Run arbitrary commands when files change using entr

entr is a really useful tool, I have used entr for automatically building and testing Go programs, when coding these and many other things.

First example is for Go, where I use entr to run the test suite whenever I save changes to the source code.

Every time a file named *.go changes, entr will run the command you specify. In this example I guild and run the test suite.

ls *.go | entr -c sh -c 'go build && go test'

ls *.go

Identify the files, you point to a single file (see examples below) or even use find.

-c sh

Starts a new shell, which is needed to run multiple commands.

'go build && go test'

Builds and runs the test suite.

Do note:

• quotes are important, since the command is passed to sh as a single argument and we do not want to have && interpreted by the shell and interfering with the entr command.
• the -c flag is used to clear the screen before running the command.

Another example is for perlcritic, the dominant linter for Perl.

When you alter your Perl::Critic configuration outlined in the perlcriticrc file, you can run check your source code against the new configuration. This is a good practice when adopting a new Perl::Critic configuration or when starting to use Perl::Critic on a project, which has not been criticized before.

ls t/perlcritic.rc |entr -c sh -c 'perlcritic --profile /_ lib'

Here we just point to a single file.

ls t/perlcritic.rc

And we use nifty feature, we use the --profile flag to point to the file we want to use for perlcritic and entr lets us point to the file via the /_ construct.

ls t/perlcritic.rc

And for building a basic Perl project, it sort of resembles the Go example above.

ls *.pm| entr -c sh -c './Build && ./Build test'

Another example could be Markdownlint

find . -name "*.md" |entr markdownlint /_

I really like jq and the combination with entry is a good one, since jq the command line JSON processor, it not easy to use and often you need to do alot of iterations, well I have to anyway.

You can put your jq command in a file and run it with entr like this:

ls -l myjqfile.jq | entr -c sh -c 'cat myfile.json | jq -r -f myjqfile.jq'

Resources and References

• entr manpage
• superuser: “How to execute a command whenever a file changes?”
• TIL: use jq