Wiring Pi Issue - #!/bin/bash vs #!/bin/sh

Question

I am working on a project and have an application shell script. For function part, I need the GPIO access, and for that, I am using the standard wiringpi package.

The issue I am getting is with the #!/bin/bash vs #!/bin/sh use

I have been using the #!/bin/bash in past in conjunction with the wiringpi function - Raspberry Pi A+

but now when I have switched to the Raspberry Pi Jessie buster July 2019, I am having an issue running the wiring Pi function

for example

gpio mode 21 out
gpio -g write 21 0

working fine inside script if I use #!/bin/sh

but I get warnings for the associative array

Warning:

./dataLog.sh: 292: ./dataLog.sh: declare: not found
./dataLog.sh: 295: ./dataLog.sh: Bad substitution

Code:

Line 291: declare -A status="($(upsc myups | sed 's/\(.*\):\(.*\)/[\1]="\2"/'))"

Line 295: if [[ ${status[device.model]} != "" ]] ; then

if I use the #!/bin/bash, I do not get any error related to the associative array, but now my GPIO is not working.

I am trying to find the solution to the issue - Where I can use #!/bin/bash shell and wiringpi functions together

Thank you

Answer 1

Finally, after fiddling around I am able to resolve the issue.

As per the WiringPi setup guide to set the direction of a gpio u should use

    gpio mode 21 out

This method works if you are using #!/bin/sh shell

But if you want to use #!/bin/bash shell then you have to use

    gpio -g mode 21 out

After, setting up the direction with use of '-g' I am able to control the GPIOs

Answer 2

Check out this post on the difference between sh and bash:

https://stackoverflow.com/questions/5725296/difference-between-sh-and-bash

In summary of key points, sh is really a standard and bash was originally an implementation of that standard that evolved to have a lot of extensions, some of which caused it to deviate from the standard.

When you do #!/bin/sh that usually actually directs you to a bash shell, but that's implementation / configuration dependent on your flavor of Linux. (Note the recent trend toward having sh point to dash in the answer at the link.) Even when it directs to bash, however, bash may know that you called it as sh and restrict access to the extensions.

So bottom line is that sh and bash are not equivalent and they may differ in different ways depending on the exact version of Linux that you have. If you are using bash extensions, then specify that you need bash. (This seems to be your case.)