This is a sample backup script I used, it has already a lot of options. Feel free to make any modification you want. If you add some good enhancements, let me know I can put them here so everybody can profit from them.

RMAN Backup script

#!/bin/bash
##############################################################################
#                            RMAN BACKUPS
##############################################################################
#
# Description: This script create a RMAN backup based on parameter passed
#
##############################################################################
#
# Author: Cyrille MODIANO
#
# Version : 1.0 (2016-07-12)
# Version : 1.1 (2016-12-15) - Added compression
# Version : 1.2 (2017-01-18) - Added parallelism and backup of a list of databases
#
##############################################################################

usage () {
echo "Usage : SID BACKUP_TYPE COMPRESSION PARALLELISM
        SID : SID, comma separated list of databases or ALL for all databases (running)
        BACKUP_TYPE : INCR, FULL, COLD or ARCH
        COMPRESS : COMPRESS or NOCOMPRESS to compress or not the backup
        PARALLEL : defines the number of channel to use

        exemple backup full : rman_backup.sh db1 FULL COMPRESS 16
        exemple backup arch : rman_backup.sh db1 ARCH NOCOMPRESS 2
"
}

##Variables definition
BASEDIR=$(dirname "$0")
BACKUP_BASE=/Data_Domain/oracle/prod/
LOGDIR=${BASEDIR}/log


DEST_EMAIL=example@example.com
export NLS_DATE_FORMAT='dd/mm/yyyy hh24:mi:ss'
DATE=`date +"%Y%m%d_%H%M%S"`
PATH=$PATH:/usr/local/bin

# Create directorires if not exist
mkdir -p $BACKUP_BASE/
mkdir -p $LOGDIR
mkdir -p $BACKUP_BASE/autobackup


# Validating du number of parameters passed
if [ $# -lt 4 ]; then
    usage
    exit 1
fi

# Parameters provided
DB_LIST=$1
BACKUP_TYPE=$2
PARALLEL=$4

# Backup type validation
case $BACKUP_TYPE in
    FULL)
        LEVEL="incremental level 0"
    ;;
    INCR)
        LEVEL="incremental level 1"
    ;;
    ARCH)
        LEVEL=""
    ;;
    COLD)
        LEVEL=""
    ;;
    *)
        usage
    exit 1
    ;;
esac

# Compression validation
if [ $3 = 'COMPRESS' ]; then
    COMPRESS='AS COMPRESSED BACKUPSET'
else
    if [ $3 = 'NOCOMPRESS' ]; then
        COMPRESS=''
    else
        usage
        exit 1
    fi
fi

##backup function
function backup_database() {
    # Set Oracle Environment for database
    ORACLE_SID=$1
    ORAENV_ASK=NO
    . oraenv

    OUTPUT_SID=${ORACLE_SID}
    BACKUP_DIR=$BACKUP_BASE/${ORACLE_SID}
    LOGFILE=$LOGDIR/rman_backup_${ORACLE_SID}_${BACKUP_TYPE}_${DATE}.log
    

    # Controlfile backup directory
    CF_BACKUP="'$BACKUP_DIR/autobackup/cf_sp_%F'"
    FORMAT_DATA="format '${BACKUP_DIR}/data_%d_${BACKUP_TYPE}_bks%s_%T_%U.bck'"
    FORMAT_ARCHIVE="format '${BACKUP_DIR}/arch_%d_${BACKUP_TYPE}_bks%s_%T_%U.bck'"

    if [ $BACKUP_TYPE = 'COLD' ]; then
        sqlplus -s / as sysdba <<EOF
            shutdown immediate;
            startup mount;
            exit
EOF
        rman target /  << EOF >> $LOGFILE 2>&1
            CONFIGURE CONTROLFILE AUTOBACKUP ON;
            CONFIGURE CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO ${CF_BACKUP};
            CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO BACKUPSET PARALLELISM ${PARALLEL};
            run {
                backup ${COMPRESS} database $FORMAT_DATA;
                delete noprompt obsolete;
                }
            exit
EOF

        sqlplus -s / as sysdba <<EOF
            alter database open;
            exit
EOF

    else
        if [  $BACKUP_TYPE = 'ARCH' ]; then
            rman target /  << EOF >> $LOGFILE
                CONFIGURE CONTROLFILE AUTOBACKUP ON;
                CONFIGURE CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO ${CF_BACKUP};
                CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO BACKUPSET PARALLELISM ${PARALLEL};
                run {
                    backup ${COMPRESS} archivelog all $FORMAT_ARCHIVE delete input filesperset 10;
                    delete noprompt obsolete;
                    }
                exit
EOF
        else
            rman target /  << EOF >> $LOGFILE 2>&1
                CONFIGURE CONTROLFILE AUTOBACKUP ON;
                CONFIGURE CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO ${CF_BACKUP};
                CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO BACKUPSET PARALLELISM ${PARALLEL};
                run {
                    backup ${COMPRESS} archivelog all $FORMAT_ARCHIVE delete input filesperset 10;
                    backup ${COMPRESS} ${LEVEL} database $FORMAT_DATA include current controlfile;
                    backup ${COMPRESS} archivelog all $FORMAT_ARCHIVE delete input filesperset 10;
                    delete noprompt obsolete;
                    }
                exit
EOF
        fi
    fi

    # Validate Errors in the log.
    ERRORLIST=$(egrep "^RMAN-[0-9]*:|^ORA-[0-9]*:" $LOGFILE)
    ERRORLIST=$(echo $ERRORLIST)
    if [ -n "$ERRORLIST" ]
    then
        SUBJECT="$(date +%y)/$(date +%m)/$(date +%d) $(date +%H).$(date +%M).$(date +%S) - `hostname` - Backup Report ${OUTPUT_SID} - ERROR"
    else
        SUBJECT="$(date +%y)/$(date +%m)/$(date +%d) $(date +%H).$(date +%M).$(date +%S) - `hostname` - Backup Report ${OUTPUT_SID}"
    fi
    cat -v $LOGFILE | mail -s "$SUBJECT" "$DEST_EMAIL"
}

if [ $1 = 'ALL' ]; then
    for database in `ps -ef | grep pmon | egrep -v 'ASM|grep' | awk '{print $8}' | cut -d_ -f3`
    do
        backup_database $database
    done
else
    for database in $(echo $1 | sed "s/,/ /g")
    do
        backup_database $database
    done
fi