Very Computer

Hello,

Consider the following short example:

$ a="foo"
$ b="a"
$ echo $b
a
$ echo $$b
9540b

I was expecting "foo". Any ideas on how I could get the expected result?

Thanks,

Neil

Quote:> Consider the following short example:

> $ a="foo"
> $ b="a"
> $ echo $b
> a

foo

foo

Try

% eval echo '$'$b

The shell is interpreting $$ as the process ID.

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echo ${!b}

man bash
man ksh

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Linux solution for data processing and document management.

2004-04-4, 06:42(+00), William Park:

Only newer versions of bash. Not with ksh. With ksh93, you can
use name references:

typeset -n b=a
echo $b

${!b} would then expand to "a", not to the content (splitted and
expanded) of $a.

With zsh:

echo ${(P)a}

eval "echo \"\$$a\""
works with every shell.

--
Stphane                      ["Stephane.Chazelas" at "free.fr"]

In case it was not obvious, I meant to ask about how to do it without changing
the first two lines. Otherwise the answer is quite simple...

Quote:> $ eval echo '$'$b

That works. I guess that's what eval is for!?

That works too.

BTW, which of these two constructs also work in traditional System V sh shell?
It seems like there are two ways of doing the same thing. One of them ought to
be more portable across shells and hence preferable. Which one?

Thanks,

Neil

2004-04-4, 04:29(-07), Neil Zanella:
[...]

Quote:>> $ eval echo '$'$b

> That works. I guess that's what eval is for!?

eval concatenates its arguments with spaces and has the shell
evaluate that string as shell code.

Here, it takes "echo" and "$" followed by as many arguments as
$b expands to (most likely "a" here unless you modified IFS,
putting $b insite quotes would have been more correct).

The result is "echo $a". Then the shell evaluates that string.
(runs echo with the list of arguments resulting from the
expansion of $a).

Quote:>> echo ${!b}

> That works too.

> BTW, which of these two constructs also work in traditional System V sh shell?
> It seems like there are two ways of doing the same thing. One of them ought to
> be more portable across shells and hence preferable. Which one?

${!var} is bash specific. It's a syntax error in System V sh or
ksh or ash or zsh and has a different meaning in ksh93.

eval was already in Unix V7 Bourne shell (1975). There's also
one in csh.

--
Stphane                      ["Stephane.Chazelas" at "free.fr"]