entr: Re-Run a Command When Files Change

entr reads a list of file paths on stdin and runs a command each time any of those files changes, making it a tiny, portable file-watch loop.

entr is the classic Unix watcher: you feed it a file list (usually from find or fd) and a command, and it reruns on save. Like watchexec, it loops, so it is local-only.

What it does

entr takes a newline-separated list of file paths on stdin, then blocks and watches exactly those files. When one changes it runs the command you passed as arguments. -r reloads a persistent process (restart); -c clears the screen first; -d exits if a new file appears in a watched directory so a wrapping loop can re-scan.

Common usage

bash

find . -name '*.py' | entr pytest          # rerun tests on change
fd -e go | entr -r go run .                 # restart the server
ls *.tex | entr -c make                     # clear screen, rebuild
# re-scan when files are added/removed:
while true; do fd -e rs | entr -d cargo test; done

Options

Flag	What it does
-r	Reload: restart a persistent child process on change
-c	Clear the screen before each run
-d	Exit if a directory gains a new file (for a rescan loop)
-s	Run the argument as a shell command (allows pipes)
-p	Postpone the first run until the first change
/_	Placeholder replaced with the changed file name

In CI

entr is a local development loop, not a CI step: it blocks waiting for changes and never returns, which would stall the pipeline. Use it on your machine for tight test feedback. In an automated environment, run the underlying command (pytest, make) once instead.

Common errors in CI

"entr: No regular files to watch" means stdin was empty: the find/fd upstream matched nothing, often because of a wrong path or ignore rules. "entr: command not found" (install via apt-get install -y entr or brew install entr). A job that hangs forever after an entr invocation is entr doing its job; it should not be in CI.