From: Jilles Tjoelker <jilles@stack.nl>
To: Oleg Bulatov <oleg@bulatov.me>
Cc: dash@vger.kernel.org
Subject: Re: heredoc and subshell
Date: Wed, 24 Feb 2016 00:18:44 +0100 [thread overview]
Message-ID: <20160223231843.GA55161@stack.nl> (raw)
In-Reply-To: <2978711456265264@web9h.yandex.ru>
On Wed, Feb 24, 2016 at 01:07:44AM +0300, Oleg Bulatov wrote:
> trying to minimize a shell code I found an unobvious moment with
> heredocs and subshells.
> Is it specified by POSIX how next code should be parsed? dash output
> for this code differs from bash and zsh.
> --- code
> prefix() { sed -e "s/^/$1:/"; }
> DASH_CODE() { :; }
>
> prefix A <<XXX && echo "$(prefix B <<XXX
> echo line 1
> XXX
> echo line 2)" && prefix DASH_CODE <<DASH_CODE
> echo line 3
> XXX
> echo line 4)"
> echo line 5
> DASH_CODE
> --- bash 4.3.42 output:
> A:echo line 3
> B:echo line 1
> line 2
> DASH_CODE:echo line 4)"
> DASH_CODE:echo line 5
> --- dash 0.5.8 output:
> A:echo line 1
> B:echo line 2)" && prefix DASH_CODE <<DASH_CODE
> B:echo line 3
> line 4
> line 5
I think POSIX is clear that the bash/zsh behaviour is correct and the
dash behaviour is wrong. In XCU 2.6.3 Command Substitution, it says:
] With the $(command) form, all characters following the open
] parenthesis to the matching closing parenthesis constitute the
] command.
Therefore, the shell should not start reading the here-document
belonging to prefix A <<XXX while it is still inside the command
substitution $(prefix B <<XXX.
Instead, the here-document belonging to prefix B <<XXX should be read.
The line ending the command substitution contains another <<
redirection; the here-documents are read in order of the <<
redirections.
In FreeBSD sh, another ash derivative, I fixed this in
FreeBSD SVN r208655,
https://github.com/freebsd/freebsd/commit/930ce3922652c50fc8b621b14b6238b325d7f16f
Interestingly, mksh parses this in yet another way. In unmodified form,
it fails with here document 'XXX' unclosed. After appending an XXX
line, it outputs:
A:echo line 5
A:DASH_CODE
B:echo line 2)" && prefix DASH_CODE <<DASH_CODE
B:echo line 3
line 4
The here-document containing line 1 seems lost entirely.
The ksh93 93u+ 2012-08-01 from FreeBSD ports segfaults while executing
the script.
Concludingly, it seems unwise to rely on this construct in scripts to be
distributed.
--
Jilles Tjoelker
next prev parent reply other threads:[~2016-02-23 23:28 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-23 22:07 heredoc and subshell Oleg Bulatov
2016-02-23 22:49 ` Eric Blake
2016-02-23 23:16 ` Eric Blake
[not found] ` <56CCE1E3.1060805-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2016-02-24 10:58 ` Joerg Schilling
[not found] ` <56cd8cd1.LdVYbpfe+0tQMm8I%Joerg.Schilling-8LS2qeF34IpklNlQbfROjRvVK+yQ3ZXh@public.gmane.org>
2016-02-24 12:27 ` Shware Systems
2016-02-24 12:32 ` Joerg Schilling
2016-02-24 20:27 ` Oleg Bulatov
2016-02-23 23:18 ` Jilles Tjoelker [this message]
2016-02-24 8:46 ` Thorsten Glaser
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