How do I prompt the user from within a commit-msg hook?

I want to warn the user if their commit message doesn't follow a certain set of guidelines, and then give them the option to edit their commit message, ignore the warning, or cancel the commit. The problem is that I don't seem to have access to stdin.

Below is my commit-msg file:

function verify_info {
    if [ -z "$(grep '$2:.*[a-zA-Z]' $1)" ]
    then
        echo >&2 $2 information should not be omitted
        local_editor=`git config --get core.editor`
        if [ -z "${local_editor}" ]
        then
            local_editor=${EDITOR}
        fi
        echo "Do you want to"
        select CHOICE in "edit the commit message" "ignore this warning" "cancel the commit"; do
            case ${CHOICE} in
                i*) echo "Warning ignored"
                    ;;
                e*) ${local_editor} $1
                    verify_info "$1" $2
                    ;;
                *)  echo "CHOICE = ${CHOICE}"
                    exit 1
                    ;;
            esac
        done
    fi
}

verify_info "$1" "Scope"
if [ $# -ne 0 ];
then
    exit $#
fi
verify_info "$1" "Affects"
if [ $# -ne 0 ];
then
    exit $#
fi

exit 0

Here is the output when I leave the Scope information blank:

Scope information should not be omitted
Do you want to:
1) edit the commit message  3) cancel the commit
2) ignore this warning
#?

The message is correct, but it doesn't actually stop for input. I've also tried using the simpler read command, and it has the same problem. It seems that the problem is that at this point git has control of stdin and is providing its own input. How do I fix this?

Update: It seems this might be a duplicate of this question which unfortunately seems to suggest I'm out of luck.

Calling exec < /dev/tty assigns standard input to the keyboard. Works for me in a post-commit git hook:

#!/bin/sh

echo "[post-commit hook] Commit done!"

# Allows us to read user input below, assigns stdin to keyboard
exec < /dev/tty

while true; do
  read -p "[post-commit hook] Check for outdated gems? (Y/n) " yn
  if [ "$yn" = "" ]; then
    yn='Y'
  fi
  case $yn in
      [Yy] ) bundle outdated --pre; break;;
      [Nn] ) exit;;
      * ) echo "Please answer y or n for yes or no.";;
  esac
done

The commit-msg hook is not run in an interactive environment (as you have noticed).

The only way to reliable notify the user would be to write an error to stdout, place a copy of the commit message in a BAD_MSG file and instruct the user to edit the file and git commit --file=BAD_MSG

---

If you have some control over the environment you could have an alternate editor which is a wrapper script that checks the proposed message, and can restart the editor with an extra commented message.

Basically, you run the editor, check the file saved against your rules. and if it fails, prepend your warning message (with leading #) to the file and restart the editor.

You could even allow them to put in a #FORCE=true line in the message which would suppress the check and continue.

read -p "Question? [y|n] " -n 1 -r < /dev/tty
echo
if echo $REPLY | grep -E '^[Yy]$' > /dev/null; then
#do if Yes
else
#do if No
fi

this works fine when running git commit from command line. On windows (haven't tryed on linux) if you use gitk or git-gui you won't be able to prompt because you get an error on the exec < /dev/tty line.

The sollution is to call git-bash.exe in your hook:

.git/hooks/post-commit contains:

#!/bin/sh
exec /c/Program\ Files/Git/git-bash.exe /path/to/my_repo/.git/hooks/post-checkout.sh

the .git/hooks/post-commit.sh file contains:

# --------------------------------------------------------
# usage: f_askContinue "my question ?"
function f_askContinue {
  local myQuestion=$1

  while true; do
     read -p "${myQuestion} " -n 1 -r answer
     case $answer in
        [Yy]* ) printf "\nOK\n"; break;;
        [Nn]* )   printf "\nAbandon\n";
                  exit;;
        * ) printf "\nAnswer with Yes or No.\n";;
     esac
  done
}

f_askContinue "Do you want to continue ?"
echo "This command is executed after the prompt !"

How to do it in Node.js or TypeScript

EDIT: I made an npm package

---

I see people commenting on how to do it for other languages in Eliot Sykes answer, but the JavaScript solution is a bit long so I'll make a separate answer.

I'm not sure if O_NOCTTY is required, but it doesn't seem to affect anything. I don't really understand what a controlling terminal is. GNU docs description. I think what it means is that with O_NOCTTY on, you wouldn't be able to send a CTRL+C to the process (if it doesn't already have a controlling terminal). In that case, I'll leave it on so you don't control spawned processes. The main node process should already have a controlling terminal, I think.

I adapted the answer from this GitHub issue

I don't see any docs on how to use the tty.ReadStream constructor so I did a bit of trial and error / digging through Node.js source code.

You have to use Object.defineProperty because Node.js internals uses it too, and doesn't define a setter. An alternative is to do process.stdin.fd = fd, but I get duplicate output that way.

Anyway, I wanted to use this with Husky.js and it seems to work so far. I should probably turn this into an npm package when I get the time.

Node.js

#!/usr/bin/env node

const fs = require('fs');
const tty = require('tty');

if (!process.stdin.isTTY) {
  const { O_RDONLY, O_NOCTTY } = fs.constants;
  let fd;
  try {
    fd = fs.openSync('/dev/tty', O_RDONLY + O_NOCTTY);
  } catch (error) {
    console.error('Please push your code in a terminal.');
    process.exit(1);
  }

  const stdin = new tty.ReadStream(fd);

  Object.defineProperty(process, 'stdin', {
    configurable: true,
    enumerable: true,
    get: () => stdin,
  });
}

...Do your stuff...

process.stdin.destroy();
process.exit(0);

TypeScript:

#!/usr/bin/env ts-node

import fs from 'fs';
import tty from 'tty';

if (!process.stdin.isTTY) {
  const { O_RDONLY, O_NOCTTY } = fs.constants;
  let fd;
  try {
    fd = fs.openSync('/dev/tty', O_RDONLY + O_NOCTTY);
  } catch (error) {
    console.error('Please push your code in a terminal.');
    process.exit(1);
  }

  // @ts-ignore: `ReadStream` in @types/node incorrectly expects an object.
  // https://github.com/DefinitelyTyped/DefinitelyTyped/pull/37174
  const stdin = new tty.ReadStream(fd);

  Object.defineProperty(process, 'stdin', {
    configurable: true,
    enumerable: true,
    get: () => stdin,
  });
}

...Do your stuff...

process.stdin.destroy();
process.exit(0);

To make select stop for input, you may also try to redirect the stdin of select from /dev/fd/3 (See: https://stackoverflow.com/questions/6883363/read-input-in-bash-inside-a-while-loop).

# sample code using a while loop to simulate git consuming stdin
{ 
echo 'fd 0' | while read -r stdin; do
   echo "stdin: $stdin"
   echo "Do you want to"
   select CHOICE in "edit the commit message" "ignore this warning" "cancel the commit"; do
      case ${CHOICE} in
         i*) echo "Warning ignored"
             ;;
         e*) echo ${local_editor} $1
             echo verify_info "$1" $2
             ;;
         *)  echo "CHOICE = ${CHOICE}"
             exit 1
             ;;
      esac
   done 0<&3 3<&-
done
} 3<&- 3<&0