There's a nice way to halfway-fix that issue. It makes use of the fact that most bash commands that we copy are prefixed with a $ and a space. It also requires your terminal emulator to have a way to hook into the paste mechanism and edit the to-be-pasted text. To my knowledge, Kitty and iTerm both have this feature.

Basically, the terminal will replace each line of the pasted text that match (start with)

  $ rest

with

  pbash 'rest_escaped'

or more elegantly

  \$ 'rest_escaped'

We will then define a function with either of these names in fish.

This function will start (if not exists) a persistent background bash instance connected on both ends to FIFOs, with the same lifetime as your fish session. We then pass the `rest_escaped` to one FIFO, capture the output from the other FIFO, and echo it.

Because its a persistent session, stuff you copy that makes use of variables or bash aliases all Just Work. Being able to blindly copy-paste the entire bash codefence from a github readme.md is especially nice.

It all happens automatically after a one-time setup, and overall works pretty well for me. Here is a demo: https://i.imgur.com/HdqGkRk.png

This is the fish function

  function \$
      if not test -f /tmp/bash_daemon.pid; or not kill -0 (cat /tmp/bash_daemon.pid) 2>/dev/null
          echo "Starting bash daemon..."
          bash ~/scripts/bash_daemon.sh &
          sleep 0.5
      end
  
      echo "$argv" > /tmp/bash_daemon_pipe &
  
      while read -l line
          if test "$line" = "###DONE###"
              break
          end
  
          echo $line
      end < /tmp/bash_daemon_out
  end

And this is the bash script `~/scripts/bash_daemon.sh`

  #! /usr/bin/env bash
  
  rm -f /tmp/bash_daemon_pipe /tmp/bash_daemon_out
  mkfifo /tmp/bash_daemon_pipe
  mkfifo /tmp/bash_daemon_out
  
  echo $$ > /tmp/bash_daemon.pid
  
  while true; do
      read cmd < /tmp/bash_daemon_pipe
      { eval "$cmd"; echo "###DONE###"; } > /tmp/bash_daemon_out
  done