SWAP needs to know the name of the main input file (.swp) which should be present in the folder where you start SWAP. Please read the manual, Section 1.4 where it is explained how to properly start SWAP.

SWAP can be downlaoded and used free of charges under the following License Agreement.
SWAP version 4 is free software and is distributed under the terms of the GNU GENERAL PUBLIC LICENSE Version 2, June 1991 (see at https://www.gnu.org/copyleft/gpl.html). A small number of files (TTUTIL427.LIB) are distributed under the LESSER GNU GENERAL PUBLIC LICENSE version 2.1 (see at https://www.gnu.org/copyleft/lgpl.html). SWAP version 4 is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Public License for more details. You should have received a copy of the GNU Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA or see https://www.gnu.org/.

The development and testing of SWAP occurs under Windows. However, as of Nov. 2020 we also provide a Linux executable for version 4.0.1 (and later releases). Linux tests are performed irregularly (tested: Ubuntu 18.04; Scientific Linux). However, Windows and Linux executables are both created based on exactly the same source code.

Note that in Linux filenames are case-sensitive. Internally SWAP converts (most, if not all) filenames to lowercase. So you must use lowercase filenames, as well as to have lowercase file names given in the SWAP input files and command line.

In principle there is no need to make changes in the source code. Just simply add the weather records of the forecast in the main weather input file and tell SWAP to simulate to the new end-date.

Irrigation option tcs = 5 consisted of 2 sub-methods: irrigate when water content or pressure head exceeded a certain threshold value. As of SWAP version 4.1.29 option tcs = 5 is obsolete and replaced by either tcs = 7 (pressure head) or tcs = 8 (volumetric water content).

No. The Soil-Water-Atmosphere-Plant model SWAP inludes the WOFOST version 7.1 as a special case for considering crop growth. The implementation is such that limitations in the soil (e.g. drought stress, oxygen stress, salinity stress) result in diminished (actual) crop production. This likely results in a different prediction of actual crop growth compared to that predicted by WOFOST including its own simple soil module. Potential crop production is identical for SWAP-WOFOST and WOFOST.