What is differences between arguments and inputs?

Question

I am in trouble with xargs and pipe commands. Here is one example =

echo 'a' | xargs mkdir

This command creates a directory which name is a. So I can understand that this commmand is equivalent to mkdir a

But why doesn't this command work = echo 'a' | mkdir ?

I read that pipe command transfer left command's output as a input to right one , when | xargs command transfers left command's output as a argument to right one.

1-What are differences between input and argument ?

2-echo 'a' | mkdir why doesn't this command work and what is equivalent of this command ?

Thank you...

Answer 1

Simple answer

The arguments are an input.

Longer answer

There are also other types of input.

In your example echo 'a' | mkdir you are expecting mkdir to read its standard input. However mkdir is not written to do this.

What you are calling input is standard-input (stdin). And output is standard-out (stdout). There is all standard-error (stderr). And an exit code.

Even longer answer -- so what is the difference?

The arguments must be available before the program is started. It is typically: short; contains options, filenames, and short text.

Standard-in can be written to as the program is running; data can be sent to it, that did not exist when the program was started; It is not guaranteed that you can read it out of order (so read it sequentially); It can be of arbitrary length; There is only one standard-in per process, if you need more send file-names as arguments;

How to do echo 'a' | xargs mkdir without xargs

mkdir "$(echo 'a')"

The $() means run the contained command, then replace the $() and its content with the standard-out of that command.

Answer 2

I was confused about this too, for a very long time. I'll take a slightly may be simpler example (I am by far no bash expert):

wc "test" 
wc <<< "test" (same as: echo "test" | wc)

The result of these two are very different. The first one tries to read a file test, the second one shows 1 1 5.

The issue here is that wc "test" gets the argument "test" as input, while wc <<< "test" takes stdin.

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So when you issue echo 'a' | xargs mkdir (xargs stands for: execute as an argument), you are passing an argument to mkdir; it's like doing: mkdir a.

On the other hand, when you issue echo 'a' | mkdir (simpler would be : mkdir <<< 'a'), you pass a as stdin.

mkdir treats these things very differently, thus the result you see.