Testing for the existence of STDIN?

darkomen has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: Testing for the existence of STDIN? by wog (Curate) on Jan 24, 2002 at 03:22 UTC
In addition to the code above, you could use the -t filetest operator to check if `STDIN` was opened to a terminal (and thus not a pipe, etc): `if (-t STDIN) { # we're running from a shell, without input being # piped to us. } else { # ... }` [download] Or you could also try a non-blocking read of STDIN, which most directly answers your original question of seeing if there is any input waiting without blocking: `use IO::Handle; STDIN->blocking(0); # and try to read from it, like you would normally. my $line = <STDIN>; if (defined $line) { # we have input, read succeeded } else { # no input at the moment. }` [download] (Note that this technique may give false negatives if pipes take some time to start writing data.)	[reply] [d/l] [select]
Re: Testing for the existence of STDIN? by Zaxo (Archbishop) on Jan 24, 2002 at 03:28 UTC
Four argument `select 1, undef, undef, $timeout` is what you're after. After Compline, Zaxo	[reply]
(jeffa) Re: Testing for the existence of STDIN? by jeffa (Bishop) on Jan 24, 2002 at 02:58 UTC
This is expected behaviour. If you ask for something, Perl waits until it gets it. Your alternative is to pass arguments to the script, like the name of a file to open. If the file is empty, then you could exit cleanly. UPDATE: Actually, you _could_ use a time out with $SIG{ALRM}: `$SIG{ALRM} = sub { die 'STDIN' }; eval { alarm(1); print while <>; alarm(0); }; print "no STDIN\n" if $@ =~ /STDIN/;` [download] You cannot get finer grained than one second with this way, but you can with select. Or .... just use wog's much more elegant solution. wog++ jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply] [d/l]
Re: (jeffa) Re: Testing for the existence of STDIN? by darkomen (Acolyte) on Jan 24, 2002 at 03:05 UTC
I have a piece of working code using sigalrm, i was just hoping there was more of a clean way to do so... It would be nice if you could use the -s test operator on STDIN...	[reply]