Variable expansion in shell commands should work in substrings

[fperez]

Consider:

b=10
!echo f$b
!echo f${b}.txt

the output is:

f10
f.txt

Our expansion logic works only if the variable to be expanded is either alone or at the end of a string. It would be more convenient if we also covered the second case, with the output being f10.txt, as it would make many common shell scenarios easier to type.

[fperez]

Never mind, f{b}.txt works, I'd forgotten. Closing.