A fun gotcha with magic <>

Despite liberally sprinkling my shell scripts with -e1 and -o pipefail2, and adding defense with use autodie; in Perl scripts, there’s a still some logic in Perl’s core that allows an error to slip by unnoticed.


Consider a typical Perl script with a read loop:

use strict;
use warnings;
use autodie ':io';

while (<>) {
    my ($size, $toppings) = split /\t/, $_, 2;

Due to the magic <>, the script can be called either as order-pizza pizzalist (where pizzalist is a file) or order-pizza < pizzalist (via standard input).

The problem occurs when pizzalist does not exist: Perl will complain:

Can't open pizzalist: No such file or directory ...

…but the script will continue as if <> returned undef (exiting the loop) and terminate with zero exit status! Indicating success, despite the error.

This caused a problem for me when a file went missing and this script suddenly did nothing, but we didn’t pick up on it immediately because the script exited just fine!


This is actually documented in the readline perldoc for newer Perl versions; the workaround is to read the files yourself, but this doesn’t handle standard input. (It also uses two argument open3!) You’ll need to handle standard input (and - being used as a placeholder) specially.

Something like:

use autodie ':io';

@ARGV = ('-') if !@ARGV;
foreach my $arg (@ARGV) {
    my $fh;
    if ($arg eq '-') {
        $fh = \*STDIN;
    else {
        open $fh, '<', $arg;
    while ( ! eof($fh) ) {
        $_ = <$fh>;

Not quite as simple as while (<>) { ... }, huh? :(

  1. If a command fails, abort the script rather than chugging onwards. 

  2. In a pipeline, if an earlier part of the pipe fails, don’t allow the non-zero exit code to be masked by commands later in the pipe that exit successfully. Critical! 

  3. With two argument open, special characters such as > and | will be interpreted respectively as ‘open the file for writing’ and ‘open a shell with the following command for input/output’. Leading whitespace is trimmed, too. This causes subtle bugs and/or security flaws in your script.