Variables can be assigned within arithmetic expressions.

Notation	Description
parameter = value	Simple assignment. Assigns value to parameter.
paramater += value	parameter = parameter + value
parameter -= value	parameter = parameter - value
parameter *= value	parameter = parameter * value
parameter /= value	parameter = parameter / value
parameter %= value	parameter = parameter % value
parameter++	Variable post-increment.
	parameter = parameter + 1
parameter--	Variable post-decrement.
	parameter = parameter - 1
++parameter	Variable pre-increment.
	parameter = parameter + 1
--parameter	Variable pre-decrement.
	/parameter = paramter - 1

Post-increment/decrement and Pre-increment/decrement

Post-increment and pre-increment (decrement) borrowed from C programming language. If pre the parameter is incremented/decremented before the parameter is returned. If post the operation is performed after the parameter is returned.

# post-incremented
[me@linuxbox ~]$ foo=1
[me@linuxbox ~]$ echo $((foo++))
1
[me@linuxbox ~]$ echo $foo
2
# pre-incremented
[me@linuxbox ~]$ foo=1
[me@linuxbox ~]$ echo $((++foo))
2
[me@linuxbox ~]$ echo $foo
2