Thread: When to print to stderr?

Of course, when there is an error, that much I understand. But what about these cases?

• User doesn't know how to run the program, so he (or she) simply runs it without parameters to get information. The program replies:
  Usage: TheProgram [Options] Infile Outfile.
  Stderr or stdout? My guess is stderr.
• User wants help and runs
  Code:

  TheProgram --help

  Output to stderr or stdout? I would guess stderr, but stdout makes sense too, I think.
• User wants to know the program's version and runs something like
  Code:

  TheProgram --version

  Is the output supposed to go to stdout or stderr? I would guess stdout for this one.

Just trying to do things right…

Think about running this with various options:

Code:

The Program > out.txt

What do you want to show up in out.txt?

I really think the answer is kinda obvious. A switch like --version or --help usually means that the user is requesting the output on purpose.

Without any redirection, the three standard predefined files input and output as follows:

stdin: Normally input from the keyboard

stdout: Normal output of your program to the screen

stderr: Error messages you write will go to the screen, even when stdout is redirected, unless stderr is also redirected.

Providing you are running on Linux/Unix, see `man stdin` for details.

That all make sense to me now. Thanks!

And yes, 100 % Linux here.