I know that in Magento 1 the directory "
/tmp/magento" was used as fall-back in case it existed.
I would like to know if that is still the case in Magento 2.
As @MagenX says, /tmp is an environment variable. It is the temporary folder that php is using for various tasks, like handling uploads.
The temporary folder can be changed in php configuration (via
Magento2 uses the temp folder too:
$this->directories[self::SYS_TMP] = [self::PATH => realpath(sys_get_temp_dir())];
but only for handling files, not for cache when var is not available (like magento1 did).