This chapter shows you how to manipulate the files themselves, not merely the data contained within. Perl uses UNIX semantics for providing access to files and directories. Some of these names will be familiar to Win32 programmers who have used the C run-time library, while others may not. Perl provides a rich set of file and directory manipulation routines, and not all of these are implemented on Win32 platforms, but we’ll cover the most useful ones here.[83]
Earlier, you learned how to create a file from within Perl by opening it for output with a filehandle. Now, we’ll get dangerous and learn how to remove a file (very appropriate for Chapter 13, don’t you think?).
The Perl
unlink
function (named for the POSIX system call) deletes a file. This is
exactly what the command prompt
del
command does. Here’s how to remove a
file called fred and then remove a file
specified during program execution:
unlink ("fred"); # say goodbye to fred print "what file do you want to delete? "; chomp($name = <STDIN>); unlink ($name);
The unlink
function can take a list of names to be
unlinked as well:
unlink ("spottedowl","meadowlark"); # kill two birds unlink <*.bak>; # just like "del *.bak" in the command prompt
The glob is evaluated in a list context,
creating a list of filenames that match the pattern. This list is
exactly what we need to feed unlink
.
The return value of unlink
is the number of files
successfully deleted. If only one argument exists, and it is deleted,
the result is one; otherwise, the result is zero. If there are three
filenames but only two could be deleted, the result is two. You
can’t tell which two, so if you need to figure out which
deletion failed, you must do them one at a time. Here’s how to
delete all of the backup files (ending in .bak)
while reporting an error for any file that cannot be deleted:
foreach $file (<*.bak>) { # step through a list of .bak files unlink($file) || warn "having trouble deleting $file: $!"; }
If unlink
returns one (meaning the one file
specified was indeed deleted), the true result skips the
warn
function. If the filename cannot be deleted,
the zero result is false, so the warn
is executed.
Once again, this can be read abstractly as “unlink this file or
tell me about it.”
If the unlink
function is given no arguments, the
$_
variable is once
again used as a default. Thus, we could have written the preceding
loop as:
foreach (<*.bak>) { # step through a list of .bak files unlink || warn "having trouble deleting $_: $!"; }
[83] In particular, we’ve omitted discussion
of the link()
and
symlink()
functions, used to create hard and
symbolic links under Unix, because these functions are unimplemented
in the Windows NT filesystems and thus in Perl for Win32, as
well.