diff --git a/tools/m2ctx/m2ctx.sh b/tools/m2ctx/m2ctx.sh
index 24c201165..861de5ee4 100755
--- a/tools/m2ctx/m2ctx.sh
+++ b/tools/m2ctx/m2ctx.sh
@@ -6,6 +6,7 @@ GCC=gcc
 FLAGS="-E -P -dD -undef"
 INCLUDES="-Iinclude -Iinclude/library -Ifiles -Ilib/include -include global.h"
 DEFINES="-DHEARTGOLD -DGAME_REMASTER=0 -DENGLISH -DPM_KEEP_ASSERTS -DSDK_ARM9 -DSDK_CODE_ARM -DSDK_FINALROM"
+SRCS=()
 
 if [ "$(uname -s)" == "Darwin" ]; then
 	SED="$(which gsed)"
@@ -17,13 +18,7 @@ generate-ctx () {
     # Remove any line containing a predefined macro. If not removed, mwccarm
     # generates compiler warnings.
 
-    if [ "$OUT_FILE" = "-" ]; then
-        PIPE_CTX=
-    else
-        PIPE_CTX="> $OUT_FILE"
-    fi
-
-    grep "^#include " "$1" | $GCC $FLAGS $INCLUDES $DEFINES  -x c - | ${SED} '/__STDC__\|__STDC_VERSION__\|__STDC_VERSION__\|__STDC_HOSTED__/d' $PIPE_CTX
+    grep "^#include " "$1" | $GCC $FLAGS $INCLUDES $DEFINES  -x c - | ${SED} '/__STDC__\|__STDC_VERSION__\|__STDC_VERSION__\|__STDC_HOSTED__/d'
 }
 
 usage () {
@@ -48,8 +43,20 @@ while [[ $# -gt 0 ]]; do
     OUT_FILE="$2"
     shift 2 ;;
   *)
-    generate-ctx "$1"
-    exit 0
-    ;;
+    SRCS+=("$1")
+    shift ;;
   esac
 done
+
+if [ "${#SRCS[@]}" -ne 1 ]; then
+  echo "error: specify exactly one source file"
+  exit 255
+else
+  src="${SRCS[0]}"
+fi
+
+if [ "$OUT_FILE" != "-" ]; then
+  exec 1>$OUT_FILE
+fi
+
+generate-ctx "$src"