Bug report: bash/dash
Hi, I won't pretend that this is simple to fix, but it is a bug.
The setup scripts and some of the other scripts used to install ISP Config have in their sh-bang #!/bin/sh but in reality they rely on /bin/bash. If a script is a bash script (instead of a posix-compatible shell script) then they should specify #/bin/bash since on newer Debian and Ubuntu systems /bin/sh != /bin/bash
For the record, bash takes a lot longer to start up than the simpler dash shell so dash is the default now which had a tremendous impact on boot speed. Dash is a posix compatible shell so shell scripts which conform to posix standards will work in dash or bash or numerous other shells. Posix scripts can be a pain in the butt when you're used to scripting for bash, so it's best in those cases where you don't want to rewrite/relearn to just use #!/bin/bash.
This has the additional benefit of failing outright on systems that don't have bash installed, rather than doing a half-install with tons of confusing errors and possibly leaving the system in an unusable state.