add env command

pages/common/env.md

@@ -0,0 +1,19 @@
+# env
+
+> Show the environment or run a program in a modified environment
+
+- Show the environment
+
+`env`
+
+- Clear the environment and run a program
+
+`env -i {{program}}`
+
+- Remove variable from the environment and run a program
+
+`env -u {{variable}} {{program}}`
+
+- Set a variable and run a program
+
+`env {{variable}}={{value}} {{program}}`