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Chapter 12. Directory Operations

The files we created in the previous chapter were generally in the same place as our program. But modern operating systems let us organize files into directories, allowing us to keep our Beatles MP3s away from our important Llama book chapter sources so that we don't accidentally send an MP3 file to the publisher. Perl lets you manipulate these directories directly, in ways that are even fairly portable from one operating system to another.

12.1. Moving Around the Directory Tree

Your program runs with a "working directory," which is the starting point for relative pathnames. That is, if you refer to the file fred, that means "fred in the current working directory."

The chdir operator changes the working directory. It's just like the Unix shell's cd command:

chdir "/etc" or die "cannot chdir to /etc: $!";

Because this is a system request, the value of $! will be set if an error occurs. You should normally check $! when a false value is returned from chdir, since that indicates that something has not gone as requested.

The working directory is inherited by all processes that Perl starts (we'll talk more about that in Chapter 14, "Process Management"). However, the change in working directory cannot affect the process that invoked Perl, such as the shell.[275] So you can't make a Perl program to replace your shell's cd command.

[275]This isn't a limitation on Perl's part; it's actually a feature of Unix, Windows, and other systems. If you really need to change the shell's working directory, see the documentation of your shell.

If you omit the parameter, Perl determines your home directory as best as possible and attempts to set the working directory to your home directory, similar to using the cd command at the shell without a parameter. This is one of the few places where omitting the parameter doesn't use $_.

Some shells permit you to use a tilde-prefixed path with cd to use another user's home directory as a starting point (like cd ~merlyn). This is a function of the shell, not the operating system, and Perl is calling the operating system directly. Thus, a tilde-prefix will not work with chdir.

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