[SOLVED] Why does '<<<' in Bash works only when a command is executed?

Issue

If I run the first command, it outputs a="A", b="B", but if I run the second command, it doesn't output anything.

first

while read a && read b; do echo "a=$a, b=$b"; done <<<$(echo '["A", "B"]' | jq '.[]')

second

 while read a && read b; do echo "a=$a, b=$b"; done <<<"A B"

Why can't I read the string supplied unless it comes from a command output?

Solution

In the second example, read a consumes the entire here string. read b has nothing left to read, so it fails. This causes the while loop condition to fail, which means the body is never executed.

The difference in the first command is that jq produces two lines of output, so each read command reads one of them. (The newlines separating them is preserved because the result of the command substitution is not subject to word-splitting, as the value of the here string.)

$ echo '["A", "B"]' | jq '.[]'
"A"
"B"

Answered By - chepner