A lock includes a body with a lock cylinder and a knob on two ends of the body. A cam is located between the lock cylinder and the knob. The lock cylinder includes a transmission member is pushed by resilient element toward the cam. The knob includes a shaft. A spring, an engaging member and a driving member are located to one end of the shaft. The engaging member includes a contact portion with a head. A first housing is connected to the body and includes a stop unit. In an initial position, the driving member contacts the transmission member. The end block resiliently contacts the head. In a locked position, the end block is located between the shaft and the head to restrict the engaging member from being pushed by the lock cylinder so that the lock is automatically locked. The knob is axially moved to unlock position.