Process substitution with heredoc works in zsh. How to make it work in bash?

Question

I want the ability to pass a heredoc to the source command for local interpretation. The body of the heredoc document will be injected later before calling the script, and can be multiline.

I came up with this trick, which works in zsh:

source <(<<'heredoc'
echo foo
sleep 10
echo done
heredoc
) && echo ok || echo no

it prints foo, sleeps 10 seconds, then prints done, then ok. So far, so good.

Now, the script needs to run in bash. When I feed bash (or sh) the exact same thing, I only get ok, with no wait at all, as if the command I was providing with the heredoc was just true.

I cannot understand why, and how I can build the same behavior in bash and sh.

Answer 1

How to make it work in bash?

Remove any here documents and execute the commands. You could preserve the group.

{
   echo foo
   sleep 10
   echo done
} && echo ok || echo no

You can use, both in zsh and bash, source /dev/stdin if you want to execute commands in the current execution environment coming from a steam, like a here document. No need for <( process substitution in either case. So here you go:

source /dev/stdin <<heredoc
echo foo
sleep 10
echo done
heredoc

Note: this looks very odd thing to do and most probably there are better solutions to what you are doing.

why

Because in bash empty redirection like a here document without a command does nothing. You have to for example cat the content of the stream created by here document to output it to standard output for process substation to capture it.

Answer 2

Redirections in bash aren't processes, so it doesn't make sense to use a process substitution to capture them. But you can certainly do it if you use a process to capture the redirection. cat does that.

The following works in bash 4 and up:

source <(cat <<'heredoc'
echo foo
sleep 10
echo done
heredoc
) && echo ok || echo no

You cannot source a process substitution in bash 3.

Answer 3

You could use eval ("evil"); in this context, it's likely not a security issue.

eval '
echo foo
sleep 10
echo done
' && echo ok || echo no

You could also just, you know, not use dynamic code execution:

{
    echo foo
    sleep 10
    echo done
} && echo ok || echo no