Bash – Process Substitution with Perl Script

You can pretty easily get the return from any subshelled process by echoing its return out over its stdout. The same is true of process substitution:

while IFS= read -r -d $'\0' FILE || 
    ! return=$FILE
do    ARGS[ARGID++]="$FILE"
done < <(find . -type f -print0; printf "$?")

If I run that then the very last line - (or \0 delimited section as the case may be) is going to be find's return status. read is going to return 1 when it gets an EOF - so the only time $return is set to $FILE is for the very last bit of information read in.

I use printf to keep from adding an extra \newline - this is important because even a read performed regularly - one in which you do not delimit on \0 NULs - is going to return other than 0 in cases when the data it has just read in does not end in a \newline. So if your last line does not end with a \newline, the last value in your read in variable is going to be your return.

Running command above and then:

echo "$return"

OUTPUT

0

And if I alter the process substitution part...

...
done < <(! find . -type f -print0; printf "$?")
echo "$return"

OUTPUT

1

A more simple demonstration:

printf \\n%s list of lines printed to pipe |
while read v || ! echo "$v"
do :; done

OUTPUT

pipe

And in fact, so long as the return you want is the last thing you write to stdout from within the process substitution - or any subshelled process from which you read in this way - then $FILE is always going to be the return status you want when it is through. And so the || ! return=... part is not strictly necessary - it is used to demonstrate the concept only.

You were really close:

tr "o" "a" < <(echo "Foo")

The substitution <() makes a file descriptor and just pastes the path to the shell. For comprehension just execute:

<(echo blubb)

You will see the error:

-bash: /dev/fd/63: Permission denied

That's why it just pastes /dev/fd/63 into the shell and /dev/fd/63 is not excutable, because it's a simple pipe. In the tr-example above, it's echo "Foo" that writes to the pipe and via input redirection < it's the tr command that reads from the file descriptor.