[susam]

Indeed, Bash is always available on Debian! After all, Debian uses Bash as the default interactive shell. But that's not the point of writing system scripts for dash. They are written for dash because that's still the default non-interactive shell. And it is so because dash is leaner and faster. Quoting from the link I posted in my previous comment:

> Since it executes scripts faster than bash, and has fewer library dependencies (making it more robust against software or hardware failures), it is used as the default system shell on Debian systems.