Using a variable in between ' 's

Question

I have a bash script with a line that was originally this

convert '%d.jpg[1-300]' combined.pdf

Uses convert from Imagemagick to strap a load of sequentially numbered jpgs in to a PDF.

I've written a basic script to accept user input for a changeable number of pages and tried to swap the 300 for a variable set by the user when the script runs using read -p to make this:

convert '%d.jpg[1-$pages]' combined.pdf

The rest of the script works fine but it fails on this line and I'm not sure why. Any suggestions?

I suspect its something to do with incorrect delimiters but I don't know the correct thing to look for.

Answer 1

Parameter expansion is not performed inside single-quotes. It is performed inside double quotes though, so:

convert "%d.jpg[1-$pages]" combined.pdf

In any case, you need some form of quoting as [ is special in most shells as a glob operator (fish being a notable exception, but in fish (older versions anyway) it's % that's special), and in all Bourne-like shells except zsh, unquoted parameter expansion is subject to split+glob which we don't want here.