14.3.1 Integers ( declare and let Commands) The declare Command Variables can be declared as integers with the declare “i command. If you attempt to assign any string value, bash assigns 0 to the variable. Arithmetic can be performed on variables that have been declared as integers. (If the variable has not been declared as an integer, the built-in let command allows arithmetic operations. See "The let Command" on page 873.) If you attempt to assign a floating-point number, bash reports a syntax error . Numbers can also be represented in different bases such as binary, octal , and hex . Example 14.6. 1 $ declare i num 2 $ num=hello $ echo $num 3 $ num=5 + 5 bash: +: command not found 4 $ num=5+5 $ echo $num 10 5 $ num=4*6 $ echo $num 24 6 $ num="4 * 6" $ echo $num 24 7 $ num=6.5 bash: num: 6.5: syntax error in expression (remainder of expression is ".5")
EXPLANATION -
The declare command with the “i option creates an integer variable num . -
Trying to assign the string hello to the integer variable num causes the string to be stored as 0. -
The whitespace must be quoted or removed unless the let command is used. -
The whitespace is removed and arithmetic is performed. -
Multiplication is performed and the result assigned to num . -
The whitespace is quoted so that the multiplication can be performed and to keep the shell from expanding the wildcard ( * ). -
Because the variable is set to integer, adding a fractional part causes a bash syntax error. Listing Integers The declare command with only the “i argument will list all preset integers and their values, as shown in the following display. $ declare i declare -ir EU # effective user id declare -ir PP # parent process id declare -ir U # user id
Representing and Using Different Bases Numbers can be represented in decimal (base 10, the default), octal (base 8), hexadecimal (base 16), and a range from base 2 to 36. FORMAT variable=base#number-in-that-base
Example 14.7. n=2#101 # Base is 2; number 101 is in base 2
Example 14.8. (The Command Line) 1 $ declare -i x=017 $ echo $x 15 2 $ x=2#101 $ echo $x 5 3 $ x=8#17 $ echo $x 15 4 $ x=16#b $ echo $x 11
EXPLANATION -
The declare function is used to assign an integer variable x the octal value 017 . Octal numbers must start with a leading 0. 15 , the decimal value of 017 , is printed. -
The variable x is assigned the value of 101 (binary). 2 represents the base, separated by a # , and the number in that base, 101 . The value of x is printed as decimal, 5 . -
The variable x is assigned the value of 17 (octal). The value of x is printed as decimal, 15 . -
The variable x is assigned the value of b (hexadecimal). The value of x is decimal 11 . The let Command The let command is a bash built-in command that is used to perform integer arithmetic and numeric expression testing. To see what let operators your version of bash supports, type at the prompt: help let
A list of the let operators is also found in Table 14.4 on page 884. Example 14.9. (The Command Line) 1 $ i=5 or let i=5 2 $ let i=i+1 $ echo $i 6 3 $ let "i = i + 2" $ echo $i 8 4 $ let "i+=1" $ echo $i 9 5 $ i=3 6 $ ((i+=4)) $ echo $i 7 7 $ ((i=i-2)) $ echo $i 5
Table 14.4. The let Command Operators [a] | Operator | Meaning | | “ | Unary minus | | + | Unary plus | | ! | Logical NOT | | ~ | Bitwise NOT (negation) | | * | Multiply | | / | Divide | | % | Remainder | | + | Add | | “ | Subtract | | let Operators Not Implemented Prior to bash 2.x | | << | Bitwise left shift | | >> | Bitwise right shift | | <= >= < > | Comparison operators | | == != | Equal to and not equal to | | & | Bitwise AND | | ^ | Bitwise exclusive OR | | | Bitwise OR | | && | Logical AND | | | Logical OR | | = *= /= %= += “= <<= >>= &= ^= = | Assignment and shortcut assignment |
[a] See Example 14.18. EXPLANATION -
The variable i is assigned the value 5 . -
The let command will add 1 to the value of i . The $ (dollar sign) is not required for variable substitution when performing arithmetic. -
The quotes are needed if the arguments contain whitespace. -
The shortcut operator, += , is used to add 1 to the value of i . -
The variable i is assigned the value 3. -
The double parentheses can be used to replace let . [a] 4 is added and assigned to i . [a] Implemented in versions of bash 2.x. -
2 is subtracted from i . We could have also written i “=2 14.3.2 Floating-Point Arithmetic Bash supports only integer arithmetic, but the bc , awk , and nawk utilities are useful if you need to perform more complex calculations. Example 14.10. (The Command Line) 1 $ n=`echo "scale=3; 13 / 2" bc` $ echo $n 6.500 2 product=`gawk -v x=2.45 -v y=3.123 'BEGIN{printf "%.2f\n",x*y}'` $ echo $product 7.65
EXPLANATION -
The output of the echo command is piped to the bc program. The scale is set to 3 , which is the number of significant digits to the right of the decimal point that will be printed. The calculation is to divide 13 by 2 . The entire pipeline is enclosed in backquotes. Command substitution will be performed and the output assigned to the variable n . -
The gawk program gets its values from the argument list passed in at the command line, x=2.45 y=3.123 . After the numbers are multiplied, the printf function formats and prints the result with a precision of two places to the right of the decimal point. The output is assigned to the variable product . |
