I say this as somebody who has written shell scripts for almost 20 years.

Constructs like

    if [ "X${str1}" = "X${str2}" ]

Not to mention the hell that is trying to use a user supplied filename safely in shellscript. (for the uninitiated, a unix filename can contain any character, and any sequence of bytes besides / and 0x00, including newlines, backspaces, shell meta-charachters and so many more exciting things)

Similarly, not every Linux distribution puts bash in the same place, e.g. NixOS. So blindly putting `#!/bin/bash` as your shebang will result in a broken script on those systems.

You can just install bash or move it around to suit your script, but maybe you're shipping this script for other people to use and can't control their environments. It's easier to make a handful of changes to the script instead. This is why some people care about portability.

But the modern reason is https://en.wikipedia.org/wiki/Almquist_shell (ash) and its descendant, dash. Busybox environments ship with ash as the only available shell, unless you manually build another shell into the busybox chroot. As well, many initrd/initramfs environments have only one shell, usually dash. When scripting either of these, you have to write pure POSIX scripts.

And—mostly due to inertia—this tends to apply to all such environments, no matter their size. My Synology NAS (because it’s technically a busybox env) doesn’t have bash, despite having huge honking disks! Neither does CoreOS (because it’s technically a PXE initramfs env), despite not being intended as a “link-bandwidth-constrained” PXE environment at all. They’re just descended from these “lineages” of OSes that were used on more constrained devices in more constrained times, so their maintainers have kept to the old embedded-system ways.

On top of this, as the ash wiki article says, Debian made a choice to make dash their /bin/sh, and then forced all system scripts to use it, thus forcing them to be rewritten to be POSIX-compatible. (I think this was done with goal of lower memory-usage for constrained Debian environments that might still need to install some arbitrary subset of Debian system services.)

There was also another kind of pressure, 10-20 years ago, with Linux single-floppy or low-link-bandwidth PXE boot environments wanting to be as slim as possible, and therefore not shipping with bash.

https://www.gnu.org/software/bash/manual/html_node/Bash-POSI...