Transaction AL11 will allow you to browse through the system's backend files and directories. If you haven't seen this be sure to try it. The directories it presents originate from directory parameters, e.g. DIR_HOME would typically be the home directory like /usr/sap/DEV/DVSOMESYSTEMID/work. Would you like to use these directories without hardcoding them ? Here's how: the DIR_* parameters are in fact system parameters which can be viewed with RSPARAM.
To get the value of e.g. DIR_HOME just call function module RSAN_SYSTEM_PARAMETER_READ.
DATA: lw_pars TYPE spfpflpar.
lw_pars-parname = 'DIR_HOME'.
CALL FUNCTION 'RSAN_SYSTEM_PARAMETER_READ'
EXPORTING
i_name = lw_pars-parname
IMPORTING
e_value = lw_pars-pvalue
EXCEPTIONS
OTHERS = 0.
concatenate lw_pars-pvalue 'SAP{S}_tables_{D}_{T}.csv'
INTO pa_file2 SEPARATED BY '/'.
So why is DIR_INTERFACE listed on AL11 and not available as parameter on RSPARAM ? Good question.