How to run a package's testthat tests

R doesn't install the testthat tests by default. To do so try :

install.packages('eplusr', INSTALL_opts="--install-tests", type='source')

Then, either:

testthat::test_package('eplusr')

or alternatively using the built-in testInstalledPackage:

tools::testInstalledPackage('eplusr')

You can't (unless you reinstall overriding default behaviour as shown in Brodie's answer).

It's a design flaw^Hchoice. testthat, in all its wisdom, decided to not install tests by default by enforcing the placement in the tests/ directory. R offers an option to override it (as shown) which is generally not turned on. Hence the dilemma.

Of course, RUnit did it correctly all along having tests below inst/. So if a package uses RUnit you can run its tests once installed. Without having to reinstall it.

Edit: 1 1/2 years later, many of us now use the tinytest package which arrived within the last year. Among other advantages, also allows very easy testing of installed packages.