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Python: Commandline parameters

The circuit used in this chapter

Switching LEDa and a servo via a GPIO using Python

Figure 1:
Two LEDs and a servo are switched in the examples of this chapter. The servo should be a microservo (also called a 9g servo), as these draw less current than standard servos.
More about how to connect LEDs to GPIOs is written in the chapter on switching LEDs via GPIOs.
Which pins have which function can be found in the chapter on the hardware of the Raspberry Pi.

Hand parameters to a script from the command line

In addition to reading parameters from a file, as demonstrated in the previous chapter, these can also be handed to a script while started from the commandline. The following example processes parameters for two LEDs and a servo:

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#!/usr/bin/env python

import sys # Needed for handling command line parameters

from gpiozero import Device, Servo, PWMLED # Needed for PWM control

# The older Raspberry Pi 1 or Pi Zero require to set the pin factory

from gpiozero.pins.pigpio import PiGPIOFactory

# Pin factory for older Raspberry Pi models

Device.pin_factory = PiGPIOFactory()

argNumber = 0

for arg in sys.argv: # Loop through all command line parameters

argNumber = argNumber + 1

if arg == "-h" or arg == "--help" or len(sys.argv) < 2:

print ("Script can set GPIO (physical pin numbers) 16 or 22 to pwm signal from 0.0 to 1.0")

print ("or set GPIO 12 to servo angle from -1.0 to 1.0")

print ("commandline.py -pwm16 <pulse-width> -pwm22 <pulse-width> -servo12 <servo angle>")

sys.exit()

elif arg == "-pwm16" or arg == "--pulse-width-pin-16":

pulseWidth16 = 0

if(argNumber < len(sys.argv)):

pulseWidth16 = float(sys.argv[argNumber])

if (pulseWidth16 < 0.0 or pulseWidth16 > 1.0):

print("Pulse-width 16 " + str(pulseWidth16) + " out of range (0.0 to 1.0)!")

sys.exit(2)

else:

pwm16 = PWMLED("BOARD16", True, 0, 100)

pwm16.value = pulseWidth16

elif arg == "-pwm22" or arg == "--pulse-width-pin-22":

pulseWidth22 = 0

if(argNumber < len(sys.argv)):

pulseWidth22 = float(sys.argv[argNumber])

if (pulseWidth22 < 0.0 or pulseWidth22 > 1.0):

print("Pulse-width 22 " + str(pulseWidth22) + " out of range (0.0 to 1.0)!")

sys.exit(2)

else:

pwm22 = PWMLED("BOARD22", True, 0, 100)

pwm22.value = pulseWidth22

elif arg == "-servo12" or arg == "--servo-angle-pin-12":

servoAngle12 = 0

if(argNumber < len(sys.argv)):

servoAngle12 = float(sys.argv[argNumber])

if (servoAngle12 < -1.0 or servoAngle12 > 1.0):

print("Servo angle 12 " + str(servoAngle12) + " out of range (-1.0 to 1.0)!")

sys.exit(2)

else:

servo12 = Servo("BOARD12")

servo12.value = servoAngle12

#!/usr/bin/env python

import sys # Needed for handling command line parameters
from gpiozero import Device, Servo, PWMLED # Needed for PWM control

# The older Raspberry Pi 1 or Pi Zero require to set the pin factory
from gpiozero.pins.pigpio import PiGPIOFactory

# Pin factory for older Raspberry Pi models
Device.pin_factory = PiGPIOFactory()

argNumber = 0

for arg in sys.argv: # Loop through all command line parameters

argNumber = argNumber + 1

if arg == "-h" or arg == "--help" or len(sys.argv) < 2:
        print ("Script can set GPIO (physical pin numbers) 16 or 22 to pwm signal from 0.0 to 1.0")
        print ("or set GPIO 12 to servo angle from -1.0 to 1.0")
        print ("commandline.py -pwm16 <pulse-width>  -pwm22 <pulse-width> -servo12 <servo angle>")
        sys.exit()
    elif arg == "-pwm16" or arg == "--pulse-width-pin-16":
        pulseWidth16 = 0
        if(argNumber < len(sys.argv)):
            pulseWidth16 = float(sys.argv[argNumber])
        if (pulseWidth16 < 0.0 or pulseWidth16 > 1.0):
            print("Pulse-width 16 " + str(pulseWidth16) + " out of range (0.0 to 1.0)!")
            sys.exit(2)
        else:
            pwm16 = PWMLED("BOARD16", True, 0, 100)
            pwm16.value = pulseWidth16
    elif arg == "-pwm22" or arg == "--pulse-width-pin-22":
        pulseWidth22 = 0
        if(argNumber < len(sys.argv)):
            pulseWidth22 = float(sys.argv[argNumber])
        if (pulseWidth22 < 0.0 or pulseWidth22 > 1.0):
            print("Pulse-width 22 " + str(pulseWidth22) + " out of range (0.0 to 1.0)!")
            sys.exit(2)
        else:
            pwm22 = PWMLED("BOARD22", True, 0, 100)
            pwm22.value = pulseWidth22
    elif arg == "-servo12" or arg == "--servo-angle-pin-12":
        servoAngle12 = 0
        if(argNumber < len(sys.argv)):
            servoAngle12 = float(sys.argv[argNumber])
        if (servoAngle12 < -1.0 or servoAngle12 > 1.0):
            print("Servo angle 12 " + str(servoAngle12) + " out of range (-1.0 to 1.0)!")
            sys.exit(2)
        else:
            servo12 = Servo("BOARD12")
            servo12.value = servoAngle12

Save the script as "/home/pi/commandline.py" and make it executable:
chmod a+x background.py

If you call the script, you can hand the servo lever position or the duty cycle of the LEDs as parameter. The following example sets the pulse width of LED 16 to 40%:
./commandline.py -pwm16 0.4

The command to move the servo lever to 45 degrees:
./commandline.py -servo12 0.5

or with the long version:
./commandline.py --servo-angle-pin-12 0.5

Several parameters can also be transferred at the same time:
./commandline.py -servo12 -0.5 -pwm16 0.1 -pwm22 0.3

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