If I have a variable, say xvar, which can take both string and integer value and I want to perform an operation in following 2 conditions:
1. Either xvar is null
2. xvar equals 2

[ -z "$xvar" ] || [ $xvar -eq 2 ] && <some-code>

Doesn't seem to work if xvar takes string values

I know that since I have no restriction on xvar, I can get away with string comparison in second test too, ie
[ -z "$xvar" ] || [ "$xvar" = "2" ] && <some-code>

But, 'Sams teach yourself shell script in 24 hrs' says that [ expr1 -eq expr2 ], if either is string, it assumes 0 value

Is it true ??

Can I get it working with 'integer' comparison ??

1. Yes, "-eq $some_alpha_value" will equate to 0.

2. Yes, this makes it impossible to distinguish between "zero" and "not-a-number"

3. Reasonable solution:

1 members found this post helpful.

Or you could use the correct enclosures for arithmetic in bash:

1 members found this post helpful.

Thanks grail for the suggestion, i'll use that
Though, I am pretty confused abt the double enclosure stuff

Can you point me to some good links to understand these
I read that this is new and inbuilt in bash but am not able to figure out the exact difference

@ moose

If I do this, this won't equate to 0 I guess
[ 2 -eq "u" ]
bash: [: u: integer expression expected

Then is it correct to say that "-eq $some_alpha_value" will equate to 0

But this seems to work, [[ 2 -eq u ]], is this what was meant ??

[ is synonymous with test
[[ is basically an extended version of [ and generally handles a little better with unset variables and the like
(()) is used for arithmetic comparisons and calculations

This will increase the x variable integer value by 1

Have a look here for some info.

A simpler bash idiom to determine whether a variable contains an integer isThe pattern can be translated as "contains only one or more digits 0-9".

EDIT: not only simpler but faster running -- significant if it is repeated many times.

1 members found this post helpful.