Re: While loop and stdin

[Dennis Williamson]

On Wed, Feb 14, 2024, 3:25 PM Mohammad Akhlaghi <mohammad@akhlaghi.org>
wrote:

> Dear GNU Bash developers,
>
> We recently confronted the following situation which we do not
> understand. Any insight would be highly appreciated:
>
> The following command works as expected (and prints "1" and "4"):
>
> $ printf "1 2 3\n4 5 6\n" | while read a b c; do echo $a; done
> 1
> 4
>
> However, if a command within the while loop reads from standard input,
> the while loop doesn't read that line any more:
>
> $ printf "1 2 3\n4 5 6\n" \
>           | while read a b c; do cat /dev/stdin; echo $a; done
> 4 5 6
> 1
>
> We were expecting that 'while' reads the full standard input and that
> the 'cat' command would have returned an empty string (for an empty
> standard input). But when 'cat' has read from the standard input, the
> 'while' loop has no longer seen the second line that was given to it.
>
> I just wanted to confirm if the result above is the expected way for
> 'while' to read from standard input?
>
> The original problem happened for a program in GNU Astronomy Utilities
> (that I am maintaining) which read from standard input by default. If
> the behavior above is expected, I need to modify the behavior of that
> program.
>
> Thank you very much,
> Mohammad
>
>


Another way to use a different file descriptor as Greg showed is with the
Bash-specific -u option:

while read -u 3 -r a b c; do
    cat /dev/stdin
    echo "$a"
done 3< my-input-file

This is shown at the bottom of the page Greg linked to as is a way to avoid
hard-coding the file descriptor.