How to determine inside the program where the output goes to the terminal or to the file?

Question

C++ / C#

Hello.
If you write ls in the terminal, for example, it will display something like this:

Makefile ft_ls libft output2.txt
author inc output.txt src

If you write in the terminal ls > out.txt, for example, it will display something like this:

Makefile
author
ft_ls
inc
libft
output.txt
output2.txt
src

That is, in the second case, it inserts only a line break, in the first case it also inserts spaces.
Maybe someone knows how it determines where it writes to a file or terminal in order to act differently?

Answer 1

The author noticed that the output is different, that he himself sent the output to the file, he is aware :)
You can find out if we are writing to the terminal like this: istty is in io.h, and fileno is in stdio.h And ls, it really looks if the output is in the terminal - then in one line, otherwise - in several: https://github.com/coreutils/coreutils/blob/master...

Answer 2

It doesn't define anything. It is you who determines where the output will go, cmd > file You wrote)))
In the usual case, the command will spit out everything to the standard output.

Answer 3

So you yourself let her know that you are redirecting the output with the symbol ">"
I / O redirection in Linux

Answer 4

man ls

-1 (The numeric digit ``one''.) Force output to be one entry per line. This is the default when output is not to a terminal