Run a command when files change

If you usually develop your software without an IDE, it may come in handy to be able to run a custom command or two whenever a file or a group of files in the file system is modified. This post discusses entr, a small event notify test runner which might just be what you need to fill an inconvenient gap in your mouseless development environment.

entr

entr does just that; it watches files and whenever they change, it runs a command. It is super easy to use. It reads a list of files from the standard input and runs an arbitrary command whenever any of the files change. For example, you can pair it with ripgrep to watch your whole home and print something whenever a file changes.

cd ~
rg --files ~ | entr echo "A file just changed in $HOME"

Practical uses

The example above is cool, it lets you know whenever a file changes in home. However, if a new file is added you won’t notice as it was not initially listed by rg --files ~. entr shines brightest when you need to track changes in a limited number of files. For example, a code base. In my Rust development environment, I usually have a window in a corner where I run the following command.

ls src/**/*.rs | entr cargo build

This tracks changes in all Rust source code *.rs files in the src/ directory and its sub-directories, and builds the Rust project whenever a change happens.

I also use it to automatically rebuild my latex papers and documents when editing them. This enables immediate feedback and automatic building. Open the document in a minimal document viewer like zathura and you have an automatic display which is always in sync with your code.

Options and arguments

A few command line arguments are available to tune the behavior of entr. Below are the most important.

• -c clears the screen before running the command each time.
• -d accepts directories as input, and the program exits whenever a new file is detected.
• -p do not run the command until the first file is modified. By default entr will run the given command when it starts.