From: Eric Blake <eblake@redhat.com>
To: solsTiCe d'Hiver <solstice.dhiver@gmail.com>, dash@vger.kernel.org
Subject: Re: "if [ s1 > s2 ]" broken, writing a s2 file
Date: Mon, 08 Dec 2014 10:39:41 -0700 [thread overview]
Message-ID: <5485E25D.9080100@redhat.com> (raw)
In-Reply-To: <CAE4vF2dip455tXT4K3XwJy1Pp_qrw0fupd3xoAKUgbCG7vcFzg@mail.gmail.com>
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On 12/08/2014 10:32 AM, solsTiCe d'Hiver wrote:
> hello,
> folowing that bug
> https://bugs.launchpad.net/ubuntu/+source/update-notifier/+bug/1400357,
> I follow through to investigate and I found out
> that whatever I try, when comparing 2 strings I always end up with a
> file written to disk
>
>>From the man page
> test expression
> [ expression ]
> [...]
> s1 > s2 True if string s1 comes after s2 based on the ASCII
> value of their characters.
You HAVE to escape the > so that it is interpreted as an argument and
not a redirection operator. The bug is not in dash, but in your usage.
>
> when I try to use it:
> a="ert"
> b="aze"
> if [ $a > $b ] ; then
Wrong. Use:
if [ "$a" ">" "$b" ]; then
>
> so this "if syntax" is broken or I don't knwo how to use it.
The latter.
>
> Also it is really dangerous to use a syntax similar to file
> redirection and this is exactly what is happening here.
POSIX is proposing the addition of the shell builtin [[ ]], where
because it is a syntactical part of the shell, it would have safe
semantics (that is, [[ $a > $b ]] would be perfectly safe and do the
right thing). But until the POSIX standardization is complete, dash does
not implement [[; and as long as only '[' is portable (with its
unfortunate but historically-mandated semantics of operating as if it
were NOT a builtin, in that shell parsing happens before test sees its
arguments), then you have to quote anything that might otherwise be
misinterpreted during parsing.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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next prev parent reply other threads:[~2014-12-08 17:39 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-12-08 17:32 "if [ s1 > s2 ]" broken, writing a s2 file solsTiCe d'Hiver
2014-12-08 17:39 ` Eric Blake [this message]
2014-12-08 17:46 ` solsTiCe d'Hiver
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