Yesterday’s pair pro­gram­ming ses­sion had Gábor Szabó and I thrash­ing around for a bit try­ing to fig­ure out how to get test cov­er­age sta­tis­tics for the appli­ca­tion. The Devel::Cover doc­u­men­ta­tion lists how to run the mod­ule sev­er­al ways, but it does­n’t exact­ly describe how to run prove by itself rather than run­ning a Makefile​’s tests. I worked out how to do it today, and with the Baughs’ help on Twitter I worked out a few more methods.

All exam­ples below use the bash or zsh com­mand shells and were test­ed on macOS Catalina 10.15.7 run­ning zsh 5.7.1 and Perl 5.32.1. If you’re using some­thing very dif­fer­ent (e.g., Microsoft Windows’ CMD or PowerShell), you may have to set envi­ron­ment vari­ables differently.

Ad-​hoc test coverage

If all you want to do is run one shell com­mand, here it is:

$ prove -vlre 'perl -MDevel::Cover -Ilib' t

This takes advan­tage of prove​’s --exec option (abbre­vi­at­ed as -e) to run a dif­fer­ent exe­cutable for tests. It recur­sive­ly (-r) runs all your tests ver­bose­ly (-v) from the t direc­to­ry while load­ing your appli­ca­tion’s libraries (-l), while the perl exe­cutable uses (-M) Devel::Cover and the lib sub­di­rec­to­ry. I use a sim­i­lar tech­nique when debug­ging tests.

$ HARNESS_PERL_SWITCHES=-MDevel::Cover prove -vlr t

This does almost the same thing as above with­out run­ning a dif­fer­ent exe­cutable. It sets Test::Harness’ HARNESS_PERL_SWITCHES envi­ron­ment vari­able for the dura­tion of the prove com­mand. You won’t get the text out­put of your test cov­er­age at the end, though, and will still have to run Devel::Cover​’s cover com­mand to both see the cov­er­age and gen­er­ate web pages.

In a dedicated test session, window, or tab

If you have a ter­mi­nal ses­sion, win­dow, or tab ded­i­cat­ed sole­ly to run­ning your tests, set one of the envi­ron­ment vari­ables above for that session:

$ export HARNESS_PERL_SWITCHES=-MDevel::Cover

Now all of your test scripts will pick up that option. You can add more options by enclos­ing the envi­ron­ment vari­able’s val­ue in 'quotes'. For exam­ple, you might also want to load Devel::NYTProf for code profiling:

$ export HARNESS_PERL_SWITCHES='-MDevel::Cover -MDevel::NYTProf'

Why not PERL5OPT?

Setting the PERL5OPT envi­ron­ment vari­able also sets options for the perl run­ning prove, which means that your test cov­er­age, pro­fil­ing, etc. will pick up prove​’s exe­cu­tion as well as your test scripts.

What about yath?

I don’t know for sure; I don’t use the Test2 suite. But it looks like it has a --cover option for load­ing and pass­ing option to Devel::Cover.