Expressions

Arithmetic, boolean, and bit operations

1. can use standard C binary operators for equality (= =, !=) (equals, not equal)
   • note the difference between equality (= =) and assignment (=)
2. arithmetic (+, -, *, /, %) (plus, minus, times, divide, remainder)
3. boolean (||, & & ) (or, and)
4. bit operations (|, & , ^, > > , < < ) (bit-or, bit-and, bit-xor, shift-right, shift-left)
   • syntax of > > and < < is "< int> op < var> "
5. unary operators
   • arithmetic (-, ++, --) (negate, increment, decrement)
   • boolean (!) (not)
   • bit (> > , < < ) (syntactic sugar for "1 > > < var> ", "1 < < < var> "

String Expressions

• matching a pattern,

         < string>  [ == | != ] < string>  

• tests for literal equality

         < string>  [ ~= | !~ ] < pattern> 

• tests for equality, matching wildcards in < pattern>

File-test predicates

• Are of the form -< flag> < filename> and are all boolean tests. (see page 208)

Assignment operators

• In addition, we have the assignment operators

          =           evaluate RHS and store in LHS

          +=          evaluate RHS and add to current LHS

          -=          evaluate RHS and subtract from current LHS

          *=          evaluate RHS and multiply to current LHS

          /=          evaluate RHS and use to divide current LHS

Assignment

1. Where the RHS is a constant or a variable expression, it can be assigned to a variable by "set" or "setenv", depending on the type of variable. "set" can also be used to assign a simple RHS to an element of a list.
   • Extending a list requires a form like

         set < var>  = ( $< var>  new_entries )

• where new_entries can be constants or variables, or a mix, and can include lists.
• Lists are flat; that is, cannot contain lists as elements. if you put a list in a list, the lists are instead appended.
• If the RHS is an expression, we have to use the syntax

          @ < var>  < op>  < expr> 

• where < op> is one of the assignment operators above.
• Note that assignment operators have precedence like that of equality operators, so if you use any of ( ==, !=, & , ^, |, & & , || ) in < expr> , you will have to place the expression in parens for correct evaluation.