Port of the week: entr

entr is a command line tool that let you run arbitrary command on file change. This is useful when you are doing something that requires some processing when you modify it.

Recently, I have used it to edit a man page. At first, I had to run mandoc each time I modified to file to check the render. This was the first time I edited a man page so I had to modify it a lot to get what I wanted. I remembered about entr and this is how you use it:

$ ls stagit.1 | entr mandoc /_

This simple command will run “mandoc stagit.1” each time stagit.1 is modified. The file names must be given by stdin to entr, and then use the characters sequence /_ to replace the names (like {} in find).

The man page of entr is very well documented if you need more examples.