13. Arithmetic Expansion

Arithmetic expansion provides a powerful tool for performing (integer) arithmetic operations in scripts. Translating a string into a numerical expression is relatively straightforward using backticks, double parentheses, or let.

Variations

Arithmetic expansion with  backticks  (often used in conjunction with expr) 

z=`expr $z + 3`          # The 'expr' command performs the expansion.

Arithmetic expansion with  double parentheses   , and using   let

The use of backticks (backquotes) in arithmetic expansion has been superseded by double parentheses  -- ((...))  and $((...)) -- and also by  the very convenient let construction.

z=$(($z+3))
z=$((z+3))                                  #  Also correct.
                                            #  Within double parentheses,
                                            #+ parameter dereferencing
                                            #+ is optional.

# $((EXPRESSION)) is arithmetic expansion.  #  Not to be confused with
                                            #+ command substitution.



# You may also use operations within double parentheses without assignment.

n=0
echo "n = $n"                               # n = 0

  (( n += 1 ))                              # Increment.
# (( $n += 1 )) is incorrect!
echo "n = $n"                               # n = 1


let z=z+3
let "z += 3"  #  Quotes permit the use of spaces in variable assignment.
              #  The 'let' operator actually performs arithmetic evaluation,
              #+ rather than expansion.

Examples of arithmetic expansion in scripts:

1. Example 16-9
    
2. Example 11-14
    
3. Example 27-1
    
4. Example 27-11
    
5. Example A-16