature that actually creates a sort of temporary file:
也就是说，它实际上尝试将123234345（加上换行符）作为filename传递，而不是作为文件的内容传递。从技术上讲，您可以通过使用 Bash 的“进程替换”功能来实现后者，该功能实际上创建了一种临时文件：
diff file <(cat file | grep -E '^[0-9]+$')
but in your case it's not needed, because of diff's support for -.
但在你的情况下，它是没有必要的，因为，diff的支持-。
回答by ashbyp
grep -E '^[0-9]+$' file | diff - file
where -means "read from standard input".
where-表示“从标准输入读取”。
回答by chepner
Try process substitution:
尝试进程替换：
$ diff file <(grep -E "^[0-9]+$" file)
From the bash manpage:
从 bash 联机帮助页：
Process Substitution
Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files. It takes the form of <(list) or >(list). The process list is run with its input or output connected to a FIFO or some file in /dev/fd. The name of this file is passed as an argument to the current command as the result of the expansion. If the >(list) form is used, writing to the file will provide input for list. If the <(list) form is used, the file passed as an argument should be read to obtain the output of list.
过程替换
支持命名管道 (FIFO) 或命名打开文件的 /dev/fd 方法的系统支持进程替换。它采用 <(list) 或 >(list) 的形式。进程列表运行时其输入或输出连接到 FIFO 或 /dev/fd 中的某个文件。该文件的名称作为扩展的结果作为参数传递给当前命令。如果使用 >(list) 形式，写入文件将为列表提供输入。如果使用 <(list) 形式，则应读取作为参数传递的文件以获得 list 的输出。
回答by mob
In bash, the syntax is
在 bash 中，语法是
diff file <(cat file | grep -E ^[0-9]+$)
