Newlines in a script are almost always equivalent to semicolons:

mycmd.sh; ret=$?; rm -rf temp_files; exit $ret

Newlines in a script are almost always equivalent to semicolons:

mycmd.sh; ret=$?; rm -rf temp_files; exit $ret

In response to the edit:

Alternatively, you could also use a trap and a subshell:

( trap 'rm -rf temp_files' EXIT; mycmd.sh )