Remove a fixed prefix/suffix from a string in Bash

$ prefix="hell"
$ suffix="ld"
$ string="hello-world"
$ foo=${string#"$prefix"}
$ foo=${foo%"$suffix"}
$ echo "${foo}"
o-wor

This is documented in the Shell Parameter Expansion section of the manual:

${parameter#word}
${parameter##word}

The word is expanded to produce a pattern and matched according to the rules described below (see Pattern Matching). If the pattern matches the beginning of the expanded value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the # case) or the longest matching pattern (the ## case) deleted. […]

${parameter%word}
${parameter%%word}

The word is expanded to produce a pattern and matched according to the rules described below (see Pattern Matching). If the pattern matches a trailing portion of the expanded value of parameter, then the result of the expansion is the value of parameter with the shortest matching pattern (the % case) or the longest matching pattern (the %% case) deleted. […]

Using sed:

$ echo "$string" | sed -e "s/^$prefix//" -e "s/$suffix$//"
o-wor

Within the sed command, the ^ character matches text beginning with $prefix, and the trailing $ matches text ending with $suffix.

Adrian Frühwirth makes some good points in the comments below, but sed for this purpose can be very useful. The fact that the contents of $prefix and $suffix are interpreted by sed can be either good OR bad- as long as you pay attention, you should be fine. The beauty is, you can do something like this:

$ prefix='^.*ll'
$ suffix='ld$'
$ echo "$string" | sed -e "s/^$prefix//" -e "s/$suffix$//"
o-wor

which may be what you want, and is both fancier and more powerful than bash variable substitution. If you remember that with great power comes great responsibility (as Spiderman says), you should be fine.

A quick introduction to sed can be found at http://evc-cit.info/cit052/sed_tutorial.html

A note regarding the shell and its use of strings:

For the particular example given, the following would work as well:

$ echo $string | sed -e s/^$prefix// -e s/$suffix$//

...but only because:

1. echo doesn't care how many strings are in its argument list, and
2. There are no spaces in $prefix and $suffix

It's generally good practice to quote a string on the command line because even if it contains spaces it will be presented to the command as a single argument. We quote $prefix and $suffix for the same reason: each edit command to sed will be passed as one string. We use double quotes because they allow for variable interpolation; had we used single quotes the sed command would have gotten a literal $prefix and $suffix which is certainly not what we wanted.

Notice, too, my use of single quotes when setting the variables prefix and suffix. We certainly don't want anything in the strings to be interpreted, so we single quote them so no interpolation takes place. Again, it may not be necessary in this example but it's a very good habit to get into.