Calculations in Bash
Doing calculations in Bash, is full of surprises. Like so many things in Bash or on Linux
What I am usually looking for:
$ echo $((1+1)) 2
Simple calculations - The problem
$ 1+1 bash: 1+1: command not found $ 1 + 1 bash: 1: command not found $ echo 1+1 1+1 $ echo (1+1) bash: syntax error near unexpected token `1+1' $ echo $(1+1) bash: 1+1: command not found $ echo $(1 + 1) bash: 1: command not found $(1+1) bash: 1+1: command not found $ expr 1+1 1+1 $ echo $((1+1)) 2 $ echo `expr 1 + 1` 2
Calculation + echo
Zie deze post voor diverse mogelijkheden. Het gaat mij hier om het retourneren van de uitkomst van een bewerking via echo. Dat is van invloed op de mogelijkheden. Wat basically lijkt te werken:
echo $((1+1)) 2 echo `expr 1 + 1` 2
De eerste formulering met behulp van parenthesis command substitution is waarschijnlijk te prefereren: Command substitution (Bash).
Samengestelde berekeningen
$ echo $((12+1)) 13 $ echo $((12+1)*2) -bash: command substitution: line 1: syntax error near unexpected token `*2' -bash: command substitution: line 1: `(12+1)*2' $ echo $(((12+1)*2)) 26 $ a=12; b=1; echo $((a+b)) 13 $ a=12; b=1; c=2; echo $(((a+b)*c)) 26 $ a=12; b=1; c=2; echo $(((a+b)*c+1)) 27
Bestaande variabelen aanpassen
Bv.:
number_of_sites=29 echo $number_of_sites number_of_sites=$((number_of_sites-2)) echo $number_of_sites 29 27
Floating point arithmatic
Surprise: Bash can only handle integers!
Example: Conversion from mm to inch:
$ a=12; echo $((a*0.03937008)) bash: a*0.03937008: syntax error: invalid arithmetic operator (error token is ".03937008")
$ a=12; b=0.03937008; echo $a $b $((a*b)) bash: 0.03937008: syntax error: invalid arithmetic operator (error token is ".03937008")
However, there are lots of command line tools to take this job from Bash [1], with bc being a commonly named tool, that seems to be part of a default Linux installation. I suspect it's name is an abbreviation of Bash Calculator.
See bc for details