How to verify if an extended attribute exist?
The following xttar command causes both the attribute names and corresponding values to be displayed, if any exist.

xattr -l file


You can also use the ls command:

ls -@ file




That said, the ... No such xattr: com.apple.quarantine message is a non-fatal message, meaning there no real need to make it conditional, and if your goal is to remove the com.apple.quarantine extended attribute from a lot of files at once in a given directory, then cd to the target directory and use the following command:

for f in *; do xattr -dr com.apple.quarantine "$f" 2&gt;/dev/null; done


The above one-liner will delete the com.apple.quarantine extended attribute off every file that has it and act as if the entire contents of the directory recursively were also specified (so that every file in the directory tree is acted upon), thats the -r option, while not showing errors for files that don't have the target extended attribute. (That's what the 2&gt;/dev/null does.)