Why `ls` list multiple files per line, but `ls pipe/redirect` list just 1 file per line? [closed]

Question

Just curious, this is normal-expected behavior of ls:

user@host:~$ ls
Codes    Documents  Music   Pictures  Templates
Desktop  Downloads  Papers  Public    Videos

But when I use ls with pipe/redirection, it behave like ls -1:

user@host:~$ ls | cat
Codes
Desktop
Documents
Downloads
Music
Papers
Pictures
Public
Templates
Videos

Why? (and how to write such program that gives difference output between stdout and pipe like this?)

---

P.S. I also set alias l='ls -F', and this time pipe/redirection is no longer ls -1 style:

user@host:~$ l | cat
Codes/    Documents/  Music/   Pictures/  Templates/
Desktop/  Downloads/  Papers/  Public/    Videos/

Without using the alias, it does the command in ls -1 style, however:

$ ls -F | cat
Codes/
Desktop/
Documents/
Downloads/
Music/
Papers/
Pictures/
Public/
Templates/
Videos/

Answer 1

You can check this line from the source:

if (format == long_format)
  format = (isatty (STDOUT_FILENO) ? many_per_line : one_per_line);

It uses the isatty function to check if stdout points to a tty, and to print many_per_line if it does or one_per_line if it does not.

Answer 2

Here is how GNU ls does it (ls.c):

  if (isatty (STDOUT_FILENO))
    {
      format = many_per_line;
    }
  else
    {
      format = one_per_line;
    }