how to add floating number in shell?

Question

a=-1.4
b=42273.85
awk "BEGIN {print ($a + $b)}"

42272.4

I am expecting result as 42272.45, what is wrong here?

Answer 1

You didn't specify a precision so awk picked one for you (%.6g, the default for CONVFMT, see https://www.gnu.org/software/gawk/manual/gawk.html#Built_002din-Variables):

a=-1.4
b=42273.85
awk -v a="$a" -v b="$b" 'BEGIN {printf "%.2f\n", (a + b)}'
42272.45

I'm also correcting your use of shell variables in an awk script above, see How do I use shell variables in an awk script?.

Answer 2

One solution to do floating point math in sh, bash, zsh and more is using bc

echo "$a + $b" | bc
42272.45

Using awk you can override the internal default output format by using printf

awk -v a="$a" -v b="$b" 'BEGIN{printf("%.2f\n", a + b)}' 
42272.45

Answer 3

You might either use printf or set OFMT variable, which will affect print as follows

a=-1.4
b=42273.85
awk -v a=$a -v b=$b 'BEGIN{OFMT="%.2f";print a + b}'

gives output

42272.45

Explanation: OFMT is supposed to be floating-point conversion specification which print uses when it encounters number, default is %.6g. Observe that OFMT is handy when you have multiple prints which are supposed to output numbers in uniform way.