a.k.a. @alxndr
a.k.a. @drwxrxrx

For the next time I forget what this tool is called…

entr (“event notify test runner”) is a neat little tool that monitors files and runs a command when they change.

You can hit space to re-run the command even when the watched files haven’t changed, and exit by hitting q or Ctrl-C.

I’ve been using it for over a decade now, and decided it’s worth documenting why!

basic use case

Run a test file whenever a source file changes!

$ find . -name '*.ex' -o -name '*.exs' | entr mix test

This looks for all the files in any subdirectories that end in .ex or exs, and runs mix test when any of them changes.

running multiple commands piped or chained together

Trying to run multiple commands using | or ; or && or || will not work as expected, because those character sequences get interpreted by the shell first. Instead, reach for the -s flag:

$ ls package*.json | entr -crs 'npm install && npm start'

…to watch the package.json and package-lock.json files in the current directory, and then npm install and npm start when they change.

  • list the filenames to watch and pipe them to entr’s stdin
  • the -c flag ‘clears’ the screen before running the command (but preserves the output too so you can still scroll up and read it!)
  • the -r flag indicates the command is long-running and won’t exit (e.g. it’s a server process or a daemon)
  • -s <command-string> allows passing a string for the command to run, which means it can contain semicolons / ampersands / pipes

echo or use the modified filepath

…the string /_ within the command will be replaced with the first filepath which triggers the command, e.g.:

find . -name '*' | entr echo modified: /_

Running that and then editing a file in that directory will log out the name of the just-edited file.

combining /_ with the -s flag

In order to get the modified filepath into a complex -s <command-string> use the param $0 within the command string.

find . -name '*' | entr -s 'echo modified: $0'

gotchas

Note that if you use a glob to select files and then watch them with entr, and then you add a new file which should match that glob, the file creation and later edits of the file will not trigger the entr command, because the new file was not present when the glob was evaluated. Using the -d flag might help remind you of this; the -d flag will exit entr when a new file shows up in any of the directories it is watching.

Updated: