Re: reading from external command

[Chet Ramey]

On 3/31/22 7:40 PM, Peng Yu wrote:
> I don't get how to modify the while-loop (with unnamed pipes in the
> body) to get the exit code in the loop input unnamed pipe. Could you
> make it clear by showing a working example? Thanks.

The key is realizing that the process substitution that's redirecting into
the loop is created just before the loop executes, so $! is set to its pid
the first time through the loop. Whether it stays valid for any other loop
iterations depends on the loop body.

So make sure you set the variable you want to hold the desired value of $!
once, the first time through the loop, and use wait on it after the loop
terminates.

while read x y; do
        [ -z "$pid" ] && pid=$!
        echo $x $y      # or other loop body
done < <(echo one two ; exit 42 )

wait $pid
echo $?


--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/