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11.10 Parentheses in Shell Scripts
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Beware of two opening parentheses in a row, as many shell
implementations treat them specially, and Posix says that a portable
script cannot use ‘((’ outside the ‘$((’ form used for shell arithmetic.
In traditional shells, ‘((cat))’ behaves like ‘(cat)’; but many shells,
including Bash and the Korn shell, treat ‘((cat))’ as an arithmetic
expression equivalent to ‘let "cat"’, and may or may not report an error
when they detect that ‘cat’ is not a number.  As another example,
‘pdksh’ 5.2.14 does not treat the following code as a traditional shell
would:

     if ((true) || false); then
       echo ok
     fi

To work around this problem, insert a space between the two opening
parentheses.  There is a similar problem and workaround with ‘$((’; see
*note Shell Substitutions::.