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.

dired on Alán's blog

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