How to use psql with no psql password prompt?

 I wrote a script to REINDEX indexes in a database. Here is one of them:

echo -e "nreindex for unq_vbvdata_vehicle started at: `date "+%F %T"`" >> ${LOG_FILE}
psql -U ${USERNAME} -h ${HOSTNAME} -d ${DBNAME} -c "REINDEX INDEX scm_main.unq_vbvdata_vehicle;"
if [[ ${?} -eq 0 ]]; then
    echo "reindex for unq_vbvdata_vehicle finished at: `date "+%F %T"`" >> ${LOG_FILE}

else
    echo "reindex for unq_vbvdata_vehicle failed" >> ${LOG_FILE}
    exit 1

fi

The problem is I can not run this script in standalone mode. psql is prompting password every time it runs. There is also two limitations:

I can not create a user on a database with no password.

Because REINDEX locks tables, I should use sleep  between each REINDEX.

Is there any automatic solution?