Globbing (Learning Perl, 3rd Edition)

12.2. Globbing

Normally, the shell expands any filename patterns on each command line into the matching filenames. This is called globbing. For example, if you give a filename pattern of *.pm to the echo command, the shell expands this list to a list of names that match:

$ echo *.pm
barney.pm dino.pm fred.pm wilma.pm
$

The echo command doesn't have to know anything about expanding *.pm, because the shell has already expanded it. This works even for your Perl programs:

$ cat >show-args
foreach $arg (@ARGV) {
  print "one arg is $arg\n";
}
^D
$ perl show-args *.pm
one arg is barney.pm
one arg is dino.pm
one arg is fred.pm
one arg is wilma.pm
$

Note that show-args didn't need to know anything about globbing -- the names were already expanded in @ARGV.

But sometimes we end up with a pattern like *.pm inside our Perl program. Can we expand this pattern into the matching filenames without working very hard? Sure -- just use the glob operator:

my @all_files = glob "*";
my @pm_files = glob "*.pm";

Here, @all_files gets all the files in the current directory, alphabetically sorted, and not including the files beginning with a period, just like the shell. And @pm_files gets the same list as we got before by using *.pm on the command line.

In fact, anything you can say on the command line, you can also put as the (single) argument to glob, including multiple patterns separated by spaces:

my @all_files_including_dot = glob ".* *";

Here, we've included an additional "dot star" parameter to get the filenames that begin with a dot as well as the ones that don't. Please note that the space between these two items inside the quoted string is significant, as it separates two different items to be globbed.[276]

[276]Windows users may be accustomed to using a glob of *.* to mean "all files". But that actually means "all files with a dot in their names," even in Perl on Windows.

The reason this works exactly as the shell works is that prior to Perl Version 5.6, the glob operator simply called /bin/csh[277] behind the scenes to perform the expansion. Because of this, globs were time-consuming and could break in large directories, or in some other cases. Conscientious Perl hackers avoided globbing in favor of directory handles, which will be discussed in Section 12.4, "Directory Handles" later in this chapter. However, if you're using a modern version of Perl, you should no longer be concerned about such things.

[277]Or it will call a valid substitute if a C-shell wasn't available.


12. Directory Operations		12.3. An Alternate Syntax for Globbing