eval and export behave differently together in dash and ash than in bash, zsh, and ksh

eval and export behave differently together in dash and ash than in bash, zsh, and ksh

[Dan Kegel]

The script

eval export dir=~$LOGNAME
echo $dir

eval dir2=~$LOGNAME
echo $dir2

produces different results on different shells.

bash  (4.2-2ubuntu2.1) (with and without --posix)
zsh (4.3.17-1ubuntu1)
ksh (93u-1)
produce

/home/buildbot
/home/buildbot

dash (0.5.7-2ubuntu2)
ash (0.5.7-2ubuntu2)
produce

~buildbot
/home/buildbot

The easy workaround is to use the older form

eval dir=~$LOGNAME
export dir

but it'd be nice if that weren't needed.

Re: eval and export behave differently together in dash and ash than in bash, zsh, and ksh

[Eric Blake]

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On 02/14/2014 11:29 AM, Dan Kegel wrote:
> The script
> 
> eval export dir=~$LOGNAME
> echo $dir

This has been discussed by the POSIX folks.  See:
http://austingroupbugs.net/view.php?id=351

The standard added the definition of a 'declaration utility', which can
evaluate its arguments in assignment context, and marked that 'export'
is one such utility:

>> At line 74276 (XCU export DESCRIPTION), add:
>> The export special built-in shall be a declaration utility. Therefore,
>> if export is recognized as the command word of a simple command, then
>> subsequent words of the form name=word shall be expanded in an
>> assignment context. See Section 2.9.1.

Meanwhile, 'eval' is not allowed to be a declaration utility.  Thus,
once this POSIX interpretation is made live in Issue 8 (currently still
unreleased), a conforming shell would have to evaluate this as:

eval export dir=~$LOGNAME      # not in assignment context
eval 'export' 'dir=~buildbot'  # actual arguments passed to eval
export dir=~buildbot           # declaration context, do tilde expansion
dir gets /home/buildbot

ksh and bash are compliant, dash needs to be patched.

However, as Issue 8 POSIX is not yet released, dash is not yet
non-compliant, and you are correct that workarounds exist while waiting
for someone to write the patch for dash.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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