Warn if STDIN pipe is missing or unwanted

[Anonymous Monk]

I have a script that can accept input from STDIN, but only if the proper option is specified (-i -). I want the script to warn if STDIN is not specified but the script is being piped to (echo test | ./script.pl) or if input is redirected (./script < input.txt). I also want it to warn in the opposite case, where STDIN is specified, but the script is not being piped or input is not redirected.

I can detect if STDIN is a pipe with -p STDIN and if STDIN is redirected using (stat STDIN)[0] != 0, but when the script is run under cron, STDIN is a pipe even if the command is not being piped to. The only case under cron where STDIN is not a pipe is if it is redirected input. And testing if STDIN is empty is inadequate, because it's possible that the writing command in a pipeline will not have any output.

Is it possible to do what I'm attempting, or is a bad idea?

use Getopt::Long qw(:config bundling);
GetOptions('input-file|i=s' => \my @file);

my ($stdin_is_redir, $want_stdin) = !! (stat STDIN)[0];
for my $arg (@file) {
    my $fh;
    if ('-' eq $arg) {
        $want_stdin = 1;
        # This is never triggered under cron.
        warn "stdin isn't connected- missing pipe?\n" and next
            unless -p STDIN or ($stdin_is_redir and ! -t STDIN);
        $fh = *STDIN{IO};
    }
    else {
        require Path::Tiny;
        my $file = path($arg);
        $fh = eval { $file->openr } or warn "$file: $@->{err}\n" and n
+ext;
    }
    # do_something($_) while <$fh>;
}

# This is always triggered under cron unless $stdin_is_redir
warn "stdin is connected, missing `-i -`?\n"
    if ! $want_stdin and ! -t STDIN and ($stdin_is_redir or -p STDIN);

# Tested with:
# ./script.pl
# ./script.pl -i -
# ./script.pl -i /dev/null
# ./script.pl -i - < /dev/null
# echo test | ./script.pl
# echo test | ./script.pl -i -

[haukex]

Why? If feels to me to go against the *NIX principles to worry about something like that. Perhaps this is an X/Y Problem, so if you could explain why you need this that would be useful.

If you need enough rope to shoot yourself in the foot, perhaps there's some trickery you can try like fileno(*ARGV)==fileno(*STDIN) or something, though I don't give any guarantees on that being reliable either. Also see IO::Interactive.

[Anonymous Monk]

Because different commands have different syntaxes/semtantics and after running other commands, I sometimes use the wrong syntax on the script in question. When that happens the script can either exit without processing the onput or hang waiting for input. Both cases seem like an error to me and if possible, should warn or fail.

[haukex]

I sometimes use the wrong syntax on the script in question.

In that case perhaps the simplest solution is to require the -i option in your script, thereby always requring one to specify a filename or "-"?

[talexb]

To reiterate: the UNIX philosophy for a program that handles input is to get that a) from the input filename that is specified as part of the command line, or, if there is no filename specified on the command line, b) from the input stream. The most obvious example I can think of (and there are many others) is the utility less. You can specify a filename (or a file mask), and less will open and display each of those files. If no filename is supplied, it waits for an input stream (like the command ls -lrt | less) and displays that, a page at a time.

[ikegami]

but only if the proper option is specified

Why break convention and add needless restrictions?

STDIN is a pipe even if the command is not being piped to.

It *is* being piped to. It may not have been done are your request, but it was still done.

if STDIN is redirected using (stat STDIN)[0] != 0

No.

$ perl -Mv5.14 -e'say( (stat STDIN)[0] )'
60

$ perl -Mv5.14 -e'say( (stat STDIN)[0] )' </dev/null
5