Create the war file in a different directory to where the content is otherwise the jar command might try to zip up the file it is creating.
#!/bin/bash
set -euo pipefail
war=app.war
src=contents
# Clean last war build
if [ -e ${war} ]; then
echo "Removing old war ${war}"
rm -rf ${war}
fi
# Build war
if [ -d ${src} ]; then
echo "Found source at ${src}"
cd ${src}
jar -cvf ../${war} *
cd ..
fi
# Show war details
ls -la ${war}