As far as the bash shell is concerned, you have:

1. The [ and the test builtin commands. In this context (after the [ and the test command) parameter expansion, word splitting and pathname expansion are performed[1]. So beware of the following situation when using the [ and the test commands:

- missing quotes
-- unset or null values:

Code:

$ unset var
$ [ $var -eq 1 ]
bash: [: -eq: unary operator expected

-- values containing IFS characters:

Code:

$ var="a b"
$ [ $var = "a b" ]
bash: [: too many arguments

-- pathname expansion:

Code:

$ touch x
$ var=\*
$ set -x
$ [ $var = "*" ]
+ '[' x = '*' ']'

- particular syntax for logical comparison:

Code:

$ [ value = value -a 1 -lt 2 ]&&echo true
true

2. The [[ non standard (ksh93, zsh and bash) compound command. Parameter expansion is performed inside [[ ]].
No word splitting and pathname expansion are performed[2]. Support more operators.
Consider the following:

- no need to quote unset variables or variables with null values or values containing IFS characters:

Code:

$ unset var
$ set -x              
$ [[ $var -eq value ]]
+ [[ '' -eq value ]]

Code:

$ var=a\ b
$ set -x
[[ $var == "a b" ]]
+ [[ a b == \a\ \b ]]

- new versions of bash support new operators:

Code:

$ var=abc              
[[ $var =~ ^a ]]&&echo OK
OK
$ echo "${BASH_REMATCH[@]}"
a

- readable operators for logical comparison:

Code:

$ [[ value == value && 2 < 1 ]]||echo false
false

3. The compound command ((expression)). From the manual pages for bash[3]:

Code:

$ unset a
$ ((++a))&&echo $a # version >= 3
1

And as far as your particular problem is concerned it seems like a limitation of the old version of bash you're using,
your code works with recent versions of the bash shell.

[1] For more details see the manual pages for bash, the section Shell Builtin Commands and CONDITIONAL EXPRESSIONS.
[2] For more details see the manual pages for bash, the section Compound Commands. Tilde expansion, arithmetic expansion, command substitution, process substitution, and quote removal are also performed.
[3] For more information see:

Code:

man bash|less -p^ARITHMETIC\ EVALUATION

Hope this helps.