问题
I am having trouble with this simple task:
cat file | grep -E ^[0-9]+$ > file_grep
diff file file_grep
Problem is, I want to do this without file_grep
I have tried:
diff file `cat file | grep -E ^[0-9]+$`
and
diff file "`cat file | grep -E ^[0-9]+$`"
and a few other combinations :-) but I can't get it to work.
I always get an error, when the diff gets extra argument which is content of file filtered by grep.
Something similar always worked for me, when I wanted to echo command outputs from within a script like this (using backtick escapes):
echo `ls`
Thanks
回答1:
If you're using bash:
diff file <(grep -E '^[0-9]+$') file
The <(COMMAND) sequence expands to the name of a pseudo-file (such as /dev/fd/63) from which you can read the output of the command.
But for this particular case, ruakh's solution is simpler. It takes advantage of the fact that - as an argument to diff causes it to read its standard input. The <(COMMAND) syntax becomes more useful when both arguments to diff are command output, such as:
diff <(this_command) <(that_command)
回答2:
The simplest approach is:
grep -E '^[0-9]+$' file | diff file -
The hyphen - as the filename is a specific notation that tells diff "use standard input"; it's documented in the diff man-page. (Most of the common utilities support the same notation.)
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:
cat `echo file`
is equivalent to this:
cat file
and this:
diff file "`cat file | grep -E ^[0-9]+$`"
is equivalent to something like this:
diff file "123
234
456"
That is, it actually tries to pass 123234345 (plus newlines) as a filename, rather than as the contents of a file. Technically, you could achieve the latter by using Bash's "process substitution" feature that actually creates a sort of temporary file:
diff file <(cat file | grep -E '^[0-9]+$')
but in your case it's not needed, because of diff's support for -.
回答3:
grep -E '^[0-9]+$' file | diff - file
where - means "read from standard input".
回答4:
Try process substitution:
$ diff file <(grep -E "^[0-9]+$" file)
From the bash manpage:
Process Substitution
Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files. It takes the form of <(list) or >(list). The process list is run with its input or output connected to a FIFO or some file in /dev/fd. The name of this file is passed as an argument to the current command as the result of the expansion. If the >(list) form is used, writing to the file will provide input for list. If the <(list) form is used, the file passed as an argument should be read to obtain the output of list.
回答5:
In bash, the syntax is
diff file <(cat file | grep -E ^[0-9]+$)
来源：https://stackoverflow.com/questions/9847479/use-pipe-of-commands-as-argument-for-diff