Shell script updating same file code is looping through - shell

Question

The below code is currently working to update a property file with values from set environment variables. However, I'm wondering are there any issues/problems with updating the same file the code is looping through (at the same time)? https://github.com/koalaman/shellcheck/wiki/SC2094
The below code:
Loops through each line of a property file
Finds the key=value pair
Converts the key property to an ENVIRONMENT_VARIABLE name
Updates the same file with the new value from the ENVIRONMENT_VARIABLE so that now -- > key=ENVIRONMENT_VARIABLE
Before code is run:
MY_COLOR_PROP=yellow
my.color.prop=red
After code is run:
my.color.prop=yellow
FILE_NAME="./path/to/folder/my-app-${ENV}.properties"
echo "Update $FILE_NAME..."
OIFS=$IFS
while IFS="=" read -r key value; do
case "$key" in
'#'*)
echo "Skipping comment line..."
;;
*)
echo "Property from file: $key=$value"
ENV_VAR=$(echo "$key" | sed 's/\./_/g' | tr '[:lower:]' '[:upper:]')
ENV_VAR_VALUE=$(printf '%s\n' "${!ENV_VAR}")
echo "Environment variable: $ENV_VAR=$ENV_VAR_VALUE"
if [ -n "$ENV_VAR_VALUE" ]; then
echo "Setting $key=$ENV_VAR_VALUE in $FILE_NAME..."
sed -i "/$key=/ s^=.*^=$ENV_VAR_VALUE^" "$FILE_NAME"
else
echo "Skipping $key property - No value set for $ENV_VAR..."
fi
;;
esac
done <"$FILE_NAME"
IFS=$OIFS
***** UPDATE with what I implemented from below answer *****
My property file has comments in it and only a subset of the properties will need to be updated, the rest should remain untouched.
input="path/to/file/my.properties"
while IFS="=" read -r key value; do
case "$key" in
'#'*)
echo "$key"
;;
*)
ENV_VAR=$(echo "$key" | sed 's/\./_/g' | tr '[:lower:]' '[:upper:]')
echo "$key"="${!ENV_VAR-$value}"
;;
esac
done < "$input" > tmp; mv tmp "$input"
where the my.properties file looks like...
# Comment number one
bob=tom
organization.name=Chris Peter Org
# Comment number two
# Comment number three
db.url=jdbc:mysql://dburl/dbname
# Comment number four
test.parameter.ccf=always

Answer 1

Just do:
while IFS== read k v ; do echo $k=${!k-$v}; done < input > tmp && mv tmp input
eg:
$ cat input
foo=bar
baz=qux
$ unset baz
$ foo=foovalue
$ while IFS== read k v ; do echo $k=${!k-$v}; done < input > tmp && mv tmp input
$ cat input
foo=foovalue
baz=qux
Note that $!{k} is a bashism, and if you're not using bash you might want to do something like:
while IFS== read k v ; do eval echo \$k=\${$k:-$v}; done < input