问题
What is the design rationale that cat doesn't take list of file names from pipe input? Why did the designers choose that the following does not work?
ls *.txt | cat
Instead of this, they chose that we need to pass the file names as argument to cat as:
ls *.txt | xargs cat
回答1:
When you say ls *.txt | cat doesn't work, you should say that doesn't work as you expect. In fact, that works in the way it was thought to work.
From man:
cat - Concatenate FILE(s), or standard input, to standard output
Suppose the next output:
$ ls *.txt
file1.txt
file2.txt
... the input to cat will be:
file1.txt
file2.txt
...and that's exactly what cat output in the standard output
In some shells, it's equivalent to:
cat <(ls *.txt)
or
ls *.txt > tmpfile; cat tmpfile
So, cat is really working as their designers expected to do so.
On the other hand, what you are expecting is that cat interprets its input as a set of filenames to read and concatenate their content, but when you pipe to cat, that input works as a lonely file.
回答2:
To make it short, cat is a command, like echo or cp, and few others, which cannot convert pipe redirected input stream into arguments.
So, xargs, is used, to pass the input stream as an argument to the command.
More details here: http://en.wikipedia.org/wiki/Xargs
As a former unix SA, and now, Python developer, I believe I could compare xargs, to StringIO/CStringIO, in Python, as it kind of helps the same way.
When it comes to your question: Why didn't they allow stream input? Here is what I think
Nobody but them could answer this.
I believe, however, than cat is meant to print to stdout the content of a file, while the command echo, was meant to print to stdout the content of a string.
Each of these commands, had a specific role, when created.
来源：https://stackoverflow.com/questions/26443951/why-not-pipe-list-of-file-names-into-cat