Dynamic precision in awk printf using shell variable

awk's printf, like C's, lets you use a * instead of a hardcoded width or precision number. The value is instead taken from the next argument to the function. So instead of messing with shell substitution into the awk script, you can just pass the precision as another variable:

$ result=2050
$ prec=2
$ awk -v res="$result" -v p="$prec" 'BEGIN { printf "%.*f\n", p, res/1000 }'
2.05

However, if you're going to do a lot with floating point numbers, I'd suggest moving away from bash to another shell like zsh or ksh that supports floating-point math natively, or to a non-shell scripting language (perl, tcl, ruby, etc.). Tends to be more efficient than having to run external programs to do basic things like division.

I suggest:

result="123456"
var=2            # decimal places

awk -v res="$result" -v dp="$var" 'BEGIN {printf "%." dp "f", res / 1000}'

or

awk -v res="$result" -v CONVFMT="%.${var}f" 'BEGIN {printf res / 1000}'

Output:

123.46

From man awk:

CONVFMT: The conversion format for numbers, "%.6g", by default.

If available/acceptable, using a pure bash solution with version 5.3 with the loadable builtin fltexpr and the new form of Command Substitution , and bash builtin printf with the -v option/flag

#!/usr/bin/env bash

enable fltexpr || exit

var=2
result=12345

LC_NUMERIC=C printf -v float '%.*f' "$var" "${ fltexpr -p "$result / 1000"; }"

echo "$float"

Or

#!/usr/bin/env bash

enable fltexpr || exit

var=2
result=12345

echo "${| LC_NUMERIC=C printf -v REPLY '%.*f' "$var" "${ fltexpr -p "$result / 1000"; }"; }"

Both should print/output

12.35

According to help fltexpr

fltexpr: fltexpr [-p] expression
    Evaluate floating-point arithmetic expression.

    Evaluate EXPRESSION as a floating-point arithmetic expression and,
    if the -p option is supplied, print the value to the standard output.

    Exit Status:
    If the EXPRESSION evaluates to 0, the return status is 1; 0 otherwise.

According to help printf

printf: printf [-v var] format [arguments]
    Formats and prints ARGUMENTS under control of the FORMAT.
    
    Options:
      -v var    assign the output to shell variable VAR rather than
                display it on the standard output
    
    FORMAT is a character string which contains three types of objects: plain
    characters, which are simply copied to standard output; character escape
    sequences, which are converted and copied to the standard output; and
    format specifications, each of which causes printing of the next successive
    argument.
    
    In addition to the standard format characters csndiouxXeEfFgGaA described
    in printf(3), printf interprets:
    
      %b        expand backslash escape sequences in the corresponding argument
      %q        quote the argument in a way that can be reused as shell input
      %Q        like %q, but apply any precision to the unquoted argument before
                quoting
      %(fmt)T   output the date-time string resulting from using FMT as a format
                string for strftime(3)
    
    The format is re-used as necessary to consume all of the arguments.  If
    there are fewer arguments than the format requires,  extra format
    specifications behave as if a zero value or null string, as appropriate,
    had been supplied.
    
    Exit Status:
    Returns success unless an invalid option is given or a write or assignment
    error occurs.

As per Léa Gris , added the LC_NUMERIC=C since it is affected by the systems locale