[TAG] need some help

[Ben Okopnik]

On Tue, Jul 10, 2007 at 12:34:05PM +0530, Nayanam,Sarsij wrote:
>    I am writing a shell script to run on MC/SG cluster , and I am facing an
>    issue as mentioned below:
> 
>    if we have a package with a dash in the name say sgpkg-cust :
> 
>    # PKGsgpkg-cust=hello
>    sh: PKGsgpkg-cust=hello:  not found.
>    # PKGsgpkg=hello
>    # echo $PKGsgpkg-cust
>    hello-cust

This is not surprising; a dash is not a valid character in a variable
for Bourne-derived shells.
 
>    I have a function get_package_fqdn which starts as below:
> 
>        37  get_package_fqdn()
>        38  {
>        39      eval var=$`echo PKG$1`
>        40      if [[ -z $var ]]; then
>                [...]
>        64      fi
>        65  }
> 
>    we will notice that if $1=sgpkg-cust, var will be equal to "-cust" and the
>    rest of the funcion "if[[ -z $var ]];" will not be used and nothing will
>    be introduced in the PKG$packagename variable.

This isn't shell-specific, but an excellent Perl programmer named
Mark-Jason Dominus has a writeup called "Why it's stupid to use a
variable as a variable name" (http://perl.plover.com/varvarname.html).
The above problem is explicitly cited. In short: since variable names
are restricted to a specific set of characters, and the set of
characters that could be contained in your '$1' is essentially
arbitrary, you're creating a problem when you do that. So don't do that.

In Perl, the answer is "use a hash". In Bash, well, you need to rethink
what it is that you're trying to do and use different functionality.  As
a general approach, you could try "flattening" that arbitrary character
set - be sure to do do in both populating _and_ reading the strings:

```
package_name=$(echo -n "PKG$1"|tr -c 'a-zA-Z0-9_' '_')
'''

``
ben at Tyr:~$ var=$(echo -n "xyzabc@$%^&*()_++sadbjkfjdf" | tr -c 'a-zA-Z0-9_' '_')
ben at Tyr:~$ echo $var
xyzabc____________sadbjkfjdf
''

Do note that this will _still_ break if a "special" Bash character
(e.g., '!') appears in the string. Overall, you just need to rethink
your approach to this problem. MJD is right: it's a bad idea to use a
variable as a variable name.


-- 
* Ben Okopnik * Editor-in-Chief, Linux Gazette * http://LinuxGazette.NET *