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Perl::Critic::Policy::ValuesAndExpressions::ProhibitFiletest_f(3pm) | User Contributed Perl Documentation | Perl::Critic::Policy::ValuesAndExpressions::ProhibitFiletest_f(3pm) |
NAME¶
Perl::Critic::Policy::ValuesAndExpressions::ProhibitFiletest_f - don't use the -f file testDESCRIPTION¶
This policy is part of the "Perl::Critic::Pulp" add-on. It asks you not to use the "-f" file test because doing so is usually wrong or unnecessarily restrictive. On that basis this policy is under the "bugs" theme, see "POLICY THEMES" in Perl::Critic.- "-f" is not the opposite of "-d"
- If you're traversing a tree and want to distinguish files from directories
to descend into then "-d" should be used
so device files or pipes can be processed.
if (-f $filename) { # bad process ($filename); } else { descend ($filename); } if (-d $filename) { # better descend ($filename); } else { process ($filename); }
- "-f" doesn't mean readable/writable/seekable
- Char specials and named pipes are perfectly good for reading and writing,
and char specials can support seeking. Demanding
"-f" is an unnecessary restriction. You
might only ever use ordinary files normally, but there's no need to
prevent someone else running it on a tape drive, /dev/null, etc.
You always have to test each "seek()"
etc for success anyway, and that will tell you if a file is seekable.
seek HANDLE, 123, 0 or die "Cannot seek: $!";
- "-e" is better than "-f"
- A few inflexible functions or operations may not have good "file not
found" behaviour and may force you to check for a file before
invoking. Using "-e" is better than
"-f" since as described above it doesn't
unnecessarily disallow device files.
if (-f $filename) { # bad require $filename; } if (-e $filename) { # better require $filename; }
- "-f" before opening is a race condition
- Testing a filename before opening is bad. Any test before opening is
useless because the file can change or be removed in between the test and
the open.
if (-f $filename) { # bad open HANDLE, '<', $filename }
If you want to know if the file can be opened then open the file! The error return from "open()" must be checked, so a test beforehand only duplicates that, and is an opportunity to wrongly presume what the system or the user's permissions can or can't do.
When opening "ENOENT" will say if there was no such file, or "EISDIR" if it's in fact a directory.
if (! open HANDLE, '<', $filename) { # better if ($! == POSIX::ENOENT()) { ... } }
If you really do want to enquire into the nature of the file, in order to only accept ordinary files, then open first and then "-f" on the handle. But that's unusual except for an archiving or backup program.
Incidentally, for error message in $! is normally the best thing to print. It can be slightly technical, but its wording will at least be familiar from other programs and will be translated into the user's locale language.
Disabling¶
Most uses of "-f" tend to shell script style code written in Perl. In the shell it's usually not possible to do better than such tests (though "-d" or "-e" is generally still better than "-f"), but in Perl it is.A blanket prohibition like this policy is harsh, but is meant as a building block or at least to make you think carefully whether "-f" is really right. As always though you can disable "ProhibitFiletest_f" from your .perlcriticrc in the usual way (see "CONFIGURATION" in Perl::Critic),
[-ValuesAndExpressions::ProhibitFiletest_f]
SEE ALSO¶
Perl::Critic::Pulp, Perl::Critic, "open" in perlfunc, "ERRNO" in POSIX, Errno, errno(3)HOME PAGE¶
http://user42.tuxfamily.org/perl-critic-pulp/index.htmlCOPYRIGHT¶
Copyright 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017 Kevin RydePerl-Critic-Pulp is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.
Perl-Critic-Pulp is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with Perl-Critic-Pulp. If not, see <http://www.gnu.org/licenses>.
2018-05-30 | perl v5.26.2 |