What are all the ways of affecting where Perl 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.
Best Answer
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.
Default
@INC
Perl interpreter is compiled with a specific
@INC
default value. To find out this value, runenv -i perl -V
command (env -i
ignores thePERL5LIB
environmental variable - see #2) and in the output you will see something like this: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
:Environmental variable
PERL5LIB
(orPERLLIB
)Perl pre-pends
@INC
with a list of directories (colon-separated) contained inPERL5LIB
(if it is not defined,PERLLIB
is used) environment variable of your shell. To see the contents of@INC
afterPERL5LIB
andPERLLIB
environment variables have taken effect, runperl -V
.-I
command-line optionPerl 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:
Pass it via the first line (shebang) of your Perl script:
Pass it as part of
PERL5OPT
(orPERLOPT
) environment variable (see chapter 19.02 in Programming Perl)Pass it via the
lib
pragmaPerl pre-pends
@INC
with a list of directories passed in to it viause lib
.In a program:
On the command line:
You can also remove the directories from
@INC
viano 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 of aBEGIN {}
block, which precedes theuse 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 byPERL5LIB
, preceded by-I
, preceded byuse lib
and direct@INC
manipulation, the latter two mixed in whichever order they are in Perl code.References:
@INC
?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 ofuse lib
/no lib
pragmas, then use direct@INC
manipulation insideBEGIN {}
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?).