What happens if you redirect stderr to a file that already exists?

When you redirect stderr to a file that already exists, the existing file will be overwritten by default. This means that any content previously in the file will be lost, and new error messages sent to stderr will be written to that file instead.

For example, using the following command:

command 2> error.txt

If error.txt already exists, it will be cleared, and any error messages generated by command will be written to error.txt.

If you want to append the error messages to the existing file instead of overwriting it, you can use the >> operator:

command 2>> error.txt

In this case, new error messages will be added to the end of error.txt without deleting the existing content.