berfr blog

watch-diff: Watch command output and get notified on changes

Posted on March 01, 2020.

For a few months now, I have been using my watch-diff project to get notified on command output changes. The beauty of this simple tool is that it works with any command that can be executed in the shell. This means that once you figure out the exact command you need to monitor, you simply plug it into watch-diff and you automatically get email notifications on updates.

Example: monitoring current releases

Here is the output for monitoring the current version of this package on the PyPI website while a release is being made:

$ watch-diff "curl -s | \
    sed -n '/<h1 class=\"package-header__name\">/,/<\/h1>/p'"
[2020-03-01 01:04:11.697054] first_run:
      <h1 class="package-header__name">
        watch-diff 1.0.2
[2020-03-01 01:04:16.839766] no diff
[2020-03-01 01:04:21.986465] no diff
[2020-03-01 01:04:27.122121] no diff
[2020-03-01 01:04:32.278194] no diff
[2020-03-01 01:04:37.424123] diff:
--- Previous    2020-03-01 01:04:32.278194
+++ Current     2020-03-01 01:04:37.424123
@@ -1,3 +1,3 @@
       <h1 class="package-header__name">
-        watch-diff 1.0.2
+        watch-diff 1.0.3
[2020-03-01 01:04:42.710685] no diff

No extra dependencies

This Python package does not require any extra PyPI packages to function. Instead, it relies on a few packages from the Standard Library. These include:

Since there aren’t any additional packages installed with watch-diff, there isn’t much danger in installing it globally with pip as a regular user. This has the advantage of being able to call it from wherever without having to worry about a virtual environment.

# install as user in home directory
pip install --user watch-diff

Email thread handling

By setting the Message-ID and In-Reply-To in the emails sent, they are easily grouped together in their respective runs. This makes it possible to start multiple instances of watch-diff all watching different commands while keeping a tidy inbox.

Mar 01 2020 watch-diff (2.8K)   ┌─>
Mar 01 2020 watch-diff (2.9K) ┌─>watch-diff diff: ls -la
Mar 01 2020 watch-diff (7.5K) watch-diff first_run: ls -la
Mar 01 2020 watch-diff (1.6K)                   ┌─>
Mar 01 2020 watch-diff (1.6K)                 ┌─>
Mar 01 2020 watch-diff (1.6K)               ┌─>
Mar 01 2020 watch-diff (1.6K)             ┌─>
Mar 01 2020 watch-diff (1.6K)           ┌─>
Mar 01 2020 watch-diff (1.6K)         ┌─>
Mar 01 2020 watch-diff (1.6K)       ┌─>
Mar 01 2020 watch-diff (1.6K)     ┌─>
Mar 01 2020 watch-diff (1.6K)   ┌─>
Mar 01 2020 watch-diff (1.6K) ┌─>watch-diff diff: date
Mar 01 2020 watch-diff (1.3K) watch-diff first_run: date


Configuration of watch-diff is only needed if the -r/--recipient option is used. This triggers email notifications and so SMTP settings are needed. If used, the program will either prompt the user for the needed values or use the ones set in environment variables if available. The following variables are used:

These can be exported in the ~/.bashrc file so they are available at any time.

A good strategy for more secure configuration is to create an email account that is used exclusively for sending emails. If the credentials get compromised, it is has less impact than if your main email credentials get stolen. Also, it is a good idea to use secret-tool lookup ... to get the SMTP password so that it is not written in plain text in a config file.

Further work

I am very satisfied with this tool right now; it is super simple to use with whatever shell command, the emails are tidy and they look good. If you have any issues or want to suggest or contribute a feature, feel free to contact me!