What command is used to dump the current kernel ring buffer to standard output?

The command used to dump the current kernel ring buffer to standard output is dmesg. This command allows users to view messages that are logged by the kernel, which can include important information about system events, device drivers, and hardware status. The kernel ring buffer is a circular buffer that stores log messages generated by the kernel; therefore, running dmesg without any arguments will display the most recent messages stored in this buffer.

In contrast, the other options do not serve the same purpose. The cat command is typically used to display the contents of files rather than kernel messages. echo is used to output text or variable values to the standard output but does not read from the kernel ring buffer. logread is specific to certain Linux distributions (like OpenWrt) and is used to display the system log messages rather than the kernel ring buffer. Thus, dmesg is the most appropriate command for this task.