Tricks about Raspberry Pi

[AllanChain]

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Handy tricks when playing with raspberry pi. You should know them.

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Get the Temperature

cat /sys/class/thermal/thermal_zone0/temp

Outputs:

29482

And here is bash script to create temperature PS1

if [ -e /sys/class/thermal/thermal_zone0/temp ]; then
    temp=$(cat /sys/class/thermal/thermal_zone0/temp)
    if [ $temp -lt 30000 ]; then color=2
    elif [ $temp -lt 50000 ]; then color=3
    else color=1
    fi
    prompt_section $color "${temp::${#temp}-3}.${temp:${#temp}-3}"
fi

Adding a dot is just mixing bash substrings (${string:offset[:length]}) with bash string lengths (${#string})

vcgencmd

From https://www.raspberrypi.org/documentation/raspbian/applications/vcgencmd.md

To get all commands, use:

vcgencmd commands

Voltage

vcgencmd get_throttled

If voltage is okay, you will get 0x0

Else, say 0x50000, convert it to binary, and see each bit:

19 18 17 16 15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0
 0  1  0  1  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0

Bit	Meaning
0	Under-voltage detected
1	Arm frequency capped
2	Currently throttled
3	Soft temperature limit active
16	Under-voltage has occurred
17	Arm frequency capping has occurred
18	Throttling has occurred
19	Soft temperature limit has occurred

Generally, the 3 0s in the middle do not have specific meaning. Just need to care about the first 5 and the last 0.

Screen on / off

When you start the Raspberry Pi with display / monitor / screen on, it will automatically turn on display power. If not, display power will never turn on even if you later connect it to a screen. So it is necessary to control the display power.

vcgencmd display_power 1

1 for on and 0 for off. bare vcgencmd display_power shows current state.

It is possible to specify a certain display ID. For more info, visit the doc above.