So for example, if it detects that you passed the --socket flag if would hit that condition and then look for the ${1} which would be the following parameter and sets the socket variable to that value and tells it to move on or shift to the next argument and continue the loop.

Trying anything I can think of, this part looks promising to maybe be the issue...

WSREP_SST: [INFO] Removing the sst_in_progress file (20150615 23:31:45.369) 150615 23:31:45 [ERROR] WSREP: Process completed with error: wsrep_sst_xtrabackup --role 'joiner' --address 'x.x.x.x' --auth 'no:no' --datadir '/db/mysql/' --defaults-file '/etc/mysql/my.cnf' --parent '16465': 32 (Broken pipe) 150615 23:31:45 [ERROR] WSREP: Failed to read uuid:seqno from joiner script. 150615 23:31:45 [ERROR] WSREP: SST failed: 32 (Broken pipe) 150615 23:31:45 [ERROR] Aborting