RPI vcgencmd usage
Shows a list of possible (needs verification) commands.
vcgencmd measure_clock <clock>
Shows clock frequency, clock can be one of arm, core, h264, isp, v3d, uart, pwm, emmc, pixel, vec, hdmi, dpi.
vcgencmd measure_volts <id>
Shows voltage, id can be one core, sdram_c, sdram_i, sdram_p, and defaults to core if not specified.
Shows core temperature of BCM2835 SoC.
vcgencmd codec_enabled <codec>
Shows if the specified codec is enabled, codec can be one of H264 MPG2 WVC1
Parameters and function of other commands are not known.