In most cases dhclient scripts are written in bash. In any case they are executed by dhclient-script, the shebang of which - at least in Ubuntu - calls for it t

In most cases dhclient scripts are written in bash. In any case they are executed by dhclient-script, the shebang of which - at least in Ubuntu - calls for it to be executed by /bin/bash for the specific reason that the maintainer did not want to rewrite out bash-isms when the default system shell was switched to dash.