/bin/bash is not portable

[jirutka]

It’s wrong to assume that bash is installed in /bin or even installed at all – /bin/bash in shebang is not well portable. Using bash for every shell script is a bad practice that even caused several security issues in the history (that’s why Debian/Ubuntu now uses dash as /bin/sh). Bash is just one of many Shell language implementations and… just look under the hood of its development, into its source repository, you cannot seriously trust it…

You can safely assume that /bin/sh exists (specified in POSIX), as well as /usr/bin/env(the command itself is specified in POSIX, the path is not, but in practice, every *nix system has /usr/bin/env).

Please, don’t teach students that shell = bash, it’s a pretty bad practice.

[encukou]

I'll be sure to point out that shell ≠ bash.
On the other hand, AFAIK on many distros Bash is the default shell for users, so the course works with this. (The argument that I "cannot seriously trust it" should be made against distros; I don't think it's my place to second-guess the defaults and tell people to configure their shell before starting.)
The next point is that scripts are "just" CLI input saved in a file. I think this point is extremely important for beginners – more so than POSIX and cross-distro compatibility.

So, I'll say that creating portable scripts is a bit outside the scope of the beginners' course. I think it's best taught when the need is encountered, because then people will have a better understanding of why it's important.

[jirutka]

I'll be sure to point out that shell ≠ bash.

Thanks!

AFAIK on many distros Bash is the default shell for users

The default interactive shell, not for scripts. For example, /bin/sh (the default shell) on Debian is dash, /bin/sh on Alpine Linux is busybox ash and bash is not installed by default.

The argument that I "cannot seriously trust it" should be made against distros

How are distros responsible for bash? Distros give users a choice of which shell to use; you’re undermining this flexibility by forcing particular shell implementation in your scripts.

I don't think it's my place to second-guess the defaults and tell people to configure their shell before starting.

I think that you misunderstood the point. You’re not supposed to guest the defaults, just use /bin/sh and avoid bashisms for simple shell scripts.

The next point is that scripts are "just" CLI input saved in a file.

Bash is a superset of the POSIX command language, i.e. every POSIX compatible shell script will work on Bash, that’s the point. It’s not about writing scrips in a different syntax than commands in interactive session, it’s just about not using non-portable Bash/ZSH extensions in your commands.

So, I'll say that creating portable scripts is a bit outside the scope of the beginners' course

I disagree. You don’t have to teach beginners how to write portable scripts (i.e. what syntactic features are portables etc.), but you can and should avoid unnecessary bashisms in your examples for didactic reason. When beginners see #!/bin/bash in all educational materials, they will naturally repeat it in their own scripts – all scripts, despite they actually use/need bash-specific features – which leads to a limitation of choice for users of their scripts.