I want to be able to send the command "AT+CSQ" to /dev/ttyUSB1 every 5 seconds, and log out response to a text file. I can do it manually via screen (screen /dev/ttyUSB1 and type in AT+CSQ and see the response), but how do I do this as a bash script?

Below is an example script with some logic for logging and cleanup when the script terminates (the trap command). Important trick is to keep the input from the serial port open otherwise the input from the time the port was closed could get lost.

#!/bin/bash

LOGFILE=/var/tmp/modemlog
MODEM=/dev/ttyUSB1
MODEMCMD=AT+CSQ
INTERVAL=5

LOGDATE="date +%Y%m%dT%H%M%S"

# write command to modem
wrmodem () {
    echo -ne "$*"'\r\n' >"$MODEM"
    log "--- sent: $*"
}

# log message to a logfile
log () {
    echo "$($LOGDATE) $@" >>"$LOGFILE"
}

# logger
{
    trap 'log "=== logger stopped $BASHPID"' EXIT
    log "=== logger started: $BASHPID"
    while true ; do
        if read ; then
            log "$(tr -d \\r <<<"$REPLY")"
        fi
    done
} <"$MODEM" &

LOGGERPID=$!
trap 'kill $LOGGERPID' EXIT

# requester
while true ; do
    wrmodem "$MODEMCMD"
    sleep $INTERVAL
done

The expect tool is designed for just this sort of task. man expect will get you started.

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