string interpolation / variable substitution

test.sh

#!/bin/sh -ex
# I have a variable that’s a string, that includes the expansion for another variable, albeit quited: `\${foo}`
# how can I get a string that is that original string, but with the variable expanded?
# no bashisms allowed, this needs to be stock-sh.


var1=foo
var2=\${var1}/bar

# insert magic here
eval var3=$var2
echo $var3 # prints `foo/bar

Think what you want is:

var1=foo;
varx=var1;
echo "${!varx}/bar";

But that's a bashism afaik. See: http://stackoverflow.com/a/11065196, in particular:

eval is not used very often. In some shells, the most common use is to obtain the value of a variable whose name is not known until runtime. In bash, this is not necessary thanks to the ${!VAR} syntax.