I use a power bank to power my Raspberry Pi, and I want to create script to turn off the Raspberry Pi when the battery level is low, but I cannot find any command to visualize the battery level.
I don't know if is possible. Does such a command exist?
No Raspberry Pi can correctly measure its input voltage without at least one additional component.
https://github.com/aboudou/picheckvoltage
Some Raspberries can detect whether the input voltage is below 4.65 V or not - and provide a simple answer of yes/no , true/false. They use this binary distinction to display the undervoltage symbol.
Ideally you would need access to the battery terminals (not the 5V) to be able to measure it and feed into the software. For that I have been using the analog-to-digital converter ADS1115 and this tutorial from Adafruit https://learn.adafruit.com/adafruit-4-channel-adc-breakouts/python-circuitpython .
You would the need to know which voltage is "low enough" and then issue a shutdown command to the Pi. I've seen some solutions posted here, like this one: How to check how much battery is left on my Raspberry Pi in the terminal?