Most recently, I had a need to write some unit test scripts in RPG to compare a dynamically generated XML file to an existing XML document to determine whether or not they were exactly the same. I could have gone the route of coding everything in RPG using the IFS APIs, but I decided against that as the functionality already exists in Qshell's cmp (compare) command. By using the cmp command, I was able to interface with an API that has been tested by thousands of other developers, thus giving me a high level of confidence that the API will appropriately compare one file to another. Reusing what already exists is true SOA in action and is what we developers need to do to retain investment in our machines and take them to the next level in a fraction of the time.

All right! Enough intro. Let's see some code!

An obvious approach to using the Qshell APIs would be to first wrap them in an RPG program for ease of use. The example program below shows different parameters used to invoke the QSH_cmpSame RPG subprocedure.

The first line of interest is the /copy, where we bring in the prototype to make the call to QSH_cmpSame. The result variable is declared as a Boolean and will hold the result of each call to QSH_cmpSame.

Moving on to the mainline, we can see that both IFS files and source members can be compared. I didn't think this would work with the source members, but I couldn't pass up the opportunity to try! And work it did! See the code below for the contents of compare1.txt, compare2.txt, compare3.txt, compare1.mbr, compare2.mbr, and compare3.mbr. You will notice that they are incredibly small files and source members, and they are such because it serves the purpose of brevity and example. Note that compare1.txt and compare2.txt are exactly the same, so we could show an example of an exact match. On the flip side, compare1.txt and compare3.txt are slightly different and will produce the *OFF result. The .mbr files follow this same approach.

The following code shows the contents of QSH_cmpSame. The first thing that needs to happen is the redirecting of what's called standard input, standard output, and standard error. Subroutine SetupIO opens files with specific descriptors to do the Qshell redirection. (Go to the LiveFireLabs site for a good, concise tutorial of UNIX file descriptors.) You could think of the redirecting being similar to an OVRDBF used to alter the member used when accessing a PF for I/O.

The second piece of interesting code is the composing of the CMP command and the subsequent calling of QzshSystem to process it. Exhaustive information on the QzshSystem API can be found here. But for the sake of layman conversation, it's simply an enabler of sorts that allows the execution of commands you would traditionally enter into a Qshell prompt interactively. To run an interactive Qshell session, simply enter STRQSH on the command line. You can find detailed information about the CMP here, but essentially, it simply does a byte-for-byte binary comparison of two files; and as I found out, it can also work in the QSYS.LIB side of the IFS.

The way this program determines success is by checking to see if any errors appeared in the standard out files or error files that were redirected to it earlier. This is done using the stat IFS API. If either file has content, then we know something out of the ordinary occurred during the comparison and *OFF should be returned. Run this program through debug and you can see what's in those files using DSPF '/tmp/stderr-999999', where 999999 is the job number.

The last order of action is to close all of the Qshell file descriptor redirects by executing sub routine CloseIO.

Additional References

Article: "TechTip: Qshell vs. PASE"
Article: "TechTip: Link Up with Qshell"
Article: "Exploring iSeries QSHELL"
Presentation: "Qshell and OpenSSH for IBM System i"
IBM Information Center Web Page: Using Qshell