for/if/while: add

pages/common/for.md

@@ -0,0 +1,11 @@
+# for
+
+> Shell loop over parameters
+
+- Perform a command with different arguments.
+
+`for argument in 1 2 3; do {{command $argument}}; done`
+
+- Perform a command in every directory.
+
+`for d in *; do (cd $d; {{command}}); done`