Trapping Command Errors

The most efficient way to detect errors from commands is by creating condition traps, using the SIGNAL ON and CALL ON instructions, with either the ERROR or the FAILURE condition. When used in a program, these instructions enable, or switch on, a detector in Rexx that tests the result of every command. Then, if a command signals an error, Rexx stops usual program processing, searches the program for the appropriate label (ERROR:, or FAILURE:, or a label that you created), and resumes processing there.

SIGNAL ON and CALL ON also tell Rexx to store the line number (in the Rexx program) of the command instruction that triggered the condition. Rexx assigns that line number to the special variable SIGL. Your program can get more information about what caused the command error through the built-in function CONDITION.

Using the SIGNAL and CALL instructions to handle errors has several advantages; namely, that programs:

• Are easier to read because you can confine error-trapping to a single, common routine
• Are more flexible because they can respond to errors by clause ( SIGL), by return code (RC), or by other information (CONDITION method or built-in function)
• Can catch problems and react to them before the environment issues an error message
• Are easier to correct because you can turn the traps on and off (SIGNAL OFF and CALL OFF)

For other conditions that can be detected using SIGNAL ON and CALL ON, see the Open Object Rexx: Reference.