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Re: Severe Bash Bug with Arrays


From: DJ Mills
Subject: Re: Severe Bash Bug with Arrays
Date: Wed, 25 Apr 2012 13:50:42 -0400

On Wed, Apr 25, 2012 at 1:36 PM, Ted Okuzumi <address@hidden> wrote:
> Please note that if I use a different command in the while loop it works.
>
> So instead of
> echo "$mydata" | while read -r line; do myarray+=( "$line" ); done
>
> if I say:
> echo "$mydata" | while read -r line; do echo "$line" ; done
>
> Then it works.  To pipe output into a while statement works, normally,
> however it fails when the array+=( )  is used.  It doesn't fail on other
> commands that I've seen.
>
> This is why I am saying it is definitely a bug.  Do you not agree?
>
> Thanks,
> Ted



It is NOT a bug. POSIX specifies this behavior. The echo is still run
in a subshell...

The issue is that commands in a pipeline are run in a subshell. This
means that assignments and changes in PWD do not persist afterwards.
See http://mywiki.wooledge.org/BashFAQ/024.

The lastpipe shopt will work, as discussed before. The other options
are process substitution, or (POSIXly) a fifo.

Examples:

# Process substitution:
while read -r line; do
  array+=("$line")
done < <(some_command)


# FIFO:
mkfifo myfifo || exit
trap 'rm -f myfifo' EXIT

some_command > myfifo &
while read -r line; do
  array+=("$line")
done < myfifo


# Or, since it's a string you're using in your example, you could use
a here string or here document
while read -r line; do array+=("$line"); done <<<"$mydata"

while read -r line; do array+=("$line"); done <<EOF
$mydata
EOF

mapfile, with bash4, is also an option for this particular case.



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