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nano /mnt/sda1/filesize.sh 

#!/bin/sh
/bin/ls -l $1 |/usr/bin/awk '{print $5}'

chmod 755 /mnt/sda1/filesize.sh

/mnt/sda1/filesize.sh /mnt/sda1/test.txt

By using bridge you could call filesize.sh, pass file path as argument and get back file size.