til on Alán's blog

Use dired-do-shell to explore the parquet schema from Emacs

Thu, 23 Oct 2025 15:21:00 -0400

I use dired-do-shell command in Emacs to run CLI commands from within its file manager dired. This workflow makes it easy to perform batch operations on files that would be annoying otherwise. The trouble arose when trying to use the duckdb CLI to print the schema of a parquet file, as the notation for wildcards in emacs (* and ?) conflicts with duckdb’s usage of the former. Thus running the following after M-x dired-do-shell (bound to ! in dired-mode) did not work:

duckdb -c "DESCRIBE * FROM read_parquet('?')"

I am trying to get Emacs to substitute ? for the selected file(s) in dired and retain * as a normal duckdb wildcard (for which it means “get all columns”), but Emacs interprets the star as a wildcard and not the question mark. Thus Emacs yapped about it:

dired-do-shell-command: You can not combine ‘*’ and ‘?’ substitution marks

The trick is that we need to surround ? with backquotes (`), otherwise the spaces will be in the file name that read_parquet ingests. To fix the “naked star” problem (which Emacs will try to substitute) we can envelop it with the COLUMNS() command, and the lack of spaces around * will prevent Emacs from substituting its value. Thus the following command in the Emacs command line does the trick:

duckdb -c "DESCRIBE COLUMNS(*) FROM read_parquet('`?`')"

Which is a bit verbose but I can easily reuse from my command history to explore the schemas or parquets without leaving the dired buffer.

Recursive search and replace

Tue, 12 Aug 2025 13:07:00 -0400

I needed to rename all occurrences of a pattern with another, where I knew there was no ambiguous situations. This uses ripgrep, xargs and GNU sed. source.

rg old_pattern --files-with-matches | xargs sed -i 's/old_pattern/new_pattern/g'

Github code review on existing code base

Tue, 26 Nov 2024 13:06:00 -0500

Create an empty branch with one empty commit

Create new branch git checkout --orphan review-1-target
Reset git reset .
Clean branch git clean -df
Add empty commit git commit --allow-empty -m 'Empty commit'

Rebase a branch to put this commit at the root

Push to your fork git push -u origin review-1-target
Move to branch to review git checkout origin/main
Spin-off branch from here git checkout -b review-1
Rebase to empty branch git rebase -i review-1-target, the empty commit must be at the start
Push git push -u origin review-1

That should make a pull request possible, providing the code review tooling. source