Some unexpected thing with my shell script - shell

Question

This script is used for detecting USB drive input any copy some log files to USB.
on line 47,
cp "/home/root/io/log/${end}.txt" "${USB}/log" && let sfile++ || let err3=1
three commands are all executed, whether the cp command is succeed or not. And if I swap this line with line 49
cp "/home/root/io/log/${begin}.txt" "${USB}/log" && let sfile++ || let err3=1
, only the upper line of codes didn't work as expected.
echo $USB > /dev/null
echo $FILE > /dev/null
echo $begin > /dev/null
echo $end > /dev/null
echo $destfile > /dev/null
echo $tmp > /dev/null
echo $err > /dev/null
echo $sfile > /dev/null
echo $DATE > /dev/null
echo 84 > /sys/class/gpio/export
echo out > /sys/class/gpio/gpio84/direction
while true
do
USB=""
FILE=""
begin=""
end=""
destfile=""
DATE=""
let err1=0
let err2=0
let err3=0
let err4=0
let err=0
let sfile=0
ls /dev/sda1 > /dev/null 2>&1 || rm -rf /media/sda1
ls /media/sda1 > /dev/null 2>&1 && USB="/media/sda1"
if [[ "$USB" != "" ]]; then
ls $USB | grep 'cp.txt' > /dev/null && FILE="cp.txt"
if [[ "$FILE" != "" ]]; then
ls "${USB}/log" > /dev/null 2>&1 && rm -rf "${USB}/log"
mkdir "${USB}/log" && echo 0 >> /dev/null || let err1=1
begin=$(cat "${USB}/${FILE}" | cut -d 'd' -f 2)
end=$(cat "${USB}/${FILE}" | cut -d 'd' -f 3)
cp "/home/root/io/log/${end}.txt" "${USB}/log" && let sfile++ || let err3=1
cp "/home/root/io/log/${begin}.txt" "${USB}/log" && let sfile++ || let err2=1
destfile=$(ls /home/root/io/log)
destfile=${destfile#*${begin}.txt}
destfile=${destfile%${end}.txt*}
echo $destfile
while [[ "$(echo $destfile | cut -d ' ' -f 1)" != "" ]]; do
tmp=""
tmp=$(echo $destfile | cut -d ' ' -f 1)
destfile=${destfile#*$tmp}
cp "/home/root/io/log/${tmp}" "${USB}/log" && let sfile++ || let err4=1
done
DATE=$(date +%Y%m%d-%H%M%S)
LOG="${DATE} Requested: from-${begin} to-${end}. Succeed:${sfile} err1:${err1}err2:${err2}err3:${err3}err4:${err4}"
echo $LOG >> /home/root/io/cplog/cplog.txt
echo $LOG >> /media/sda1/cplog.txt
if [[ err -eq 0 ]]; then
echo 1 > /sys/class/gpio/gpio84/value
sleep 1
echo 0 > /sys/class/gpio/gpio84/value
else
for (( i = 0; i < 10; i++ )); do
echo 1 > /sys/class/gpio/gpio84/value
sleep 0.05
echo 0 > /sys/class/gpio/gpio84/value
sleep 0.05
done
fi
umount "${USB}"
fi
fi
sleep 2
done

Answer 1

I guess you are using bash, but unless you stick with POSIX-compatibility, always indicate the shell you are using. Assuming bash, if you have a command sequence X && Y || Z, and if X returns a non-zero exit status, Y won't be executed.
Aside from this, you do have a logical flaw in your script. Note that let also sets an exit status. The exit status is 1 if the value of the expression to be evaluated, is 0, and the exit status is 0 otherwise. Now regarding your let sfile++, the value of the expression is the value which sfile had before incrementing.
In the first iteration of your loop, sfile is zero. Assuming that cp returns exit code 0, sfile will be incremented, the let command returns exit code 1, and the let err4=1 will also be executed.
In the subsequent iterations, this won't happen anymore, because let sfile++ returns exit code 0.
My advice is to not be too fancy with command sequences mixing && and ||. In this case, a simple if would be the better choice:
if cp ...
then
let ...
else
let
fi