Avoiding race conditions with file creation/modification in bash

Use the noclobber flag, enabled with set -C (or by running your script with bash -C):

set -C
outputthedata > "$filepath" || exit

If filepath already exists, the shell will write an error like yourscript: line 2: filepath: cannot overwrite existing file and exit before outputthedata is started.

If you want an early exit only when noclobber triggers but not when outputthedata fails in any other way, that might look like:

set -C                      # enable noclobber
exec 3>"$filepath" || exit # open output file
set +C                      # disable noclobber for rest of the script
outputthedata >&3         # actually run the subprocess with output to file
exec 3>&-                   # close output file

Recommended way

set -o noclobber
do something
outputthedata > "$filepath"

if [[ -e "$filepath" || -L "$filepath" ]]
then
    # Either throw an error, exit the script,
    # or something like that here.
    exit 1
else
    touch "$filepath"
fi
outputthedata > "$filepath"

if [[ -e "$filepath" || -L "$filepath" ]]
then
    # Either throw an error, exit the script,
    # or something like that here.
    exit 1
else
    echo "$$" > "$filepath"
fi
read ownpid < "$filepath"
if [ "$ownpid" -eq $$ ]; then
    outputthedata > "$filepath"
fi

You can avoid race conditions in this scenario by using file locking.
In my case, I used flock, which ensures that the entire read–modify–write sequence is executed atomically.

flock provides advisory locking where any number of processes may simultaneously hold a shared lock on a file. but only one process can hold an exclusive lock at a time.

if [[ -e "$filepath" || -L "$filepath" ]]; then
    (
        flock -x 200 || {
            echo "Error: Could not acquire lock on $filepath" >&2
            exit 1
        }
        echo "Warning: File $filepath already exists" >&2
    ) 200>"$lock_file"
fi