Caché ObjectScript Reference - InterSystems Documentation

Caché ObjectScript Commands• Log on.• Issue a USE 0 command.• Cause an error when an error trap is not set.• Close the current device.• Return to programmer mode.• Exit Caché by issuing a HALT command.USE 0 implies an OPEN command to the principal device. If another process owns the device,this process hangs on the implicit OPEN as it does when it encounters any OPEN.Although USE 0 implies OPEN 0 for the principal device, issuing a USE command for anyother device that the process does not own (due to a previous OPEN command) produces a error.Note:While most Caché platforms allow you to close your principal input device, Cachéfor UNIX does not. Therefore, when a job that is the child of another job tries toperform I/O on your login terminal, it hangs until you log off Caché. At that time,the output may or may not appear.Using the Null Device (UNIX and OpenVMS)When you issue an OPEN and USE command to the null device (/dev/null on UNIX or NL:on OpenVMS), Caché treats the null device as a dummy device. Subsequent READ commandsimmediately return a null string (""). Subsequent WRITE commands immediately returnsuccess. No actual data is read or written. On UNIX-based systems, the device /dev/nullbypasses the UNIX open, write, and read system calls entirely.Processes started by other processes with the JOB command have a principal device of/dev/null or NL: by default.If you open /dev/null other than within Caché for example, by redirecting Caché output to/dev/null from the UNIX shell the UNIX system calls operate as they do for any other device.See Also• OPEN command• CLOSE command• $IO special variable• $PRINCIPAL special variable150 Caché ObjectScript Reference