substring extraction

substring extraction

[Com MN PG P E B Consultant 3]

bash has the nice feature to extract a substring from a shell variable.
For example

   # bash example
   var=abcd
   echo ${var:1:2} # Start at offset 1, returns 2 characters

displays 

   bc

Is there an easy way to achieve a similar effect in zsh, without
reverting to external
tools such as awk or sed?

Ronald
 

Re: substring extraction

[Stephane Chazelas]

On Mon, May 08, 2006 at 09:32:50AM +0200, Com MN PG P E B Consultant 3 wrote:
> bash has the nice feature to extract a substring from a shell variable.
> For example
> 
>    # bash example
>    var=abcd
>    echo ${var:1:2} # Start at offset 1, returns 2 characters
> 
> displays 
> 
>    bc

That's a ksh feature borrowed by bash. And as of many ksh
features, it's got its flaws. That one conflicts with the
${var:-default-value} Bourne operator, so that you need to do
${var: -1} (add a space) to get the last char.

> Is there an easy way to achieve a similar effect in zsh, without
> reverting to external
> tools such as awk or sed?
[...]

In zsh:

$var[2,3]

zsh can do it because arrays and scalar variables are two
different types. In zsh, $var[2,5] gives the 2nd to 5th element
when $var is of array type, and 2nd to 5th char when $var is
scalar. In ksh/bash all variables are arrays and $var is a
shortcut for ${var[0]} (though it's not completely true of
bash).

-- 
Stéphane

Re: substring extraction

[Peter Stephenson]

"Stephane Chazelas" <Stephane_Chazelas@yahoo.fr> wrote:
> In zsh:
>
> $var[2,3]
>
> zsh can do it because arrays and scalar variables are two
> different types. In zsh, $var[2,5] gives the 2nd to 5th element
> when $var is of array type, and 2nd to 5th char when $var is
> scalar. In ksh/bash all variables are arrays and $var is a
> shortcut for ${var[0]} (though it's not completely true of
> bash).

Just to point out you can combine the two in the same expression in zsh.
As soon as you've extracted a single element, later suffixes work on
characters.  (One day this may be multibyte characters but that's not done
yet.)

% array=(one two three four)
% print ${array[3][2,-2]}
hre

Since the rule is applied after every subscript match, you can do bizarre
stuff like:

% print ${array[1,3][-1][2,-2][2]}
r

which works in the following steps:

array  one two three four -> [1,3]
array  one two three -> [-1]
scalar three -> [2,-2]
scalar hre -> [2]
scalar r

--
Peter Stephenson <pws@csr.com>                  Software Engineer
CSR PLC, Churchill House, Cambridge Business Park, Cowley Road
Cambridge, CB4 0WZ, UK                          Tel: +44 (0)1223 692070


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