how does this work ? for loop strips variable result

Question

while learning bash/ksh from a book. i came across an exercise that had a result i would love to understand.

When executing this script with bash -x to try to understand how it behaves (debugging) i noticed the line variable is number + filename. as this is the result of wc -c [file]. but as soon as the for loop sets the res variable it somehow strips the file name and only says res=23
(number result of wc -c) i can see it happening but don't really understand why ? i think i understand the for loop construct .. for i in xxx do what ever.. don't 100% understand the break in this construct i do know it's to break out of a nested loop.

this is the exercise script:

if [ ! $# -eq 2 ] ;then
    echo
    echo "usage: $0 <location> <FileName>"
    echo
    exit 1
fi
TMPFILE=/tmp/count
line=$(find "$1" -name "$2" -type f -print | tee $TMPFILE | wc -l)
aant=$line
nr=0
som=0

while [ $nr -lt "$aant" ] ; do
    nr=$(( nr +1 ))
    bestand=$(head -$nr $TMPFILE | tail -1)
    echo -n "$bestand"
    line=$(wc -c "$bestand")

    for woord in $line ; do
        res=$woord
        break
    done
    echo "  $res"
    som=$(( som + res ))
done

if [ "$aant" -eq 0 ] ; then
    echo "No files found"
else
    echo
    echo "In totaal $aant files take $som bytes of space"
fi

rm $TMPFILE
exit 0

Answer 1

In for word in $line, the contents of the variable line is split to words and then expanded for filename globs (see also these questions). The loop then runs once for each resulting value (or "word"). In this case though, there is only one iteration of the loop at most, since the break statement stops the loop from running. As a result, the value of res set on that first iteration of the loop remains, and the effect is the same as picking the first whitespace-separated word from line.

There would be better ways of writing that, we could e.g. remove the filename with shell expansions. This would output 23:

line='23 somefilename'
res="${line%% *}"
echo "$res"

Or, just have wc not output the filename to begin with, by redirecting the file to wc's stdin, instead of passing the filename to wc. Compare these two:

$ wc -c foo.txt
8 foo.txt
$ wc -c < foo.txt
8

Answer 2

If I understand your question correctly, the magic happens right here:

for woord in $line ; do
    res=$woord
    break
done

To help you understand how that works, maybe considering this will help:

for N in 1 2 3 4 5; do
    echo $N
done
echo "N is $N"

Next, add the "break" statement and consider:

for N in 1 2 3 4 5; do
    echo $N
    break
done
echo "N is $N"

Can you describe what adding the break statement did to the for loop?

In my opinion, the four lines of code I originally quoted could be pared down to just three:

for res in $line ; do
    break
done

This is essentially a "kludgy" way of saying:

res="$(awk '{print $1}' <<< "$line")"

Answer 3

Check the man page for wc -c.

It outputs bytecount filename into $line as a space separated list.

The for $woord in $line loop reads the first variable (up till the first space) from $list, which is the bytecount and stores this in $res. Then instead of going on to read the next item in $list it hits the break instruction and exits the loop.

As @ikkachu says, there are easier ways to get the bytecountthan with a loop.