gammu modem vanishes or changes tty port... Here's the fix

[deajan]

Hello,

This is not an issue, merely a workaround for USB modems that like to change ports when disconnected / reconnected.
So whenever you disconnect / reconnect a USB modem, ports vary from /dev/ttyUSB0 to /dev/ttyUSB2. If while reconnecting, some program was trying to read /dev/ttyUSB2, then the modem will be connected as /dev/ttyUSB3, rendering your gammu setup unusable.

Here's a script I launch every 10 minutes as a cron task, that detects which port has a AT modem connected, and updates the corresponding gammu file.

The script needs picocom version >=3.0 as only dependency.

log_file="/var/log/modem_update.log"
gammu_config_file="/etc/gammutc"
modem=""
# Optional command when changed modem detected
#optional_cmd="systemctl restart gammu-smsd"

function log {
    line="${1}"

    echo "${line}" >> "${log_file}"
    echo "${line}"
}

for tty in $(ls /dev/ttyUSB*); do
    log "Checking $tty"
    picocom -qrX -b 9600 $tty
    sleep 1
    result=$(echo "AT&F" | picocom -qrix 1000 $tty)
    if [ "${result}" = "AT&F" ]; then
        log "Found AT compatible modem at $tty"
        modem="${tty}"
    else
        log "No AT compatible modem at $tty"
    fi
done

curr_gammu_modem=$(cat "${gammu_config_file}" | grep "device = " | awk -F'=' '{gsub(/ /,""); print $2}')
if [ "${modem}" != "" ]; then
    if [ "${modem}" != "${curr_gammu_modem}" ]; then
        log "Updating modem from ${curr_gammu_modem} to ${modem}"
        sed -i "s%${curr_gammu_modem}%${modem}%" "${gammu_config_file}"
        if [ "${optional_cmd}" != "" ]; then
            log "Running ${optional_cmd}"
            ${optional_cmd} >> "${log_file}"
            if [ $? -ne 0 ]; then
                log "Could not run cmd, exitcode: $?"
            fi
        fi
    fi
fi

Disclaimer:

• Bear in mind that this script is supposed to fix a "one modem only" usecase. If you have multiple modems, you'll have to add a bit of udevadm magic to know which /dev/ttyUSBx belong to which USB modem.
• Don't bug me if your machine explodes while executing this script

@OsaAjani Would you mind accepting this as optional script in a PR ? I'm pretty sure this can handle loads of gammu failure related scenarios