Linux in a Nutshell, Third Edition

Expressions

Expressions are used in @, if, and while statements to perform arithmetic, string comparisons, file testing, and so on. exit and set also specify expressions, as can the tcsh built-in command filetest. Expressions are formed by combining variables and constants with operators that resemble those in the C programming language. Operator precedence is the same as in C but can be remembered as follows:

1. * / %

2. + -

Group all other expressions inside parentheses. Parentheses are required if the expression contains <, >, &, or |.

Operators

Operators can be one of the following types:

Assignment operators

Operator	Description
=	Assign value.
+= -=	Reassign after addition/subtraction.
*= /= %=	Reassign after multiplication/division/remainder.
&= ^= |=	Reassign after bitwise AND/XOR/OR.
++	Increment.
--	Decrement.

Arithmetic operators

Operator	Description
* / %	Multiplication; integer division; modulus (remainder)
+ -	Addition; subtraction

Bitwise and logical operators

Operator	Description
~	Binary inversion (one’s complement).
!	Logical negation.
<< >>	Bitwise left shift; bitwise right shift.
&	Bitwise AND.
^	Bitwise exclusive OR.
|	Bitwise OR.
&&	Logical AND.
||	Logical OR.
{ command }	Return 1 if command is successful, 0 otherwise. Note that this is the opposite of command’s normal return code. The $status variable may be more practical.

Comparison operators

Operator	Description
== !=	Equality; inequality
<= >=	Less than or equal to; greater than or equal to
< >	Less than; greater than ...