Try this:
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#! /bin/bash
# autostart.sh
export SAMPLE_RATE=48000
export FRAMES_PERIOD=64
export N_PERIODS=3
if [ -e "/proc/asound/USBPre2" ]; then
/usr/bin/jackd -P81 -d alsa -d hw: $USBPre2 -r ${SAMPLE_RATE} -p
${FRAMES_PERIOD} -n ${N_PERIODS} &
fi
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