lineinfile in Shell Script

lineinfile.sh

# Ansible 'lineinfile' like function in Shell Script.
# Works on both Bash and Zsh.

function lineinfile(){
  if [[ $# != 3 ]];then
    local THIS_FUNC_NAME="${funcstack[1]-}${FUNCNAME[0]-}"
    echo "$THIS_FUNC_NAME - 3 arguments are expected. given $#. args=[$@]" >&2
    echo "usage: $THIS_FUNC_NAME PATTERN LINE FILE" >&2
    return 1
  fi
  local PATTERN="$1"
  local LINE="$2"
  local FILE="$3"
  if grep -E -q "${PATTERN}" "${FILE}" ;then
    
    ## solution 1: works with GNU sed well, but not works with BSD sed.
    # sed -E -i '' "/${PATTERN//\//\\/}/c${LINE}" "${FILE}"

    ## solution 2: works with both (GNU|BSD) sed, but get useless *.bak file generated.
    # sed -E -i.bak "/${PATTERN//\//\\/}/c\\"$'\n'"${LINE}" "${FILE}"

    ## solution 3: give up to use sed, using perl instead.
    PATTERN="${PATTERN}" LINE="${LINE}" perl -i -nle 'if(/$ENV{"PATTERN"}/){print $ENV{"LINE"}}else{print}' "${FILE}"

  else
    echo "$LINE" >> "$FILE"
  fi
}

######################
# example
######################

# write some lines to 'test.txt'
cat <<EOF > test.txt
foo = FOO1 # first occurence
bar = BAR
foo = FOO2 # second occurence
EOF

# usage: lineinfile PATTERN LINE FILE

# if some lines in FILE matches PATTEN, all of them are replaced with LINE.
lineinfile '^foo\s*=\s*' "foo = POO # changed!" test.txt

# if no lines in FILE matches PATTERN, LINE is appended to end of FILE.
lineinfile '^baz\s*=' "baz = BAZ" test.txt

cat test.txt
# now 'test.txt' will contain:
#
#   foo = POO # changed!
#   bar = BAR
#   foo = POO # changed!
#   baz = BAZ

If the *.bak is useless, why is it there in the first place?
from GNU sed man:

-i[SUFFIX], --in-place[=SUFFIX]
edit files in place (makes backup if extension supplied).

And for BSD sed the -i '' should work as it doesn't seem to be optional there.

I don't really see a reason to keep the solution 1 and solution 2 here in place instead of combining them into one that is free from:

• but not works with BSD sed
• but get useless *.bak file generated

It seems like:

sed -E -i '' "/${PATTERN//\//\\/}/c\\"$'\n'"${LINE}" "${FILE}"

could do the trick (I haven't tested that).

Maybe there was a reason having both solutions that I miss and it's worth mentioning it?