File descriptor demo; bash; How to isolate and collect error messages for particular from function calls, across subshells etc.

fd2.sh

#!/bin/bash

# Create file to accumulate errors
errfile=$(mktemp)
echo "Error File is ${errfile}"

# Have fd 3 point to where fd 2 points currently
exec 3>&2

# wrapper function
grup () {
  # Change fd 2 to point to some file we create
  # Use >> to append to file, so that it's not truncated everytime function is called
  exec 2>> ${errfile} 
  
  # command that throws the errors; errors are collected regardless of 
  $(grep $@ )
  
  # Close 2, so it no longer points to a file
  exec 2>&-
  
  # restore normal function for stderr
  exec 2>&3
}

# Call some functions that generate errors
for opt in '--wrong-arg' '--no-such-arg' '--nooope' ; do
  grup $opt
done

# Another wrong call to show resoration of stderr outside function
grep --woops

#echo -e '\nWaiting 2s and showing errors in one go:\n\n'
#sleep 1 && cat $errfile

echo "To see the other errors, run cat ${errfile}"