diff --git a/adb/commandline.c b/adb/commandline.c
index bb77c4f..8003a64 100644
--- a/adb/commandline.c
+++ b/adb/commandline.c
@@ -170,6 +170,12 @@
         "\n"
         "  - If it is \"system\" or \"data\", only the corresponding partition\n"
         "    is updated.\n"
+        "\n"
+        "environmental variables:\n"
+        "  ADB_TRACE                    - Print debug information. A comma separated list of the following values\n"
+        "                                 1 or all, adb, sockets, packets, rwx, usb, sync, sysdeps, transport, jdwp\n"
+        "  ANDROID_SERIAL               - The serial number to connect to. -s takes priority over this if given.\n"
+        "  ANDROID_LOG_TAGS             - When used with the logcat option, only these debug tags are printed.\n"
         );
 }