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31.13. use lib
use lib "$ENV{HOME}/libperl"; # add ~/libperl
no lib "."; # remove cwd
This pragma simplifies the manipulation of @INC at compile time.
It is typically used to add extra directories to Perl's search path
so that later do, require, and use statements will find
library files that aren't located in Perl's default search path.
It's especially important with use, since that happens at compile
time too, and setting @INC normally (that is, at run time) would be too late.Parameters to use lib are prepended to the beginning of Perl's
search path. Saying
use libLIST is almost the same as saying
BEGIN { unshift(@INC,LIST) },
but use libLIST includes support for
platform-specific directories. For each given directory $dir in
its argument list, the lib pragma also checks to see whether a
directory named $dir/$archname/auto exists. If so, the
$dir/$archname directory is assumed to be a corresponding
platform-specific directory, so is added to @INC (in front of $dir).To avoid redundant additions that slow access time and waste a
small amount of memory, trailing duplicate entries in @INC are
removed when entries are added.Normally, you should only add directories to @INC. If you do need
to delete directories from @INC, take care to delete only those
that you yourself added, or those that you're somehow certain aren't needed
by other modules in your program. Other modules may have added
directories to your @INC that they need for correct operation.The no lib pragma deletes all instances of each named directory
from @INC. It also deletes any corresponding
platform-specific directory as described earlier.When the lib pragma is loaded, it saves the current value of
@INC to the array @lib::ORIG_INC, so to restore the original,
just copy that array to the real @INC.Even though @INC typically includes dot ("."),
the current directory, this really isn't as useful as you'd think. For one thing,
the dot entry comes at the end, not the start, so that modules
installed in the current directory don't suddenly override system
versions. You could say use lib "." if that's what you really
want. More annoyingly, it's the current directory of the
Perl process, not the directory that the script was installed into,
which makes it completely unreliable. If you create a program
plus some modules for that program to use, it will work
while you're developing, but it won't work when you aren't running
in the directory the files live in.One solution for this is to use the standard FindBin module:
use FindBin; # where was script installed?
use lib $FindBin::Bin; # use that dir for libs, too
The FindBin module tries to guess the full path to the directory in which
the running process's program was installed. Don't use this for security
purposes, because malicious programs can usually deceive it if they try
hard enough. But unless you're intentionally trying to break the module,
it should work as intended. The module provides a $FindBin::Bin
variable (which you may import) that contains the module's guess of where
the program was installed. You can then use the lib pragma to add
that directory to your @INC, thus producing an executable-relative path.Some programs expect to be installed in a bin directory and then find
their library modules in "cousin" files installed in a lib
directory at the same level as bin. For example, programs
might go in /usr/local/apache/bin or /opt/perl/bin, and
libraries go in /usr/local/apache/lib and /opt/perl/lib.
This code takes care of that neatly:
use FindBin qw($Bin);
use lib "$Bin/../lib";
If you find yourself specifying the same use lib in several
unrelated programs, you might consider setting the PERL5LIB environment
variable instead. See the description of the PERL5LIB
environment variable in Chapter 19, "The Command-Line Interface".
# syntax for sh, bash, ksh, or zsh
$ PERL5LIB=$HOME/perllib; export PERL5LIB
# syntax for csh or tcsh
% setenv PERL5LIB ~/perllib
If you want to use optional directories on just this program without
changing its source, look into the -I command-line switch:
% perl -I ~/perllib program-path args
See the Chapter 19, "The Command-Line Interface" for more about using -I
from the command line.