Why does `ls | cat` != `ls`?

Question

Why is there a difference in output between these two commands:

ls | cat

---

ls

The first one seems to separate filenames with a newline.

This also applies to commands suc has ls > outfile, and similar things.

I'm on Mac OSX, if that makes any difference.

Answer 1

The ls command checks the file type of its standard output. If it detects a “tty”, it emits multicolumn output. If it detects any other file type (like a disk file or a pipe), it emits single column output.

Tty is short for teletype. In the old days, you would use an actual teletype to interact with a Unix system. So if standard output was a teletype, ls would produce output optimized for humans. Otherwise, it would produce output optimized for programs.

With the advent of other ways to run an interactive shell, like telnet sessions and window systems, the Unix authors created the pseudo-teletype, or “pty”, a software-only “device” which pretends to be a teletype. One program (like the telnet server, the ssh server, or the terminal window) can use a pty to make another program (the shell) think it is talking to a teletype.

You can use the -C flag to force ls to emit multicolumn output. You can use the -1 (digit one) flag to force single column output.