From GridLAB-D Wiki
return_code - Return code value from system shell commands
Synopsis
#print ${return_code}
Remarks
System commands executed using the #system macro return values based on the exit or return code of the last command executed. This value is placed in the return code global variable. If the return code is -1, this is considered an error. On POSIX compliant systems, a return code of 127 is also considered an error indicating that there is fork error.