What is the -t used for in this Perl code
I read in the perldoc that the -t file operator is used to decide if a filehandle is opened to a tty or not. Then i read what a tty was and from what i understand it's an old term for a terminal?
My main question however is what this code is used for:
if(-t STDERR) {
die "some warning/error message here"
}
I guess i don't really understand what it means for a "filehandle to be opened to a tty". Does that mean that the particular output for that filehandle appears on the tty - or the terminal? Can tty refer to something besides the terminal?
TTYs are a type of unix device. Consoles appear as TTYs to programs, so that statement basically reads as
"If STDERR is sent to a console, ..."
But not quite. We're not talking about physical devices but logical devices such as /dev/tty*
. As such, it's possible for a program to pretend to be a terminal. For example, Expect uses IO::Pty to do this.
STDERR could also be a plain file, a pipe or a socket. -t
would be false for these.
script 2>foo # Plain file
script 2>&1 | cat # Pipe
This code wants to assert that standard error is being redirected to a file. So running
perl the_script.pl
would trigger the die
call, but
perl the_script.pl 2> error.log
would not.