Command Substitution
A command enclosed in parentheses preceded by a dollar sign, like so:
$(...) or quoted with grave accents: `...` is replaced with its
standard output. If the substitution is not enclosed in double quotes,
the output is broken into words using the IFS parameter. The
substitution $(cat foo) may be replaced by the
equivalent but faster $(<foo). In either case, if the option
GLOB_SUBST is set the
output is eligible for filename generation.
Arithmetic Expansion
A string of the form $[exp] is substituted with the value of
the arithmetic expression exp. See
ARITHMETIC EVALUATION below.
Brace Expansion
A string of the form foo{xx,yy,zz}bar is expanded to the
individual words fooxxbar, fooyybar, and
foozzbar. Left-to-right order is preserved. This construct may
be nested. Malformed brace expansion expressions, including
expressions without a comma, are left unchanged by the shell.
An expression of the form {x-y}, where x and y are single characters, is expanded to every character between x and y, inclusive.