When writing a shell script, there are essentially two scripting languages to choose from: Bourne shell and C shell. These were the first two popular shells for Unix, and all common shells that have come since are compatible with one or the other.
The most popular new shells are Bourne Again shell (bash), which is an extension of Bourne shell, Korn shell (ksh), which is another extension of Bourne shell, Z-shell, a very sophisticated extension of Bourne shell, and T-shell (TENEX C shell, tcsh), which is an extended C shell.
Both Bourne shell and C shell have their own pros and cons. C shell syntax is cleaner, more intuitive, and more similar to the C programming language (hence the name C shell). However, C shell lacks some features such as subprograms (although C shell scripts can run other C shell scripts, which is arguably a better approach in many situations).
Bourne shell is used almost universally for Unix system scripts, while C shell is fairly popular in scientific research.
/bin/sh. Hence, using vanilla Bourne shell (not bash, ksh, or zsh) for scripts maximizes their portability by ensuring that they will run on any Unix system without the need to install any additional shells.
If your script contains only external commands, then it actually won't matter which shell runs it. However, most scripts utilize the shell's internal commands, control structures, and features like redirection and pipes, which differ among shells.
More modern shells such as bash, ksh, and tcsh, are backward-compatible with Bourne shell or C shell, but add additional scripting constructs in addition to convenient interactive features. The details are beyond the scope of this text. For full details, see the documentation for each shell.