How to place / store a file in memory on linux?

On Linux, you probably already have an tmpfs filesystem that you can write to at /dev/shm.

$ >/dev/shm/foo
$ df /dev/shm/foo
Filesystem           1K-blocks      Used Available Use% Mounted on
tmpfs                   224088         0    224088   0% /dev/shm

This may use swap, however. For a true ramdisk (that won't swap), you need to use the ramfs filesystem.

mount ramfs -t ramfs /mountpoint