autoninja: fix shellcheck warnings

Change-Id: Ic7ca117435de34400fdd117741d0b321339f8cfd
Reviewed-on: https://chromium-review.googlesource.com/c/1325250
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>

autoninja

@@ -5,16 +5,16 @@
 # found in the LICENSE file.
 
 # Set unique build ID.
-export AUTONINJA_BUILD_ID="$(python -c "import uuid; print uuid.uuid4()")"
+AUTONINJA_BUILD_ID="$(python -c "import uuid; print uuid.uuid4()")"
+export AUTONINJA_BUILD_ID
 
 # Execute whatever is printed by autoninja.py.
 # Also print it to reassure that the right settings are being used.
-command=$(python $(dirname -- "$0")/autoninja.py "$@")
-echo $command
-eval $command
-if [ $? -eq 0 ]; then
+command=$(python "$(dirname -- "$0")/autoninja.py" "$@")
+echo "$command"
+if eval "$command"; then
   if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
-    python $(dirname -- "$0")/post_build_ninja_summary.py $@
+    python "$(dirname -- "$0")/post_build_ninja_summary.py" "$@"
   fi
   exit
 fi