Question

while sizes=`sizes $pgid`
do
    set -- $sizes
    sample=$((${@/#/+}))
    let peak="sample > peak ? sample : peak"
    sleep 0.1
done

i am confused about the below statement:

sample=$((${@/#/+}))

could anybody explain this?

Answer 1

The ‘${@/#/+}‘ part is a regular expression expansion:

${parameter/pattern/string}

The pattern is expanded to produce a pattern just as in filename expansion.
Parameter is expanded and the longest match of pattern against its value is
replaced with string. If pattern begins with ‘/‘, all matches of pattern are replaced
with string. Normally only the first match is replaced. If pattern begins
with ‘#‘, it must match at the beginning of the expanded value of parameter.
If pattern begins with ‘%‘, it must match at the end of the expanded value of
parameter. If string is null, matches of pattern are deleted and the / following
pattern may be omitted. If parameter is ‘@‘ or ‘*‘, the substitution operation
is applied to each positional parameter in turn, and the expansion is the resultant
list. If parameter is an array variable subscripted with ‘@‘ or ‘*‘, the
substitution operation is applied to each member of the array in turn, and the
expansion is the resultant list.

So, it looks like it replaces the empty string at the start of each value in the argument list ‘$@‘ with a ‘+‘. It’s key merit is that it prefixes each argument in one fell swoop; otherwise, it is similar to "+$var".

---

The ‘$(( ... )) part is an arithmetic expression. It performs arithmetic on the expression between the parentheses. So, in context, it adds up the values in the argument list, assuming they are all numeric. Given the expansion, it might yield:

set -- 2 3 5 7 11
sample=$((${@/#/+}))
sample1=$((+2 +3 +5 +7 +11))
echo $sample = $sample1

and hence ‘28 = 28‘.