I recently created Hugo Front Matter Tools which is described as…

A collection of Python scripts desinged to help manage Hugo .md content front matter.

I already have mechanisms in many projects, like this blog, that help me report the last time ANY content was pushed to GitHub, or the last time a Hugo site was compiled. But it would be nice, especially in the case of Rootstalk, if I could save the last git add date/time into an individual file’s front matter. That way the tools mentioned above could leverage and report that valuable information.

Creating a git Hook

While searching for possibilities this morning I ran across this post which I’ll repeat here in case the original is ever lost…

edit the .gitattributes, and create rules for the files which should trigger a dictionary update:

novel.txt filter=updateDict

Then, tell Git what the updateDict filter does on smudge (`git checkout`) and clean (`git add`):

$ git config --global filter.updateDict.clean countWords.script

$ git config --global filter.updateDict.smudge cat

My First Hook

Here are some of the details surrounding my first git hook…

• Purpose: To add or update a last_modified_at: front matter field with the current local date/time whenever a git add operation touches a .md (Markdown) file in a specific project.

• Projects: Initially I’ll try to implement this on this blog project. If that works, I’ll happily apply it to the Rootstalk project.

• Improvements: What follows will only work if the last_modified_at: field already exists in a file’s front matter. What happens if we are working with a file that does NOT already have that field?

For initial implementation I’m going to follow the advice found in Adding last modified timestamps with Git. The shell script in that post reads like this:

#!/bin/sh
# Contents of .git/hooks/pre-commit
# Replace `last_modified_at` timestamp with current time

git diff --cached --name-status | egrep -i "^(A|M).*\.(md)$" | while read a b; do
  cat $b | sed "/---.*/,/---.*/s/^last_modified_at:.*$/last_modified_at: $(date -u "+%Y-%m-%dT%H:%M:%S")/" > tmp
  mv tmp $b
  git add $b
done

I created the same pre-commit script in this project’s .git/hooks/ directory. Now to test it…

Initial Test

╭─mark@Marks-Mac-Mini ~/GitHub/dlad-blog ‹main*› 
╰─$ git status
On branch main
Your branch is up to date with 'origin/main'.
╭─mark@Marks-Mac-Mini ~/GitHub/dlad-blog ‹main*› 
╰─$ git add .
╭─mark@Marks-Mac-Mini ~/GitHub/dlad-blog ‹main*› 
╰─$ git commit -m "Testing my pre-commit hook"
hint: The '.git/hooks/pre-commit' hook was ignored because it's not set as executable.
hint: You can disable this warning with `git config advice.ignoredHook false`.
[main 0b9781d7] Testing my pre-commit hook
 3 files changed, 74 insertions(+)
 create mode 100644 content/posts/131-Creating-a-git-Hook.md

So, I changed the pre-commit hook permissions and tried again after adding a bit more to this .md file.

╭─mark@Marks-Mac-Mini ~/GitHub/dlad-blog ‹main› 
╰─$ git status
On branch main
Your branch is up to date with 'origin/main'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        modified:   content/posts/131-Creating-a-git-Hook.md

no changes added to commit (use "git add" and/or "git commit -a")
╭─mark@Marks-Mac-Mini ~/GitHub/dlad-blog ‹main*› 
╰─$ git add .
╭─mark@Marks-Mac-Mini ~/GitHub/dlad-blog ‹main*› 
╰─$ git commit -m "2nd test of pre-commit hook"
[main 49118af7] 2nd test of pre-commit hook
 1 file changed, 23 insertions(+), 1 deletion(-)

It worked! The .md file for this post now includes a line of front matter that says last_modified_at: 2022-10-27T16:02:09 in both my local AND GitHub repository versions. Hurrah!

More Testing

Now, what happens if I git add and git commit more test files including:

• test1.md - A .md file that has no last_modified_at: front matter key,
• test2.txt - A .txt file that has an empty last_modified_at: front matter key, and
• test3.png - A .png image file that, of course, has no last_modified_at: front matter key.

╭─mark@Marks-Mac-Mini ~/GitHub/dlad-blog ‹main*› 
╰─$ git status
On branch main
Your branch is up to date with 'origin/main'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        modified:   content/posts/131-Creating-a-git-Hook.md

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        content/test1.md
        content/test2.txt
        content/test3.png

no changes added to commit (use "git add" and/or "git commit -a")
╭─mark@Marks-Mac-Mini ~/GitHub/dlad-blog ‹main*› 
╰─$ git add .
╭─mark@Marks-Mac-Mini ~/GitHub/dlad-blog ‹main*› 
╰─$ git commit -m "Three-part pre-commit hook test"
[main 1b20a2a8] Three-part pre-commit hook test
 4 files changed, 202 insertions(+), 1 deletion(-)
 create mode 100644 content/test1.md
 create mode 100644 content/test2.txt
 create mode 100644 content/test3.png

Looking at the three files (actually, four files including this blog post) and… BEAUTIMOUS!

Everything worked as it should!
Now, let’s see if I can improve on the rather cryptic format of the date/time that gets added.

To do that, change the git diff... line in .git/hooks/pre-commit to use the TZ timezone setting and remove the -u flag so that we get local time like so:

git diff --cached --name-status | egrep -i "^(A|M).*\.(md)$" | while read a b; do
  cat $b | sed "/---.*/,/---.*/s/^last_modified_at:.*$/last_modified_at: $(TZ=CST6CDT date "+%F %H.%M %Z")/" > tmp

Notice also that this format does not include ANY colons (I’m using a . between hour and minute instead) so there should be no need to quote the value.

🥁 It works!

In fact, it works so well that I’m keeping a copy of the pre-commit script here in this blog’s repo.

Now I Need An Install Script

The pre-commit script works exceptionally well, but since it lives in each project’s /.git/hooks directory, one that is NEVER pushed to the project’s remote, it would be nice to have a semi-automatic way of creating it in a repo.

This notion got me wondering… is there already a suitable “git hook manager” (that is the phrase I Googled) that I can use for this? Yes, I beleive there might very well be one based on these results from my Google search:

• https://github.com/aitemr/awesome-git-hooks - Just a list of cool hooks.

• https://github.com/evilmartians/lefthook - Interesting, but perhaps more than I need?

• https://pre-commit.com/ - Also interesting, but again it’s perhaps too much.

• https://stackoverflow.com/questions/427207/can-git-hook-scripts-be-managed-along-with-the-repository - A great discussion!

• https://githooks.com/ - The discussion above led me here, and from here, anything seems possible!

• https://github.com/sds/overcommit - Nice, but again, too complex.

• https://github.com/boddenberg-it/.githooker - Maybe, but I found the sparse documentation a little confusing.

• https://medium.com/@ripoche.b/using-global-pre-commit-hook-to-prevent-committing-unwanted-code-edbbf957ad12 - My choice because it’s relatively simple with no unnecessary dependencies. See below.

Implementing a Global Pre-Commit Hook

Here we go, at 2023-01-20T13:13:17-06:00 I embarked on an effort to implement portions of Using global pre-commit hook to prevent committing unwanted code on my Mac Mini in this dlad-blog project using my existing pre-commit hook.

My relevant command history…

The last git command above opened my global git configuration file, /Users/mark/.gitconfig in nano, where I added this line to the [core] section:

• hooksPath = /Users/mark/gitconfig/hooks

Now it’s time to commit all these changes to see if my new global git config works. 🥁 Yup! The frontmatter at the top of this .md file just changed to include the current date/time: last_modified_at: 2023-01-20 13.29 CST. Bingo!

There might be more to learn here, but for now… that’s a wrap.