Question

Consider:

t=0 ; for i in 1 2 3 4 5 6 7 8 9 10 ; do t=$((t+i)) ; done ; echo $t

prints 55.

But:

totsize=0
find /home/user -type f -mmin -4860 -a -mmin +3420 | xargs du | \
while read size rest ; do
    totsize=$((totsize+size))
    echo "$totsize"
done
echo "Sum: $totsize kb"

Prints “Sum: 0 kb” even tho the interim print statement prints a reasonable sum.

I know I have encountered this issue before, but have never understood it. What is difference?

Answer 1

that’s because the pipes create a subshell so totsize is “local” inside that subshell. you can try this instead (bash)

totsize=0
while read size rest ; do
    totsize=$((totsize+size))
    echo "$totsize"
done < <(find /home/user -type f -mmin -4860 -a -mmin +3420 | xargs du)
echo "Sum: $totsize kb"

Or instead of using bash, call awk

$> find /home/user -type f -mmin -4860 -a -mmin +3420 | xargs du | awk  '{s+=$1}END{print "total size: "s}'

But are you sure you want to use just du without any options, because the size is not “accurate” (using du -b would be better). If you have GNU find, you can use -printf

find /home/user -type f -mmin -4860 -a -mmin +3420 -printf "%s\n" | awk  '{s+=$1}END{print "total size: "s" bytes"}'