Performing atomic write operations in a file in bash

After going through the bash documentation , this question and this one it's still not clear to me how can I perform atomic write (append) operations to a file in bash. I have a script that is run in multiple instances and at some point must write data to a file: echo "$RESULT" >> pwd/$TEMP_DIR/$OUT_FILE How is it possible to make all write operations from all concurrently running scripts to that file atomic (so that data from one instance doesn't overlap data from another)?