While loop - process substitution vs. here string with command substitution
bash, while-loop
Solution
while read test; do
echo $test
done <<< "$(seq 5)"
Execute `seq 5`, collecting the result into a temporary variable. Then execute the while loop, feeding it the collecting result.
while read test; do
echo $test
done < <(seq 5)
Set up a subshell to execute `seq 5` and connect its `stdout` to `stdin`. Then start the while loop. When it finishes, restore `stdin`.
What's the difference? For `seq 5`, practically nothing; however, it can still be made visible by changing `seq 5` to `seq 5; echo done generating sequence >&2`. Then you can see that in the first case, the entire `seq` execution finishes before the `while` loop starts, while in the second case they execute in parallel.
$ while read n; do echo $n > /dev/stderr; done \
> <<<"$(seq 5; echo done generating sequence >&2)"
done generating sequence
1
2
3
4
5
$ while read n; do echo $n > /dev/stderr; done \
> < <(seq 5; echo done generating sequence >&2)
1
2
done generating sequence
3
4
5
If it were `seq 10000000`, the difference would be much clearer. The `<<<"$(...)` form would use a lot more memory in order to store the temporary string.
Problem
Can someone please explain the difference between these two while loops: ``` while read test; do echo $test done <<< "$(seq 5)" ``` ``` while read test; do echo $test done < <(seq 5) ```