Re: [Help-bash] Conditional expression evaluation using [ and [[ with -eq operator

[Chet Ramey]

On 11/15/13 12:41 PM, Peggy Russell wrote:
> I saw the snippet below the other day. Validating data I would use a regular
> expression or pattern in a 'if [[' or 'case', and not the below.
> 
> But in this case, what is it about the command line processing steps for
> [ and [[, that the shell builtin test command, [, appears to have determined
> this was an integer expression, but the shell keyword compound command, [[,
> did not?

Builtin commands undergo word expansion before being executed.  In this
csae, that means the words are expanded before being parsed as a
conditional expression and evaluated.  A command such as [[, which is part
of the shell grammar, does not perform word expansion before the expression
is parsed.  That means that the operators in [[ are essentially tokens and
must be present literally in order to be recognized by the shell parser.

It's the difference between `for f in x' and `$for f in x'.  Even if the
shell variable `for' had the value `for', would you expect that to work?

Chet
-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/