On cloud testbeds, conducting reproducible and verifiable experiments cannot be done by hand. This paper describes EXECO, a library providing easy and efficient control of local or remote, standalone or parallel, processes execution, as well as tools designed for scripting distributed computer science experiments or tests on any computing platform, in particular on the Grid'5000 testbed. EXECO usage is illustrated by two experiments exploring the impact on performances of virtual machines collocation and the performances of virtual machines migrations.