Script for adding a disk to fstab if not existing

Elegant and short way:

#!/bin/bash
if ! grep -q 'init-poky' /etc/fstab ; then
    echo '# init-poky' >> /etc/fstab
    echo '/dev/sdb1    /media/poky    ext4    defaults    0    2' >> /etc/fstab
fi

It uses native Bash command exit code ($?=0 for success and >0 for error code) and if grep produces error, means NOT FOUND, it does inverse (!) of result, and adds fstab entry.

Here's a simple and hopefully idiomatic solution.

grep -q 'init-poky' /etc/fstab || 
printf '# init-poky\n/dev/sdb1    /media/poky    ext4    defaults    0    2\n' >> /etc/fstab

If the exit status from grep -q is false, execute the printf. The || shorthand can be spelled out as

if grep -q 'ínit-poky' /etc/fstab; then
    : nothing
else
    printf ...
fi

Many beginners do not realize that the argument to if is a command whose exit status determines whether the then branch or the else branch will be taken.