3.13. Arithmetic Expansion

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

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

1 z=`expr $z + 3` # 'expr' does the expansion.

Arithmetic expansion with double parentheses, and using let

The use of backticks in arithmetic expansion has been superseded by double parentheses $((...))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

1 z=$(($z+3)) 2 # $((EXPRESSION)) is arithmetic expansion. # Not to be confused with 3  # command substitution. 4  5 let z=z+3 6 let "z += 3" #If quotes, then spaces and special operators allowed. 7 # 'let' is actually arithmetic evaluation, rather than expansion.

All the above are equivalent. You may use whichever one "rings your chimes".

Examples of arithmetic expansion in scripts:

1. Example 3-88

2. Example 3-54

3. Example 3-149

4. Example 3-152

5. Example A-11