sudo in non-interactive script

Add your script to the /etc/sudoers file with the NOPASSWD attribute, so that it is permitted to run without prompting for a password. You can tie this down to a specific user (or set of users), or allow it to be run with sudo by anyone on your system.

A sample line for a script called /usr/local/bin/bossy might look something like this

ALL ALL = (root) NOPASSWD: /usr/local/bin/bossy

And you'd then use something like this

A && sudo bossy && C

For this example I assumed PATH includes /usr/local/bin. If not, then just use the full path to the script, i.e. sudo /usr/local/bin/bossy

I think the best thing that you can do is launch the script with sudo and then launch the processes you want to run as a normal user explicitly with su user or sudo -u user:

#!/usr/bin/env bash

## Detect the user who launched the script
usr=$(env | grep SUDO_USER | cut -d= -f 2)

## Exit if the script was not launched by root or through sudo
if [ -z $usr ] && [ $UID -ne 0 ]
then
    echo "The script needs to run as root" && exit 1
fi

## Run the job(s) that don't need root
sudo -u $usr commandA

## Run the job that needs to be run as root
commandB