How to format a floating point for display based on locale environment variables?

Question

Lets say I have several floating point numbers to print in a Bash script. But I want the floating points numbers displayed accordingly to the LC_NUMERIC locale environment variable.

#!/usr/bin/env bash

# For consistent/reproducible error messages in this sample code
LANGUAGE=C

# The speed of light in vacum in m.s
declare -r const_C=299792458

# Declare separately when assigning command output
declare -- const_pi

# π is 4 × arc-tangent of 1, using bc calculator with math library
typeset -r const_pi="$(bc --mathlib <<<'scale=20; 4*a(1)')"

# Do it in US's English
LC_NUMERIC=en_US.utf8
printf 'LC_NUMERIC=%s\n' "${LC_NUMERIC}"
printf 'Speed of light in vacuum is:\nC=%.f m/s\n\nπ=%.10f\n' \
  "${const_C}" \
  "${const_pi}"

echo $'\n'

# Do it in France's French
# it fails because floating point format
# changes for printf parameters
LC_NUMERIC=fr_FR.utf8
printf 'LC_NUMERIC=%s\n' "${LC_NUMERIC}"
printf 'La vitesse de la lumière dans le vide est :\nC=%.f m/s\n\nπ≈%.10f\n' \
  "${const_C}" \
  "${const_pi}"

Actual output:

LC_NUMERIC=en_US.utf8
Speed of light in vacuum is:
C=299792458 m/s

π=3.1415926536


LC_NUMERIC=fr_FR.utf8
La vitesse de la lumière dans le vide est :
C=299792458 m/s

a.sh: line 29: printf: 3.14159265358979323844: invalid number
π≈3,0000000000

This is a perfectly expected result because printf %f format expects the argument be formatted according to LC_NUMERIC.

Then how do you display arbitrary floating-point numbers that are stored in POSIX or bc's format but having display reflect the settings of LC_NUMERIC?

What if I want the French part of the code, with the following output?

Expected output for French:

La vitesse de la lumière dans le vide est :
C=299792458 m/s

π≈3,1415926536

Answer 1

This is a problem with Bash's own built in printf command. The standalone printf works OK.

LC_NUMERIC=fr_FR.UTF8     printf 'Bad  : %f\n' 3.14
env LC_NUMERIC=fr_FR.UTF8 printf 'Good : %f\n' 3.14

Output

script.sh: line 4: printf: 3.14: invalid number
Bad  : 0,000000
Good : 3,140000