You state in your comments that you know:
- How to install bash.
- Not to change the default shell away for root.
- Bash does not install into /bin and is hence not available first hand in single user mode.
- Of the toor user.
All the above is correct and good for you!
A small misconception which is not important. The toor user was created for this purpose. It has uid 0 (as root does) so you can set any shell you like for toor.
A larger misconception is that most FreeBSD scripts are written in (t)csh. They are not! System script are written in sh. Just have a look at
Bash is not part of the FreeBSD Operating System. But the system does not become impure if you install and use it. You do not even have to change the user shell. But if you do - just keep your hands of root.
There are however larger issues at hand.
Do not write scripts in csh!!!
Even though the default root shell is (t)csh you should never ever write csh scripts on any system. That is pretty common consensus and if you need the full story please read CSH PROGRAMMING CONSIDERED HARMFUL
Whenever you write a script you should start the first line with the "shebang" (see more in Bash Shebang). This tells the system which interpreter to use when executing the script. You can with no problem execute a bash (or perl/zsh/etc) script from csh.
For the best portability use:
This will look through $PATH to find your location of bash. To me this is the best solution.
Others advocate that you should never do that but specify the exact location. This then adds a requirement to the target system. Either you update shebang shen moving a script - or you ensure the target system has a link to bash in /bin.
Use sh for broad portability
If you want even better portability then use the sh shell for scripting. It has less features than bash but you can be sure it is available on all POSIX(-like) systems.
Shell scripting is hard!
Writing a proper shell script can be really hard. The first and best tool to help you is Shellcheck. You can install it locally as well.
Make it a habit to check your scripts - this will save you from many headaches.