stdin-standard-in-redirect

stdin (Standard In)

In our previous lesson we learned that we have different stdout streams we can use, such as a file or the screen. Well there are also different standard input (stdin) streams we can use as well. We know that we have stdin from devices like the keyboard, but we can use files, output from other processes and the terminal as well, let's see an example.

Let's use the peanuts.txt file in the previous lesson for this example, remember it had the text Hello World in it.

$ cat < peanuts.txt > banana.txt

Just like we had > for stdout redirection, we can use < for stdin redirection.

Normally in the cat command, you send a file to it and that file becomes the stdin, in this case, we redirected peanuts.txt to be our stdin. Then the output of cat peanuts.txt which would be Hello World gets redirected to another file called banana.txt.

Not every command reads from stdin. echo, ls and pwd all ignore it completely and produce their usual output no matter what you feed them, so redirecting into them does nothing useful. Commands like cat, sort, wc and grep do read stdin, and those are the ones worth trying this with.

Exercise

Try out a couple of commands and compare what they do with the file:



$ wc -l < peanuts.txt

$ sort < peanuts.txt

$ echo < peanuts.txt

The last one prints an empty line. Why?

Quiz Question

What redirector do you use to redirect stdin?

Show answer