We present ShellOnYou, a new Computer Science education tool, and analyze its use with four successive student cohorts. Developed to help instructors manage numerous students, this web application offers auto-graded exercises to acquire practical knowledge of Unix-like operating systems from the command line. For each answer, and almost instantly, students receive a score and detailed feedback. This reactive and iterative process encourages students to resubmit answers and progressively expand their procedural knowledge. The tool can also deliver individualized statements, thereby allowing students to improve their skills by combining personal research and peer learning. As an online tool, ShellOnYou affords students access flexibility, and also easily fits in distance learning programs. We found it particularly useful when teaching students with heterogeneous Unix backgrounds. The tool is available on request. We placed four successive student cohorts in a learning situation involving this tool, and asked them to fill a survey at the end of the learning period. We combine qualitative and quantitative methods to analyze their answers to the survey. We attempt to characterize their acquisition of procedural knowledge and the building of group dynamics. Several dimensions emerge: the benefits of using the tool in a learning situation, the learning process iteration as a catalyst for renewed commitment, the tool's entertaining format and its scoring system as a motivation for regular studying, and the inherent customization of the learning pace.