A tiny C program used to periodically execute a command.
Usage: watch [options] <cmd> Options: -q, --quiet only output stderr -i, --interval <n> interval in seconds or ms defaulting to 1 -V, --version output version number
$ make install
This version of
watch(1) support millisecond resolution
$ watch -i 300ms echo hey
300 would be seconds:
$ watch -i 300 echo hey
Watch is pretty handy, here are a few use-cases:
Ad-hoc mtime watchers are annoying to construct,
and have relatively no purpose when you can simply
execute your tests at a regular interval. For example
watch(1) as a job, running tests each second (or a
second after the program exits):
$ watch make test &  3794 ✔ bifs.components ✔ bifs.dark ✔ bifs.darken ✔ bifs.image-size ...
Your tests will happily chug away, when you want to stop watch simply foreground the job and ^C:
Auto-build CSS / JS etc
mtime and only build what's necessary, this is great.
Let's say we had some Jade templates, even some nested in sub-directories, we could list them in a Makefile quite easily.
Below JADE is a list constructed by the shell command
find templates -name "*.jade", which is usually a lot easier to manage than listing these files manually, which is also valid, and sometimes important of ordering is relevant. Following that we have HTML which simply substitutes ".jade" with ".html", giving us our HTML targets.
JADE = $(shell find templates -name "*.jade") HTML = $(JADE:.jade=.html)
Our first target is
all, becoming the default target for
make. On the right-hand side of this we specify the dependencies, which in this case is a list of all of our HTML files, not yet built. Make will see this and execute the
%.html targets, which allows use to use the
jade(1) executable to translate the dependency on the right of
:, to the target on the left.
JADE = $(shell find templates -name "*.jade") HTML = $(JADE:.jade=.html) all: $(HTML) %.html: %.jade jade < $< > $@
Now we can build all of these files with a single command
$ make jade < templates/bar.jade > templates/bar.html jade < templates/baz/raz.jade > templates/baz/raz.html jade < templates/foo.jade > templates/foo.html
We can also add a
clean pseudo-target to remove the compiled files with
make clean. Here it's listed to the right of
.PHONY:, telling make that it does not expect a file named
./clean on the fs, so it wont compare mtimes etc. Make is smart about re-executing these actions, if you
make again you'll notice that since none of the dependencies have changed it'll simply tell you "make: Nothing to be done for `all'.", now doesn't this seem familiar? kinda like all these ad-hoc file watchers hey.
JADE = $(shell find templates -name "*.jade") HTML = $(JADE:.jade=.html) all: $(HTML) %.html: %.jade jade < $< > $@ clean: rm -f $(HTML) .PHONY: clean
The one missing component is periodical action, which is where
watch(1) or similar utilities come in, this functionality coupled with Make as a build system creates a powerful duo.