Difference between bash,ksh,tcsh and zsh

Question

Consider the following shell code:

unset foo
echo "${foo-'}'}"

The result depends on which shell is used:

• bash: '}'
• ksh: ''}
• tcsh: Missing }.
• zsh: ''}

Which shell(s) behave(s) according to the POSIX standard?

Answer 1

A POSIX-compliant shell should output:

''}