feat(opts): add --shell-auto-fallback

Generates shell code that hooks into the "command not found" mechanism
and attempts to run the command with npx instead of failing. The option
has an optional argument that forces generation of a particular variant,
otherwise it attempts to autodetect the shell. However, autodetecting is
actually very flaky, so it's better to always specify the argument.

To use, place this in relevant shell config file:

For Bash:

    source <(npx --shell-auto-fallback bash)

For Zsh:

    source <(npx --shell-auto-fallback zsh)

For Fish:

    source (npx --shell-auto-fallback fish | psub)

As Seen On Twitter: https://twitter.com/passcod/status/869469928474107906

auto-fallback.js

@@ -0,0 +1,46 @@
+'use strict'
+
+const POSIX = `
+command_not_found_handler() {
+  # Do not run within a pipe
+  if test ! -t 1; then
+    echo "command not found: $1"
+    return 127
+  fi
+
+  echo "Trying with npx..."
+  npx $*
+  return $?
+}`
+
+const FISH = `
+function __fish_command_not_found_on_interactive --on-event fish_prompt
+  functions --erase __fish_command_not_found_handler
+  functions --erase __fish_command_not_found_setup
+
+  function __fish_command_not_found_handler --on-event fish_command_not_found
+    echo "Trying with npx..."
+    npx $argv
+  end
+
+  functions --erase __fish_command_not_found_on_interactive
+end`
+
+module.exports = function autoFallback (shell) {
+  const { BASH_VERSION, SHELL, ZSH_VERSION } = process.env
+
+  if (shell === 'bash' || BASH_VERSION || /bash/.test(SHELL)) {
+    return POSIX.replace('handler()', 'handle()')
+  }
+
+  if (shell === 'zsh' || ZSH_VERSION || /zsh/.test(SHELL)) {
+    return POSIX
+  }
+
+  if (shell === 'fish' || /fish/.test(SHELL)) {
+    return FISH
+  }
+
+  console.error('Only Bash, Zsh, and Fish shells are supported :(')
+  process.exit(1)
+}

index.js

@@ -3,6 +3,7 @@
 
 const BB = require('bluebird')
 
+const autoFallback = require('./auto-fallback.js')
 const cp = require('child_process')
 const getPrefix = require('./get-prefix.js')
 const parseArgs = require('./parse-args.js')
@@ -18,6 +19,12 @@ updateNotifier({pkg}).notify()
 main(parseArgs())
 
 function main (argv) {
+  const shell = argv['shell-auto-fallback']
+  if (shell || shell === '') {
+    console.log(autoFallback(shell))
+    process.exit(0)
+  }
+
   if (!argv.command || !argv.package) {
     console.error('\nERROR: You must supply a command.\n')
     yargs.showHelp()

parse-args.js

@@ -62,6 +62,14 @@ function parseArgs () {
     type: 'string',
     describe: 'execute string as if inside `npm run-script`'
   })
+  .option('shell-auto-fallback', {
+    choices: ['', 'bash', 'fish', 'zsh'],
+    describe: 'generate shell code to use npx as the "command not found" fallback',
+    default: '',
+    defaultDescription: 'detect',
+    requiresArg: false,
+    type: 'string'
+  })
   .version()
   .alias('version', 'v')