We will look at how the contents of this array are constructed and can be manipulated to affect where the Perl interpreter will find module files.
- Default
@INC
The Perl interpreter is compiled with a specific default value for @INC
. To find this value, run the command env -i perl -V
(env -i
ignores the PERL5LIB
environment 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 the .
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 compilation of the Perl binary, set the configuration option otherlibdirs
:
Configure -Dotherlibdirs=/usr/lib/perl5/site_perl/5.16.3
- Environment variable
PERL5LIB
(orPERLLIB
)
Perl prepends a list of directories (colon-separated) contained in the PERL5LIB
environment variable of your shell to @INC
(if PERL5LIB
is not defined, PERLLIB
is used). To see the contents of @INC
after the 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 .
-I
command-line option
Perl prepends a list of directories (colon-separated) passed as the parameter of the -I
command-line option to @INC
. 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 the
PERL5OPT
(orPERLOPT
) environment variable (see chapter 19.02 in Programming Perl)
- Pass it via the
lib
pragma
Perl prepends a list of directories passed in to it via use lib
to @INC
.
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
.
- You can directly manipulate
@INC
as a regular Perl array.
Note: Since @INC
is used during the compilation phase, this must be done inside 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 for changing the default
@INC
in the 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 a very nice use case). -
If the directories in
@INC
need to be manipulated according to some complicated logic, either impossible or too unwieldy to implement by a combination ofuse lib
/no lib
pragmas, then use direct@INC
manipulation inside aBEGIN {}
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?).