How is Perl's @INC constructed? (aka What are all the ways of affecting where Perl modules are searched for?)

What are all the ways of affecting where Perl 开发者_开发技巧modules are searched for? or, How is Perl's @INC constructed?

As we know, Perl uses @INC array containing directory names to determine where to search for Perl module files.

There does not seem to be a comprehensive "@INC" FAQ-type post on StackOverflow, so this question is intended as one.

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We will look at how the contents of this array are constructed and can be manipulated to affect where the Perl interpreter will find the module files.

1. Default @INC

   Perl interpreter is compiled with a specific @INC default value. To find out this value, run env -i perl -V command (env -i ignores the PERL5LIB environmental variable - see #2) and in the output you will see something like this:

   $ env -i perl -V
   ...
   @INC:
    /usr/lib/perl5/site_perl/5.18.0/x86_64-linux-thread-multi-ld
    /usr/lib/perl5/site_perl/5.18.0
    /usr/lib/perl5/5.18.0/x86_64-linux-thread-multi-ld
    /usr/lib/perl5/5.18.0
    .
   

Note . at the end; this is the current directory (which is not necessarily the same as the script's directory). It is missing in Perl 5.26+, and when Perl runs with -T (taint checks enabled).

To change the default path when configuring Perl binary compilation, set the configuration option otherlibdirs:

Configure -Dotherlibdirs=/usr/lib/perl5/site_perl/5.16.3

2. Environmental variable PERL5LIB (or PERLLIB)

   Perl pre-pends @INC with a list of directories (colon-separated) contained in PERL5LIB (if it is not defined, PERLLIB is used) environment variable of your shell. To see the contents of @INC after PERL5LIB and PERLLIB environment variables have taken effect, run perl -V.

   $ perl -V
   ...
   %ENV:
     PERL5LIB="/home/myuser/test"
   @INC:
    /home/myuser/test
    /usr/lib/perl5/site_perl/5.18.0/x86_64-linux-thread-multi-ld
    /usr/lib/perl5/site_perl/5.18.0
    /usr/lib/perl5/5.18.0/x86_64-linux-thread-multi-ld
    /usr/lib/perl5/5.18.0
    .
   

3. -I command-line option

   Perl pre-pends @INC with a list of directories (colon-separated) passed as value of the -I command-line option. This can be done in three ways, as usual with Perl options:

   • Pass it on command line:

     perl -I /my/moduledir your_script.pl
     

   • Pass it via the first line (shebang) of your Perl script:

     #!/usr/local/bin/perl -w -I /my/moduledir
     

   • Pass it as part of PERL5OPT (or PERLOPT) environment variable (see chapter 19.02 in Programming Perl)

4. Pass it via the lib pragma

   Perl pre-pends @INC with a list of directories passed in to it via use lib.

   In a program:

   use lib ("/dir1", "/dir2");
   

   On the command line:

   perl -Mlib=/dir1,/dir2
   

   You can also remove the directories from @INC via no lib.

5. You can directly manipulate @INC as a regular Perl array.

   Note: Since @INC is used during the compilation phase, this must be done inside of a BEGIN {} block, which precedes the use MyModule statement.

   • Add directories to the beginning via unshift @INC, $dir.

   • Add directories to the end via push @INC, $dir.

   • Do anything else you can do with a Perl array.

Note: The directories are unshifted onto @INC in the order listed in this answer, e.g. default @INC is last in the list, preceded by PERL5LIB, preceded by -I, preceded by use lib and direct @INC manipulation, the latter two mixed in whichever order they are in Perl code.

References:

• perldoc perlmod
• perldoc lib
• Perl Module Mechanics - a great guide containing practical HOW-TOs
• How do I 'use' a Perl module in a directory not in @INC?
• Programming Perl - chapter 31 part 13, ch 7.2.41
• How does a Perl program know where to find the file containing Perl module it uses?

There does not seem to be a comprehensive @INC FAQ-type post on Stack Overflow, so this question is intended as one.

When to use each approach?

• If the modules in a directory need to be used by many/all scripts on your site, especially run by multiple users, that directory should be included in the default @INC compiled into the Perl binary.

• If the modules in the directory will be used exclusively by a specific user for all the scripts that user runs (or if recompiling Perl is not an option to change default @INC in previous use case), set the users' PERL5LIB, usually during user login.

  Note: Please be aware of the usual Unix environment variable pitfalls - e.g. in certain cases running the scripts as a particular user does not guarantee running them with that user's environment set up, e.g. via su.

• If the modules in the directory need to be used only in specific circumstances (e.g. when the script(s) is executed in development/debug mode, you can either set PERL5LIB manually, or pass the -I option to perl.

• If the modules need to be used only for specific scripts, by all users using them, use use lib/no lib pragmas in the program itself. It also should be used when the directory to be searched needs to be dynamically determined during runtime - e.g. from the script's command line parameters or script's path (see the FindBin module for very nice use case).

• If the directories in @INC need to be manipulated according to some complicated logic, either impossible to too unwieldy to implement by combination of use lib/no lib pragmas, then use direct @INC manipulation inside BEGIN {} block or inside a special purpose library designated for @INC manipulation, which must be used by your script(s) before any other modules are used.

  An example of this is automatically switching between libraries in prod/uat/dev directories, with waterfall library pickup in prod if it's missing from dev and/or UAT (the last condition makes the standard "use lib + FindBin" solution fairly complicated. A detailed illustration of this scenario is in How do I use beta Perl modules from beta Perl scripts?.

• An additional use case for directly manipulating @INC is to be able to add subroutine references or object references (yes, Virginia, @INC can contain custom Perl code and not just directory names, as explained in When is a subroutine reference in @INC called?).

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In addition to the locations listed above, the OS X version of Perl also has two more ways:

6. The /Library/Perl/x.xx/AppendToPath file. Paths listed in this file are appended to @INC at runtime.

7. The /Library/Perl/x.xx/PrependToPath file. Paths listed in this file are prepended to @INC at runtime.

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As it was said already @INC is an array and you're free to add anything you want.

My CGI REST script looks like:

#!/usr/bin/perl
use strict;
use warnings;
BEGIN {
    push @INC, 'fully_qualified_path_to_module_wiht_our_REST.pm';
}
use Modules::Rest;
gone(@_);

Subroutine gone is exported by Rest.pm.