dd if=/dev/zero of=bigassfile bs=1M count=1 seek=9999

See what I did there?

This command, issued in a console, literally means: Duplicate data from /dev/zero (a zero generator) into bigassfile.  Do it in blocks of 1 mebibyte each, write only one block, but skip forward to 9999 blocks in the bigassfile.

So what does Linux do when it sees this?  Pretty simple: it creates a bigassfile that is ten thousand megabytes in size, but only bothers to write the very last 1 mebibyte, leaving the first 9999 MiBs empty.  Since only one mebibyte is ever written to disk, it's nearly instantaneous.  When you list the file, it will show as having ten thousand mebybites, but if you use an on-disk size analyzer, the file will only occupy one mebibyte.  As you write data to the file, Linux will use free disk space to supply the big-ass file with room to save the incoming data.

So, remember the trick: give a block size, ask the command to write only one block, and skip N - 1 blocks, where N is the size in blocks of the file you want.

There you go.

Note that this trick does not work in Windows FAT16 or FAT32 filesystems.