printenv: add page (#2400)

pages/linux/printenv.md

@@ -0,0 +1,15 @@
+# printenv 
+
+> Print values of all or specific environment variables.
+
+- Display key-value pairs of all environment variables:
+
+`printenv`
+
+- Display the value of a specific variable:
+
+`printenv {{HOME}}`
+
+- Display the value of a variable and end with NUL instead of newline:
+
+`printenv --null {{HOME}}`