I need to export the data from 36 SQL tables containing 24GB of data into flat files, copy them to the client and import them there into the existing tables in his SQL database.
And I will need this for several customers (same tables, though).
How do I mass export and import data?
Is there a command line tool for this so I can write a script for repeated use?
bcp.exe is the standard bulk import/export tool for MSSQL. Using SSIS packages is an alternative, but brings a lot of overhead with it: it's a full ETL tool. In TSQL there's also a BULK INSERT statement that you can use as an alternative to "bcp in", but I personally haven't played around to see which one is faster or more useful etc.
See "bulk exporting" and "bulk importing" in Books Online for all the details.
The basic knowledge you will find here Importing and Exporting Bulk Data