To what file formats have you tried exporting this data?
As a quick test, I just exported:
from a fileMaker text field to a tab delimited file and it was not truncated.
I think you are right. Even when I exported it into CSV, it was still right and preserved all the numbers. The only issue is if I open csv file or tab file into excel and do some changes, then the number will be truncated even though I save it back to CSV or tab file again.
Thanks for your input. Now I think I have to make any changes in the notepad now, which totally sucks.
The only issue is if I open csv file or tab file into excel and do some changes, then the number will be truncated
That suggests that the issue is a limitation in Excel rather than FileMaker. You might consider using a pair of calculation fields that split the string into two parts and exporting the data from them rather than a single field. The strings may then import into Excel correctly and should be easier to put back together now that you have them in Excel and can use Excel's tools to work with them.
But I just exported straight to an Excel format and it exported correctly...
Correct. If I don't make any changes, it will stay correct. Try making a change and save it. And then import it back. Now you will see the difference. Thanks for all the good suggestions, though.
I edited the cell in Excel to change 12345678901234567890 into 12345678901234567899. I was able to do so. I then closed and re-opened the .XLSX file and it still showed the correct data. I then opened the FileMaker file and imported this file correctly back into FileMaker.
No issues were encountered in this process.