There are practice tests online for the knowledge portion of the commercial drivers license test at websites such as Truckercountry.com, CDLTest.co and Driving-Tests.org. The knowledge test for the CDL consists of a general knowledge portion and sections for any certifications sought by the applicant.
Common certifications on CDL tests include air brakes, tanker, passenger and hazardous materials. Each of those certifications requires a different, separate knowledge test to acquire. Truckercountry.com features a comprehensive selection of practice tests online for nearly every CDL certification. CDL test formats and requirements vary depending on state and federal department of transporation jurisdictions where the test is being taken.