You need to collect all the database names into a space delimited list. Use that for mysqldump
MYSQL_USER=root
MYSQL_PASS=rootpassword
MYSQL_CONN="-u${MYSQL_USER} -p${MYSQL_PASS}"
#
# Collect all database names except for
# mysql, information_schema, and performance_schema
#
SQL="SELECT schema_name FROM information_schema.schemata WHERE schema_name NOT IN"
SQL="${SQL} ('mysql','information_schema','performance_schema')"
DBLISTFILE=/tmp/DatabasesToDump.txt
mysql ${MYSQL_CONN} -ANe"${SQL}" > ${DBLISTFILE}
DBLIST=""
for DB in `cat ${DBLISTFILE}` ; do DBLIST="${DBLIST} ${DB}" ; done
MYSQLDUMP_OPTIONS=" --default-character-set=utf8mb4 --routines --triggers --single-transaction"
mysqldump ${MYSQL_CONN} ${MYSQLDUMP_OPTIONS} --databases ${DBLIST} > all-dbs.sql
Give it a Try !!!