I think that we can all agree: programs generally read information from somewhere (like a file) and print out information to somewhere (like your terminal or a different file).

When we were first getting to know our terminals at the beginning of this course we ran pandoc and we just started typing stuff in:

pandoc
Hello, pandoc! ^D

<p>Hello, pandoc!</p>

We only did that once, then we quickly switched to passing file names as arguments to pandoc:

pandoc my.md -o my.docx

But… what was going on with Pandoc when we just typed stuff in? This is actually something you already know about (!), even if it doesn’t quite look like it: pandoc was reading from a “data stream” called “standard input”. That data stream, standard input, is where you (the user) type information using your keyboard. pandoc was writing to a data stream called “standard output”. Standard output is where you see information printed in your terminal.

You’ve almost certainly used this in programs you’ve written before:

// maybe you didn't call this "keyboard", but...
final Scanner keyboard = new Scanner(System.in);

In Java, System.in is:

• The System part is the System class. Really broadly, the System class has a bunch of properties, attributes, and methods that let your program interact with your operating system.
• The in part is the static class member in, this is an instance of InputStream, and, very specifically, this instance refers to standard input.

We use code like this to get input from our keyboard, and, as far as we are concerned right now, standard input is our keyboard.

You’ve definitely seen this code before:

System.out.println("Hello, world!");

In Java, System.out is:

• System is the same System as above, and
• The out part is the static class member out, this is an instance of an OutputStream, and, very specifically, this instance refers to standard output.

When we don’t pass pandoc the name of a file to read from or a file name to write to, pandoc defaults to reading from standard input (your keyboard) and writing to standard output (the terminal).

This is actually pretty normal behaviour for programs: grep, for example, writes to standard output by default, but it can also read from standard input by default. Go ahead, try it:

grep 'hello' # now type stuff in, some lines with hello, some
             # without, what's printed?

You can quit grep by pressing Control+C or Control+D.

So we’ve got two data streams:

• Standard input (our keyboard), and
• Standard output (our terminal).

There’s actually a third data stream called standard error. You probably haven’t written this in Java before, but:

try {
    // dangerous stuff
} catch (final Exception e) {
    System.err.println("Oh no, something terrible happened!");
}

Standard error (System.err) is another output stream, and the main purpose of standard error is for printing out errors to your terminal.

#include <stdio.h>
#include <stdlib.h> // for EXIT_SUCCESS

int main(void)
{
    char h;
#define MAX_LEN 100
    char message[MAX_LEN];

    printf("Hello, world!\n"); // standard output
    fprintf(stdout, "Hello, world!\n"); // standard output

    fgets(message, MAX_LEN, stdin); // standard input
    h = getchar(); // standard input

    fprintf(stderr, "Oh no! Errors!\n"); // standard error

    return EXIT_SUCCESS;
}

import sys
message = input("Please enter a message: ") # standard input

print("Hello, world!") # standard output
print("Hello, world!", file=sys.stdout) # standard output

print("Oh no! Errors!", file=sys.stderr) # standard error

data = sys.stdin.read() # standard input
sys.stdout.write(data)  # standard output
sys.stderr.write(data)  # standard error

Further reading

You can read a more about the concepts and history of standard input, output, and error on the Wikipedia article about Standard Streams.