diff --git a/.bash_profile b/.bash_profile
index b12a2f4..5c14e62 100644
--- a/.bash_profile
+++ b/.bash_profile
@@ -15,7 +15,7 @@ export VISUAL="$EDITOR"
 hash lesspipe >/dev/null 2>&1 && eval "$(SHELL=/bin/sh lesspipe)"
 
 # Platform specific set up.
-PLATFORM="$(uname -s)"
+PLATFORM="${PLATFORM:-$(uname -s)}"
 if [[ "$PLATFORM" = "Linux" ]]; then
   # shellcheck disable=SC2155
   export GPG_TTY="$(tty)"
@@ -45,7 +45,6 @@ elif [[ "$PLATFORM" = "Darwin" ]]; then
 else
   printf "%s: %s\\n" "${BASH_SOURCE##*/}" "unsupported platform: $PLATFORM" >&2
 fi
-unset PLATFORM
 
 # Add bin directories to PATH.
 [[ -d "$HOME/.local/bin" ]] && export PATH="$HOME/.local/bin:$PATH"