Bash – run Bash scripts in FreeBSD without modifying them

Actually the shebang lines means something as some distributions like Debian don't use bash for /bin/sh but dash.

The simplest solution to determine if a shell script is bash is to use checkbashism - see man checkbashisms for details.

checkbashisms should be packaged by different distributions, for Debian it is in the devscripts package - Fedora in devscripts-minimal (it used to be in rpmdevtools) and OpenSuSe in rpmlint-mini

does using the .bash extension actually invoke bash or does it depend on system config / 1st shebang line.

If you do not use an interpreter explicitly, then the interpreter being invoked is determined by the shebang used in the script. If you use an interpreter specifically then the interpreter doesn't care what extension you give for your script. However, the extension exists to make it very obvious for others what kind of script it is.

[sreeraj@server ~]$ cat ./ext.py
#!/bin/bash
echo "Hi. I am a bash script"

See, .py extension to the bash script does not make it a python script.

[sreeraj@server ~]$ python ./ext.py
  File "./ext.py", line 2
    echo "Hi. I am a bash script"
                                ^
SyntaxError: invalid syntax

Its always a bash script.

[sreeraj@server ~]$ ./ext.py
Hi. I am a bash script