Unix Command Internals

But what if you do care?

ls is a little tricky
When it runs, it makes an fstat call on its standard output
- Among other things, this tells ls whether stdout is a file or a terminal
If it's a terminal, ls enables the column-formatting and color options
If not, it doesn't
In Perl, the code might look something like this:

        @statinfo = stat STDOUT;
        if (    $statinfo[2] & 0020000
            && ($statinfo[6] & 0xff) == 5) { print "Terminal\n" }
        else { print "Not a terminal\n" }

Or more likely, for portability reasons, you'd use isatty:

        use POSIX 'isatty';
        if (isatty(STDOUT)) { print "Terminal\n" } 
        else { print "Not a terminal\n" }

Or even more likely, you'd use Perl's built-in -t operator

        if (-t STDOUT) { print "Terminal\n" } 
        else { print "Not a terminal\n" }

Internally, -t just uses isatty