10.8 Parentheses in Shell Scripts

Beware of two opening parentheses in a row, as many shell implementations treat them specially. Posix requires that the command `((cat))' must behave 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 misparses the following code:

     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 Shell Substitutions.