diff --git a/cipd b/cipd
index 099526bd6..b20afe5b8 100755
--- a/cipd
+++ b/cipd
@@ -8,6 +8,7 @@ set -e -o pipefail
 
 # Export for other depot_tools scripts to re-use.
 export DEPOT_TOOLS_DIR="${DEPOT_TOOLS_DIR:-$(dirname "${BASH_SOURCE[0]}")}"
+export DEPOT_TOOLS_UNAME_S="${DEPOT_TOOLS_UNAME_S:-$(uname -s | tr '[:upper:]' '[:lower:]')}"
 
 MYPATH="${DEPOT_TOOLS_DIR}"
 CYGWIN=false
@@ -15,7 +16,7 @@ CYGWIN=false
 # Make sure this starts empty
 ARCH=
 
-UNAME=`uname -s | tr '[:upper:]' '[:lower:]'`
+UNAME="${DEPOT_TOOLS_UNAME_S}"
 case "${UNAME}" in
   aix)
     OS="${UNAME}"
diff --git a/cipd_bin_setup.sh b/cipd_bin_setup.sh
index 38ec886aa..a36df7e38 100644
--- a/cipd_bin_setup.sh
+++ b/cipd_bin_setup.sh
@@ -7,7 +7,7 @@ function cipd_bin_setup {
     local ENSURE="$MYPATH/cipd_manifest.txt"
     local ROOT="$MYPATH/.cipd_bin"
 
-    UNAME=`uname -s | tr '[:upper:]' '[:lower:]'`
+    UNAME="${DEPOT_TOOLS_UNAME_S:-$(uname -s | tr '[:upper:]' '[:lower:]')}"
     case $UNAME in
       cygwin*)
         ENSURE="$(cygpath -w $ENSURE)"
diff --git a/vpython b/vpython
index 08ed05baf..42d112fa7 100755
--- a/vpython
+++ b/vpython
@@ -37,12 +37,13 @@ fi
 
 # Export for other depot_tools scripts to re-use.
 export DEPOT_TOOLS_DIR="${DEPOT_TOOLS_DIR:-$(dirname "$0")}"
+export DEPOT_TOOLS_UNAME_S="${DEPOT_TOOLS_UNAME_S:-$(uname -s | tr '[:upper:]' '[:lower:]')}"
 
 source "${DEPOT_TOOLS_DIR}/cipd_bin_setup.sh"
 cipd_bin_setup &> /dev/null
 
-case $(uname -s) in
-MINGW*|CYGWIN*)
+case "${DEPOT_TOOLS_UNAME_S}" in
+mingw*|cygwin*)
   cmd.exe //c $0.bat "$@"
   ;;
 *)
diff --git a/vpython3 b/vpython3
index 307fcbd3b..3f2373dd3 100755
--- a/vpython3
+++ b/vpython3
@@ -35,12 +35,13 @@ fi
 
 # Export for other depot_tools scripts to re-use.
 export DEPOT_TOOLS_DIR="${DEPOT_TOOLS_DIR:-$(dirname "$0")}"
+export DEPOT_TOOLS_UNAME_S="${DEPOT_TOOLS_UNAME_S:-$(uname -s | tr '[:upper:]' '[:lower:]')}"
 
 source "${DEPOT_TOOLS_DIR}/cipd_bin_setup.sh"
 cipd_bin_setup &> /dev/null
 
-case $(uname -s) in
-MINGW*|CYGWIN*)
+case "${DEPOT_TOOLS_UNAME_S}" in
+mingw*|cygwin*)
   cmd.exe //c $0.bat "$@"
   ;;
 *)