Output redirection doesn't work right with Fish functions, which makes them unusable in certain kinds of scripts. Aside from that, though, Fish is perfectly pleasant for simple scripting. I use Fish for simple stuff and PowerShell for anything serious. Some day I'd like to use something like Elvish or Nushell for both, and use the same language as my interactive shell.
bash -c 'echo "hello"'
But:
1. I have CTRL+R bound to fuzzy reverse history search via fzf in my Fish config. It works great. It's fast, and it's way better than the built-in Bash reverse history search imo, even though it works basically the same way (it should feel very familiar). fzf itself comes with the keybindings, so you don't have to sort them out yourself; you can just source 'em: https://github.com/junegunn/fzf/blob/master/shell/key-bindin...
2. For one-off one liners you see in GitHub repos or something, if you really want tab completion and syntax highlighting and whatever, you can just use `exec` twice:
⋊> ~ echo $HELLO
⋊> ~ exec bash
~ $ export HELLO=hello
~ $ exec fish
Welcome to fish, the friendly interactive shell
⋊> ~ echo $HELLO
hello
It may not work right for some of them, but it's worked on the scripts I've tried. You might also be able to bind a key that translates whatever command there is on the prompt into Fish. It seems to handle the basics well, including subshells with redirection/process substitution, e.g.:
~ $ cat <(echo hi)
hi
~ $ history | sed -E 's/^[[:space:]]+[[:digit:]]+[[:space:]]+//g' | grep -E '^cat' | tail -n1 | babelfish
cat (echo hi | psub)
~ $ fish
Welcome to fish, the friendly interactive shell
⋊> ~ cat (echo hi | psub)
hi
⋊> ~ exit
~ $
I've used several dozen shells, script languages, and programming languages - and have needed to do within the context of a single job. What's one more? For an interactive shell I prioritize writeability, and for scripts I prioritize readability and maintainability. These are often at odds - not just in what I write, but in the syntax itself - and by making my shell and script the same, I make middling compromises resulting in a jack of all trades that is a master of none.
I disagree that this is "ideal".
> and you can begin to shape scripts out of your shell history.
It's been my experience that this is quite doable even when the scripting language has different syntax than your shell. Oh, sure, it's not as simple as Copy+Paste - but it's never that simple. Even simple shell scripts inevitably end up needing error checking, sanity checks, error messages, ...
bash
PASTE_HERE
exit
echo "pwd: $(pwd)"
Beside very basic usage, a shell is really a REPL for a scripting language. You can type commands but also you can do most advanced things (and that is what makes shell powerful). For example I maybe have to convert all the video files in a directory, and then upload them to a server, I can easily do that with a for loop in the shell, without creating a script.
And since you use both the shell to type commands and to write effectively scripts as in a REPL, that means that you have to learn another completely different programming language, the fish language (of course I assume that you already know POSIX scripting), and not only that, it's probably the case that only on your computer you have fish but it's not installed on every other system, so when you are using another computer (a server, a computer of a coworker that you are helping, etc) you will make mistakes because you confuse the fish syntax with the one of POSIX shell.
Yes, it's true that bash extends POSIX, and zsh too and it's not even compatible 100% with bash, but the differences are in things that it's rare that you will use in a basic script, and all these shells can understand POSIX syntax. A for loop it's the same in sh, bash and zsh (there is a shorter syntax in zsh, but the base one is the same), while it's completely different in fish.