I was asked to take a look at this and comment on what the standards say about this ksh behavior. The standards leave unspecified what happens if both operands of the test -eq, -ne, -lt, -le, -gt, and -ge binary primaries are not integer operands.

The standards allow, but do not require the test expression and [ expression ] utilities to be implemented as shell built-ins (as they are in bash and ksh)). If test and [ ... ] aren't implemented as shell built-ins, there is no way that the behavior shown here would be possible. In ksh (at least ksh on AIX and on OS X), if a variable name is given where a number is needed in these cases, the value of the variable is substituted into the expression even when the $ is not present.

The standards specify this behavior for arithmetic expansions. That is, the sequences:

Code:

n=0
while [ $n -lt 5 ]
do      printf "%d\n" $(($n++))
done

and:

Code:

n=0
while [ $n -lt 5 ]
do      printf "%d\n" $((n++))
done

are required to do the same thing. So, it seems like a logical extension to me to not require the $ in the expression in these test exp ([ exp ]) expressions either.

Obviously, this need not be portable to other shells and might not even be portable to other versions of ksh.