How to portable read a text file line by line in bash

Question

For processing a text file in bash line by line, I usually implement a while loop like this:

function doSomething() {
    local inputFile="$1"
    local fd=""
    local line=""
    exec {fd}<"$inputFile" # open file
    echo "Opened ${inputFile} for read using descriptor ${fd}"
    while IFS='' read -r -u $fd line || [[ -n "$line" ]]; do
        echo "read = \"$line\""
    done
    exec {fd}<&- # close file
    return 0
}

This works on my Linux but unfortunately not in OSX. For OSX I currently have to change the code to something like this:

exec 3<"$inputFile" # open file
while IFS='' read -r -u 3 line || [[ -n "$line" ]]; do
    echo "read = \"$line\""
done
exec 3<&- # close file

But this has the disadvantage, that I have to manage the file descriptor numbers by myself (in the first script, I let bash choose an available file descriptor number).

Did someone have a solution for this which works for both Linux and OSX?

Note that for some reason, I don't want to use piping or I/O redirection to the complete loop like this (because I don't want to execute the loop in a different process):

while IFS='' read -r line || [[ -n "$line" ]]; do
    echo "read = \"$line\""
done < "$inputFile"

Answer 1

The last loop will not fork a new process. You can verify that by printing "$BASHPID" in and outside of the loop.

New processes are only created for pipelines. Simple redirections are handled by temporary dup s within the bash process.

Feel free to use standard stdin/stdout redirection. It's no more expensive than redirection done with the exec builtin.