Beginners • What is this and How do I use it.

Code:

#!/bin/bash -efunction usage() {    echo "Usage: $0 [options...]"    echo ""    echo "Available options:"    echo "    <-i image_file> - indicates the path to the image file (e.g. -i /home/user/Download/file.img.gz)"    echo "    <-d sdcard_dev> - indicates the path to the sdcard block device (e.g. -d /dev/mmcblk0)"    echo "    [-m modem:apn:user:pwd:pin] - configures the mobile network modem (e.g. -m ttyUSB0:internet)"    echo "    [-n ssid:psk] - sets the wireless network name and key (e.g. -n mynet:mykey1234)"    echo "    [-s ip/cidr:gw:dns] - sets a static IP configuration instead of DHCP (e.g. -s 192.168.1.101/24:192.168.1.1:8.8.8.8)"    exit 1}if [ -z "$1" ]; then    usage 1>&2fiif [[ $(id -u) -ne 0 ]]; then echo "please run as root"; exit 1; fifunction msg() {    echo " * $1"}while getopts "a:d:f:h:i:lm:n:o:p:s:w" o; do    case "$o" in        d)            SDCARD_DEV=$OPTARG            ;;        i)            DISK_IMG=$OPTARG            ;;        m)            IFS=":" SETTINGS=($OPTARG)            MODEM=${SETTINGS[0]}            APN=${SETTINGS[1]}            MUSER=${SETTINGS[2]}            MPWD=${SETTINGS[3]}            PIN=${SETTINGS[4]}            ;;        n)            IFS=":" NETWORK=($OPTARG)            SSID=${NETWORK[0]}            PSK=${NETWORK[1]}            ;;        s)            IFS=":" S_IP=($OPTARG)            IP=${S_IP[0]}            GW=${S_IP[1]}            DNS=${S_IP[2]}            ;;        *)            usage 1>&2            ;;    esacdoneif [ -z "$SDCARD_DEV" ] || [ -z "$DISK_IMG" ]; then    usage 1>&2fifunction cleanup {    set +e    # unmount sdcard    umount ${SDCARD_DEV}* >/dev/null 2>&1}trap cleanup EXITBOOT=$(dirname $0)/.bootif ! [ -f $DISK_IMG ]; then    echo "could not find image file $DISK_IMG"    exit 1figunzip=$(which unpigz 2> /dev/null || which gunzip 2> /dev/null || true)unxz=$(which unxz 2> /dev/null || true)if [[ $DISK_IMG == *.gz ]]; then    if [ -z "$gunzip" ]; then        msg "make sure you have the gzip package installed"        exit 1    fi    msg "decompressing the .gz compressed image"    $gunzip -c $DISK_IMG > ${DISK_IMG%???}    DISK_IMG=${DISK_IMG%???}elif [[ $DISK_IMG == *.xz ]]; then    if [ -z "$unxz" ]; then        msg "make sure you have the xz package installed"        exit 1    fi    msg "decompressing the .xz compressed image"    $unxz -T 0 -c $DISK_IMG > ${DISK_IMG%???}    DISK_IMG=${DISK_IMG%???}fiumount ${SDCARD_DEV}* 2>/dev/null || truemsg "writing disk image to sdcard"dd if=$DISK_IMG of=$SDCARD_DEV bs=1048576syncif which partprobe > /dev/null 2>&1; then    msg "re-reading sdcard partition table"    partprobe ${SDCARD_DEV}    sleep 1fimsg "mounting sdcard"mkdir -p $BOOTif [ `uname` == "Darwin" ]; then    BOOT_DEV=${SDCARD_DEV}s1 # e.g. /dev/disk4s1    diskutil unmountDisk ${SDCARD_DEV}    mount -ft msdos $BOOT_DEV $BOOTelse # assuming Linux    BOOT_DEV=${SDCARD_DEV}p1 # e.g. /dev/mmcblk0p1    if ! [ -e ${SDCARD_DEV}p1 ]; then        BOOT_DEV=${SDCARD_DEV}1 # e.g. /dev/sdc1    fi    mount $BOOT_DEV $BOOTfi# wifiif [ -n "$SSID" ]; then    msg "creating wireless configuration"    conf=$BOOT/wpa_supplicant.conf    echo "update_config=1" > $conf    echo "ctrl_interface=/var/run/wpa_supplicant" >> $conf    echo "network={" >> $conf    echo "    scan_ssid=1" >> $conf    echo "    ssid=\"$SSID\"" >> $conf    if [ -n "$PSK" ]; then        echo "    psk=\"$PSK\"" >> $conf    fi    echo "}" >> $conf    echo "" >> $conffi# modemif [ -n "$MODEM" ]; then    msg "creating mobile network configuration"    conf=$BOOT/ppp    mkdir -p $conf    echo $MODEM > $conf/modem    echo "AT+CGDCONT=1,\"IP\",\"$APN\"" > $conf/apn    echo > $conf/extra    echo > $conf/auth    echo > $conf/pin    if [ -n "$MUSER" ]; then        echo "user \"$MUSER\"" > $conf/auth        echo "password \"$MPWD\"" >> $conf/auth    fi    if [ -n "$PIN" ]; then        echo "AT+CPIN=$PIN" > $conf/pin    fifi# static ipif [ -n "$IP" ] && [ -n "$GW" ] && [ -n "$DNS" ]; then    msg "setting static IP configuration"    conf=$BOOT/static_ip.conf    echo "STATIC_IP=\"$IP\"" > $conf    echo "STATIC_GW=\"$GW\"" >> $conf    echo "STATIC_DNS=\"$DNS\"" >> $conffimsg "unmounting sdcard"syncumount $BOOTrmdir $BOOTmsg "you can now remove the sdcard"