Script for adding a disk to fstab if not existing

Question

First of all, I have read many posts (see at bottom) with if-clauses to search in a file for a specific string and followed these posts 1to1, however I don't manage to get my script to work. I want to make an entry in /etc/fstab if it doesn't exist yet:

#!/bin/bash
fstab=/etc/fstab

if grep -q "poky-disc" "$fstab"
then
    echo "#poky-disc" >> /etc/fstab
    echo "/dev/sdb1 /media/poky ext4 defaults 0 2" >> /etc/fstab
else
    echo "Entry in fstab exists."
fi

Thanks for your help in advance. These are the similar posts, which didnt help me further:

• How to test if string exists in file with Bash shell?
• How to check if a file contains a specific string using bash

Answer 1

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.

Answer 2

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.

Answer 3

I had the same issue. I managed to edit it with this command:

sudo su -c "echo '#test' >> /etc/fstab"