entr: add page (#2613)

pages/linux/entr.md

@@ -0,0 +1,19 @@
+# entr
+
+> Run arbitrary commands when files change.
+
+- Rebuild with `make` if any files in any subfolders change:
+
+`{{ag -l}} | entr {{make}}`
+
+- Rebuild and test with `make` if any `.c` source files in the current directory change:
+
+`{{ls *.c}} | entr {{'make && make test'}}`
+
+- Send a `SIGTERM` to any previously spawned ruby subprocesses before executing `ruby main.rb`:
+
+`{{ls *.rb}} | entr -r {{ruby main.rb}}`
+
+- Run a command with the changed file (`/_`) as an argument:
+
+`{{ls *.sql}} | entr {{psql -f}} /_`