How do I use a shell-script as Chrome Native Messaging host application

bash, chrome-native-messaging, google-chrome-extension, json

Solution

I suggest to not use (bash) shell scripts as a native messaging host, because bash is too limited to be useful.

`read` without any parameters reads a whole line before terminating, while the native messaging protocol specifies that the first four bytes specify the length of the following message (in native byte order).

Bash is a terrible tool for processing binary data. An improved version of your `read` command would specify the `-n N` parameter to stop reading after `N` characters (note: not bytes) and `-r` to remove some processing. E.g. the following would store the first four characters in a variable called `var_prefix`:

IFS= read -rn 4 var_prefix

Even if you assume that this stores the first four bytes in the variable (it does not!), then you have to convert the bytes to an integer. Did I already mention that bash automatically drops all NUL bytes? This characteristics makes Bash utterly worthless for being a fully capable native messaging host.

You could cope with this shortcoming by ignoring the first few bytes, and start parsing the result when you spot a `{` character, the beginning of the JSON-formatted request. After this, you have to read all input until the end of the input is found. You need a JSON parser that stops reading input when it encounters the end of the JSON string. Good luck with writing that.

Generating output is a easier, just use `echo -n` or `printf`.

Here is a minimal example that assumes that the input ends with a `}`, reads it (without processing) and replies with a result. Although this demo works, I strongly recommend to not use bash, but a richer (scripting) language such as Python or C++.

#!/bin/bash
# Loop forever, to deal with chrome.runtime.connectNative
while IFS= read -r -n1 c; do
    # Read the first message
    # Assuming that the message ALWAYS ends with a },
    # with no }s in the string. Adopt this piece of code if needed.
    if [ "$c" != '}' ] ; then
        continue
    fi

    message='{"message": "Hello world!"}'
    # Calculate the byte size of the string.
    # NOTE: This assumes that byte length is identical to the string length!
    # Do not use multibyte (unicode) characters, escape them instead, e.g.
    # message='"Some unicode character:\u1234"'
    messagelen=${#message}

    # Convert to an integer in native byte order.
    # If you see an error message in Chrome's stdout with
    # "Native Messaging host tried sending a message that is ... bytes long.",
    # then just swap the order, i.e. messagelen1 <-> messagelen4 and
    # messagelen2 <-> messagelen3
    messagelen1=$(( ($messagelen      ) & 0xFF ))               
    messagelen2=$(( ($messagelen >>  8) & 0xFF ))               
    messagelen3=$(( ($messagelen >> 16) & 0xFF ))               
    messagelen4=$(( ($messagelen >> 24) & 0xFF ))               

    # Print the message byte length followed by the actual message.
    printf "$(printf '\\x%x\\x%x\\x%x\\x%x' \
        $messagelen1 $messagelen2 $messagelen3 $messagelen4)%s" "$message"

done

Problem

Original source