entr - run arbitrary command when files change

entr is a UNIX utility which runs arbitrary commands when files change. It helps in automating tasks during development such as rebuilding projects, running tests, or syncing files.

Here's a simple usage example:

ls *.c | entr make

In the above example, ls *.c lists all C files in the directory. This list is piped (|) into entr. When any of these files changes, entr executes the make command.

Some key features of entr include:

• It frees up developers to focus on the code by automating rebuild tasks.
• It doesn't require a configuration file or a list of tasks to run. It just reruns the command you provide it each time a file changes.
• You can use it with any command that needs to operate on a file. This might be shell commands, like ls or echo, or any other CLI tool you have in your system.

Additional commands for entr include:

• -r : To restart a long running process like a server when a file changes.
• -p : Postpone execution until files are updated.
• -s : Evaluate the first argument using the interpreter specified by the SHELL environment variable.
• -d : Track directories recursively and include files that are created after the utility starts

Please note that entr requires a list of files as input. It does not discover files on its own, it expects to receive a list of files from stdin, which is usually supplied with command line utilities like ls, find or git ls-files.