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From: David Combs on 6 Dec 2009 01:39
In article <behtt6-qvg.ln1(a)marcumbill.bellsouth.net>,
Bill Marcum <marcumbill(a)bellsouth.net> wrote:
>On 2009-11-24, jhogan <jessehogan1976(a)gmail.com> wrote:
>> This has probably been asked before but I can't seem to find this
>> anywhere.
>>
>> $ echo $t
>> + echo -Q '"SELECT' curriculum_last_update FROM
>> 'application_settings"'
>> -Q "SELECT curriculum_last_update FROM application_settings"
>>
>> Notice the single-quote at the end of SELECT and at the beginning of
>> application_settings. Why does that happen?
>>
>The echo command is treating each word as a separate argument. You
>probably want:
>eval echo $t
>
>> # Make a call to sqlcmd. sqlcmd is a command line interface to sql
>> server. $t now contains the SQL
>> # that I would like to run, preceded by -Q which means "query"
>> $ sqlcmd -b -x -d mydb -S myserver $t
> $ eval sqlcmd -b -x -d mydb -S myserver $t

Interesting -- assuming I understood itg.

Could you elaborate a bit, on why you need the eval, what it
does for you?

THANKS!


---- SECONDARY question:

PS: Whenever I see a use of eval on groups like this, everyone
starts screaming about how DANGEROUS that is.

So, in this case, what is it that might bite you?

Thanks,

David


From: Kaz Kylheku on 6 Dec 2009 12:45
On 2009-12-06, David Combs <dkcombs(a)panix.com> wrote:
> In article <behtt6-qvg.ln1(a)marcumbill.bellsouth.net>,
> Bill Marcum <marcumbill(a)bellsouth.net> wrote:
>>On 2009-11-24, jhogan <jessehogan1976(a)gmail.com> wrote:
>>> This has probably been asked before but I can't seem to find this
>>> anywhere.
>>>
>>> $ echo $t
>>> + echo -Q '"SELECT' curriculum_last_update FROM
>>> 'application_settings"'
>>> -Q "SELECT curriculum_last_update FROM application_settings"
>>>
>>> Notice the single-quote at the end of SELECT and at the beginning of
>>> application_settings. Why does that happen?
>>>
>>The echo command is treating each word as a separate argument. You
>>probably want:
>>eval echo $t
>>
>>> # Make a call to sqlcmd. sqlcmd is a command line interface to sql
>>> server. $t now contains the SQL
>>> # that I would like to run, preceded by -Q which means "query"
>>> $ sqlcmd -b -x -d mydb -S myserver $t
>> $ eval sqlcmd -b -x -d mydb -S myserver $t
>
> Interesting -- assuming I understood itg.
>
> Could you elaborate a bit, on why you need the eval, what it
> does for you?

Why do we need eval? Because we have a variable t which conatins
shell syntax!

t="-Q \"SELECT ....\""

This t is a fragment of a shell command line which needs to be subject
to word splitting, etc, just as if it were typed into the shell.

Eval is the shell's interface to its function that processes shell
syntax, right from the character level. eval is needed here to reduce a
meta-syntactic variable that contains code, into the corresponding code.

It's more correct to protect this variable with quotes when passing it
to eval. The best is to protect the entire syntax:

eval "sqlcmd -b -x -d mydb -S myserver $t"

I.e. the thing between the quotes is the piece of shell syntax we want
to pass to eval; our $t piece is substituted into this one big string.

If we just to this, it is wrong:

eval sqlcmd -b -x -d mydb -S myserver $t

At the very least we want to protect the variable. Now
eval gets "sqlcmd", "-b", ... "myserver" as separate arguments,
which are followed by the contents of t as one big argument.

eval sqlcmd -b -x -d mydb -S myserver "$t"

The contents of our variable t must not be subject to splitting prior to
being passed to eval. You see t could for instance contain significant
spaces:

t="arg1 arg2 \"arg with significant double spaces\""

These significant spaces will be lost if we do an unquoted eval.

# with above t

$ eval echo $t # oops, bug, where did our spaces go?
arg1 arg2 arg with significant double spaces

$ eval echo "$t" # aha!
arg1 arg2 arg with significant double spaces

As you can see, quoting preserves the syntax better through eval, so
that the ultimate command (in this case echo) gets the correct data.

> PS: Whenever I see a use of eval on groups like this, everyone
> starts screaming about how DANGEROUS that is.
>
> So, in this case, what is it that might bite you?

Well, see above: quoting can bite you. Unwanted evaluations can bite
you. Escapes can bite you.

Suppose your string contains a dollar sign followed by a word.
Under eval, that becomes a variable evaluation! In addition to taking
care of quoting, you must escape all characters that are special syntax
to the shell, unless you intend them to be treated that way.

Backslashes are super awkward, because they have a maening in the
syntax which are using to specify the syntax, as well as in the
syntax that will be valuated, and their interpretetation depends
on what kind of quotes you are in.

For instance if you write "\\" to the shell, that's actually a word
consiting of a single backslash. But '\\' codes for two backslashes.

If you're using single quotes to prepare synax for eval, and you want to
encode a single backslash, you sometimes have to type four backslashes!

# set up the shell syntax
$ shell_syntax="this is a backslash: \"\\\\\""

# now /show/ me my shell syntax
$ echo $shell_syntax
this is a backslash: "\\"

# now /interpret/ my shell syntax, as arguments to echo:
$ eval echo "$shell_syntax"
this is a backslash: \

The problem with multiple evaluation levels in the same code
is the close similarity between an evaluation level and its
metalevel. Meta-syntax is like shell syntax, but not exactly,
due to the extra escaping, which, to make things more
complicated, is selectively applied: some things you want
evaluated in this round, some in the next round, some in
the third round ...
From: David Combs on 19 Dec 2009 20:14
In article <20091206091923.418(a)gmail.com>,
Kaz Kylheku <kkylheku(a)gmail.com> wrote:
>On 2009-12-06, David Combs <dkcombs(a)panix.com> wrote:


>>
>> Interesting -- assuming I understood itg.
>>
>> Could you elaborate a bit, on why you need the eval, what it
>> does for you?
>
>Why do we need eval? Because we have a variable t which conatins
>shell syntax!
>
> t="-Q \"SELECT ....\""
>
>This t is a fragment of a shell command line which needs to be subject
>to word splitting, etc, just as if it were typed into the shell.
>
>Eval is the shell's interface to its function that processes shell
>syntax, right from the character level. eval is needed here to reduce a
>meta-syntactic variable that contains code, into the corresponding code.
>
>It's more correct to protect this variable with quotes when passing it
....
....


THANKS!

Now I go print it out, put on my thinking cap (with lots of
caffein), and study it.

May take a day or two before I get back to the group --
especially with 6" forecasted for tonight and 12" for
tomorrow! (snow, shovel, snow-plows shoving it right
back into the driveway!)

David


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