is variable with variable name possible?

is variable with variable name possible?

[zzapper]

somevar=$(pwd)

but I want somevar to be a variable name

-- 
zzapper
http://zzapper.co.uk/ Technical Tips

Re: is variable with variable name possible?

[Jérémie Roquet]

Hi,

2011/4/4 zzapper <david@tvis.co.uk>:
> somevar=$(pwd)
>
> but I want somevar to be a variable name

$ eval "$somevar=$(pwd)"

Best regards,

-- 
Jérémie

Re: is variable with variable name possible?

[Jérémie Roquet]

2011/4/4 Jérémie Roquet <arkanosis@gmail.com>:
> 2011/4/4 zzapper <david@tvis.co.uk>:
>> somevar=$(pwd)
>>
>> but I want somevar to be a variable name
>
> $ eval "$somevar=$(pwd)"

Actually, the quotes aren't even needed:

$ eval $somevar=$(pwd)

-- 
Jérémie

Re: is variable with variable name possible?

[zzapper]

=?UTF-8?B?SsOpcsOpbWllIFJvcXVldA==?= wrote in 
news:BANLkTikVWk9t4h3APSkFN7tOKyjTHJgmJg@mail.gmail.com:

> eval "$somevar=$(pwd)"
> 

Thanks, that works fine, and I already knww that but just had a memory 
glitch!!

-- 
zzapper
http://zzapper.co.uk/ Technical Tips

Re: is variable with variable name possible?

[Bart Schaefer]

On Apr 4,  3:51pm, Jeremie Roquet wrote:
} Subject: Re: is variable with variable name possible?
}
} 2011/4/4 Jeremie Roquet <arkanosis@gmail.com>:
} > 2011/4/4 zzapper <david@tvis.co.uk>:
} >> somevar=$(pwd)
} >>
} >> but I want somevar to be a variable name
} >
} > $ eval "$somevar=$(pwd)"
} 
} Actually, the quotes aren't even needed:
} 
} $ eval $somevar=$(pwd)

If $(pwd) has spaces in the directory name, this is going to fail.  So
you do need quotes, but a different kind:

    eval $somevar='$(pwd)'

For scalars (but not array assignments) you can also do

    typeset -g $somevar=$(pwd)

You can also use ${somevar::=$(pwd)} anywhere that you'd reference the
value of $somevar, including the no-op command:

    : ${somevar::=$(pwd)}

That also works for array assignments if you explicitly split:

    : ${(As:/:)somevar::=$(pwd)}
    print -l $somevar

Note though that here the stuff after the = is joined with the first
character of $IFS before being split, so to copy one array to another
you have to be careful.  Also note that without the (A) the split is
applied to the expansion after the assignment.

Re: is variable with variable name possible?

[Bart Schaefer]

The reply quoted below got hung up on my server for several hours
because a CentOS upgrade clobbered my sendmail configuration, which I
didn't notice immediately.  Restored from backup (which is why I'm up
at 1:30am) and it finally went out.  In the interim the discussion has
pointed out an omission I made.

On Mon, Apr 4, 2011 at 7:26 AM, Bart Schaefer <schaefer@brasslantern.com> wrote:
>
> You can also use ${somevar::=$(pwd)} anywhere that you'd reference the
> value of $somevar, including the no-op command:
>
>    : ${somevar::=$(pwd)}
>
>    : ${(As:/:)somevar::=$(pwd)}

The bit I forgot is that to get the effect desired in the original
message, you must add the (P) flag to each of the above, as in:

: ${(P)somevar::=$(pwd)}

: ${(PAs:/:)somevar::=$(pwd)}

As long as I'm expostulating I'll add that "somevar" can even be a
positional parameter, both with and without the (P) flag.  E.g.

: ${(As:/:)1::=$(pwd)}

will splice the current directory hierarchy into the front of "$@".

Re: is variable with variable name possible?

[Stephane Chazelas]

2011-04-04 15:48:32 +0200, Jérémie Roquet:
[...]
> 2011/4/4 zzapper <david@tvis.co.uk>:
> > somevar=$(pwd)
> >
> > but I want somevar to be a variable name
> 
> $ eval "$somevar=$(pwd)"
[...]

No!

eval "$somevar=\$(pwd)"
or
eval "$somevar=\$PWD"


Or think of what would happen if $PWD was "/tmp/x;rm -rf /home"

-- 
Stephane

Re: is variable with variable name possible?

[Mikael Magnusson]

On 4 April 2011 16:06, Stephane Chazelas <stephane_chazelas@yahoo.fr> wrote:
> 2011-04-04 15:48:32 +0200, Jérémie Roquet:
> [...]
>> 2011/4/4 zzapper <david@tvis.co.uk>:
>> > somevar=$(pwd)
>> >
>> > but I want somevar to be a variable name
>>
>> $ eval "$somevar=$(pwd)"
> [...]
>
> No!
>
> eval "$somevar=\$(pwd)"
> or
> eval "$somevar=\$PWD"
>
>
> Or think of what would happen if $PWD was "/tmp/x;rm -rf /home"

How do you feel about this?
: ${(P)somevar::=$PWD}

-- 
Mikael Magnusson

Re: is variable with variable name possible?

[Stephane Chazelas]

2011-04-04 16:25:01 +0200, Mikael Magnusson:
> On 4 April 2011 16:06, Stephane Chazelas <stephane_chazelas@yahoo.fr> wrote:
> > 2011-04-04 15:48:32 +0200, Jérémie Roquet:
> > [...]
> >> 2011/4/4 zzapper <david@tvis.co.uk>:
> >> > somevar=$(pwd)
> >> >
> >> > but I want somevar to be a variable name
> >>
> >> $ eval "$somevar=$(pwd)"
> > [...]
> >
> > No!
> >
> > eval "$somevar=\$(pwd)"
> > or
> > eval "$somevar=\$PWD"
> >
> >
> > Or think of what would happen if $PWD was "/tmp/x;rm -rf /home"
> 
> How do you feel about this?
> : ${(P)somevar::=$PWD}
[...]

Try after

sudo ln -s . '/*'
cd '/*/*/*/*/*'
setopt globsubst

;-)

(if you're lucky, it won't crash your machine).

-- 
Stephane

Re: is variable with variable name possible?

[Mikael Magnusson]

On 4 April 2011 18:28, Stephane Chazelas <stephane_chazelas@yahoo.fr> wrote:
> 2011-04-04 16:25:01 +0200, Mikael Magnusson:
>> On 4 April 2011 16:06, Stephane Chazelas <stephane_chazelas@yahoo.fr> wrote:
>> > 2011-04-04 15:48:32 +0200, Jérémie Roquet:
>> > [...]
>> >> 2011/4/4 zzapper <david@tvis.co.uk>:
>> >> > somevar=$(pwd)
>> >> >
>> >> > but I want somevar to be a variable name
>> >>
>> >> $ eval "$somevar=$(pwd)"
>> > [...]
>> >
>> > No!
>> >
>> > eval "$somevar=\$(pwd)"
>> > or
>> > eval "$somevar=\$PWD"
>> >
>> >
>> > Or think of what would happen if $PWD was "/tmp/x;rm -rf /home"
>>
>> How do you feel about this?
>> : ${(P)somevar::=$PWD}
> [...]
>
> Try after
>
> sudo ln -s . '/*'
> cd '/*/*/*/*/*'
> setopt globsubst
>
> ;-)
>
> (if you're lucky, it won't crash your machine).

Pretty sure globsubst breaks your idea too, but fine,
: ${(P)~~somevar::=$PWD}
In fact, globsubst pretty much breaks every hook function i have :P.

-- 
Mikael Magnusson

Re: is variable with variable name possible?

[Stephane Chazelas]

2011-04-04 18:34:26 +0200, Mikael Magnusson:
[...]
> >> > eval "$somevar=\$(pwd)"
> >> > or
> >> > eval "$somevar=\$PWD"
[...]
> >> How do you feel about this?
> >> : ${(P)somevar::=$PWD}
> > [...]
> >
> > Try after
> >
> > sudo ln -s . '/*'
> > cd '/*/*/*/*/*'
> > setopt globsubst
> >
> > ;-)
> >
> > (if you're lucky, it won't crash your machine).
> 
> Pretty sure globsubst breaks your idea too, but fine,
> : ${(P)~~somevar::=$PWD}
> In fact, globsubst pretty much breaks every hook function i have :P.
[...]

eval "$somevar=\$(pwd)"
or
eval "$somevar=\$PWD"

are POSIX and POSIX shells have globsubst and shwordsplit on.
The above will work regardless of the status of those options.

In POSIX shells, you need to quote variables to prevent
globsubst or shwordsplit, even in arguments to ":" because
globsubst can be very very resource intensive.

You can bring a machine to its knees with

: ${var=foo}

if $var is for instance /*/*/*/../../../*/*/*/../../../*/*/*

So, you should either use instead:

var=${var-foo}
or
: "${var=foo}"

In your example, if we have to accomodate users enabling
shwordsplit or globsubst (or running zsh in sh or ksh
emulation), we need

: "${(P)somevar::=$PWD}"

But

eval "$somevar=\$PWD"

is more legible and more portable.

-- 
Stephane

Re: is variable with variable name possible?

[Mikael Magnusson]

On 4 April 2011 22:50, Stephane Chazelas <stephane_chazelas@yahoo.fr> wrote:
> 2011-04-04 18:34:26 +0200, Mikael Magnusson:
> [...]
>> >> > eval "$somevar=\$(pwd)"
>> >> > or
>> >> > eval "$somevar=\$PWD"
> [...]
>> >> How do you feel about this?
>> >> : ${(P)somevar::=$PWD}
>> > [...]
>> >
>> > Try after
>> >
>> > sudo ln -s . '/*'
>> > cd '/*/*/*/*/*'
>> > setopt globsubst
>> >
>> > ;-)
>> >
>> > (if you're lucky, it won't crash your machine).
>>
>> Pretty sure globsubst breaks your idea too, but fine,
>> : ${(P)~~somevar::=$PWD}
>> In fact, globsubst pretty much breaks every hook function i have :P.
> [...]
>
> eval "$somevar=\$(pwd)"
> or
> eval "$somevar=\$PWD"
>
> are POSIX and POSIX shells have globsubst and shwordsplit on.
> The above will work regardless of the status of those options.
>
> In POSIX shells, you need to quote variables to prevent
> globsubst or shwordsplit, even in arguments to ":" because
> globsubst can be very very resource intensive.
>
> You can bring a machine to its knees with
>
> : ${var=foo}
>
> if $var is for instance /*/*/*/../../../*/*/*/../../../*/*/*
>
> So, you should either use instead:
>
> var=${var-foo}
> or
> : "${var=foo}"
>
> In your example, if we have to accomodate users enabling
> shwordsplit or globsubst (or running zsh in sh or ksh
> emulation), we need
>
> : "${(P)somevar::=$PWD}"
>
> But
>
> eval "$somevar=\$PWD"
>
> is more legible and more portable.

But your quotes do nothing (assuming $somevar has a sane value), did you mean
eval $somevar=\"\$PWD\"
or possibly more readable version
eval $somevar='"$PWD"'
? AFAICT, it works without any quotes with globsubst and shwordsplit
set too though, as long as you quote the $ in $PWD, tried in both zsh
and bash.

-- 
Mikael Magnusson

Re: is variable with variable name possible?

[Stephane Chazelas]

2011-04-05 00:32:27 +0200, Mikael Magnusson:
[...]
> > eval "$somevar=\$PWD"
> >
> > is more legible and more portable.
> 
> But your quotes do nothing (assuming $somevar has a sane value), did you mean
> eval $somevar=\"\$PWD\"
> or possibly more readable version
> eval $somevar='"$PWD"'

Those double quotes are not necessary.

> ? AFAICT, it works without any quotes with globsubst and shwordsplit
> set too though, as long as you quote the $ in $PWD, tried in both zsh
> and bash.
[...]

If you don't quote $somevar, you ask the shell to split it which
it will do according to the current value of IFS (assuming
shwordsplit).

If you quote it, you don't have to worry what the value of $IFS
is.

-- 
Stephane

Re: is variable with variable name possible?

[Aaron Davies]

> eval "$somevar=\$PWD"

shirley we have an equiv of bash2's ${!foo}

i've always advertised zsh as a superset of everything interesting
from every other shell, please tell me i'm not wrong....

Re: is variable with variable name possible?

[Phil Pennock]

On 2011-04-04 at 22:54 -0400, Aaron Davies wrote:
> > eval "$somevar=\$PWD"
> 
> shirley we have an equiv of bash2's ${!foo}

Sure thing, and just like Bash, it's useful for de-referencing more than
it is for assignment.

${(P)foo}

> i've always advertised zsh as a superset of everything interesting
> from every other shell, please tell me i'm not wrong....

You're wrong.  zsh is a highly capable shell, but there are a myriad of
interesting features out there in other shells.  rc has generalised
file-descriptor plumbing; ksh has named background pools, namespaced
variables, generic character class types for matching (think "\d" in a
glob to match a digit), seeking to an offset in a file in a <redirection
with <#((expr)) and also >#((expr)), and seeking forward automatically
to a pattern with <#pattern.  ksh has discipline functions and probably
more I've missed.

Innovation doesn't only happen in zsh and sometimes we too blatantly
copy from others.

Hrm, anyone know why ksh opens the <>filename on stdout while zsh opens
it on stdin?

-Phil

Re: is variable with variable name possible?

[ZyX]

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Reply to message «Re: is variable with variable name possible?», 
sent 00:50:02 05 April 2011, Tuesday
by Stephane Chazelas:

> In your example, if we have to accomodate users enabling
> shwordsplit or globsubst (or running zsh in sh or ksh
> emulation), we need
> 
> : "${(P)somevar::=$PWD}"
If we are talking about script, then you should use `emulate -L zsh' somewhere 
at the top. This will deal with all possible weird options that are set by user.

> eval "$somevar=\$(pwd)"
> or
> eval "$somevar=\$PWD"
> 
> are POSIX and POSIX shells have globsubst and shwordsplit on.
> The above will work regardless of the status of those options.
Somebody said that $somevar contains nothing harmful?

Original message:
> 2011-04-04 18:34:26 +0200, Mikael Magnusson:
> [...]
> 
> > >> > eval "$somevar=\$(pwd)"
> > >> > or
> > >> > eval "$somevar=\$PWD"
> 
> [...]
> 
> > >> How do you feel about this?
> > >> 
> > >> : ${(P)somevar::=$PWD}
> > > 
> > > [...]
> > > 
> > > Try after
> > > 
> > > sudo ln -s . '/*'
> > > cd '/*/*/*/*/*'
> > > setopt globsubst
> > > 
> > > ;-)
> > > 
> > > (if you're lucky, it won't crash your machine).
> > 
> > Pretty sure globsubst breaks your idea too, but fine,
> > 
> > : ${(P)~~somevar::=$PWD}
> > 
> > In fact, globsubst pretty much breaks every hook function i have :P.
> 
> [...]
> 
> eval "$somevar=\$(pwd)"
> or
> eval "$somevar=\$PWD"
> 
> are POSIX and POSIX shells have globsubst and shwordsplit on.
> The above will work regardless of the status of those options.
> 
> In POSIX shells, you need to quote variables to prevent
> globsubst or shwordsplit, even in arguments to ":" because
> globsubst can be very very resource intensive.
> 
> You can bring a machine to its knees with
> 
> : ${var=foo}
> 
> if $var is for instance /*/*/*/../../../*/*/*/../../../*/*/*
> 
> So, you should either use instead:
> 
> var=${var-foo}
> or
> 
> : "${var=foo}"
> 
> In your example, if we have to accomodate users enabling
> shwordsplit or globsubst (or running zsh in sh or ksh
> emulation), we need
> 
> : "${(P)somevar::=$PWD}"
> 
> But
> 
> eval "$somevar=\$PWD"
> 
> is more legible and more portable.

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Re: is variable with variable name possible?

[ZyX]

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Reply to message «Re: is variable with variable name possible?», 
sent 00:50:02 05 April 2011, Tuesday
by Stephane Chazelas:

> In your example, if we have to accomodate users enabling
> shwordsplit or globsubst (or running zsh in sh or ksh
> emulation), we need
> 
> : "${(P)somevar::=$PWD}"
If we are talking about script, then you should use `emulate -L zsh' somewhere 
at the top. This will deal with all possible weird options that are set by user.

> eval "$somevar=\$(pwd)"
> or
> eval "$somevar=\$PWD"
> 
> are POSIX and POSIX shells have globsubst and shwordsplit on.
> The above will work regardless of the status of those options.
Somebody said that $somevar contains nothing harmful?

Original message:
> 2011-04-04 18:34:26 +0200, Mikael Magnusson:
> [...]
> 
> > >> > eval "$somevar=\$(pwd)"
> > >> > or
> > >> > eval "$somevar=\$PWD"
> 
> [...]
> 
> > >> How do you feel about this?
> > >> 
> > >> : ${(P)somevar::=$PWD}
> > > 
> > > [...]
> > > 
> > > Try after
> > > 
> > > sudo ln -s . '/*'
> > > cd '/*/*/*/*/*'
> > > setopt globsubst
> > > 
> > > ;-)
> > > 
> > > (if you're lucky, it won't crash your machine).
> > 
> > Pretty sure globsubst breaks your idea too, but fine,
> > 
> > : ${(P)~~somevar::=$PWD}
> > 
> > In fact, globsubst pretty much breaks every hook function i have :P.
> 
> [...]
> 
> eval "$somevar=\$(pwd)"
> or
> eval "$somevar=\$PWD"
> 
> are POSIX and POSIX shells have globsubst and shwordsplit on.
> The above will work regardless of the status of those options.
> 
> In POSIX shells, you need to quote variables to prevent
> globsubst or shwordsplit, even in arguments to ":" because
> globsubst can be very very resource intensive.
> 
> You can bring a machine to its knees with
> 
> : ${var=foo}
> 
> if $var is for instance /*/*/*/../../../*/*/*/../../../*/*/*
> 
> So, you should either use instead:
> 
> var=${var-foo}
> or
> 
> : "${var=foo}"
> 
> In your example, if we have to accomodate users enabling
> shwordsplit or globsubst (or running zsh in sh or ksh
> emulation), we need
> 
> : "${(P)somevar::=$PWD}"
> 
> But
> 
> eval "$somevar=\$PWD"
> 
> is more legible and more portable.

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Re: is variable with variable name possible?

[Jérémie Roquet]

2011/4/4 Stephane Chazelas <stephane_chazelas@yahoo.fr>:
> 2011-04-04 15:48:32 +0200, Jérémie Roquet:
> [...]
>> 2011/4/4 zzapper <david@tvis.co.uk>:
>> > somevar=$(pwd)
>> >
>> > but I want somevar to be a variable name
>>
>> $ eval "$somevar=$(pwd)"
> [...]
>
> No!
>
> eval "$somevar=\$(pwd)"
> or
> eval "$somevar=\$PWD"
>
>
> Or think of what would happen if $PWD was "/tmp/x;rm -rf /home"

Oh, right!

-- 
Jérémie