Be Sure Qshell Accesses the Right IFS Data

by Ted Holt

Today I continue my relentless Campaign to Eradicate Unreliable Software. Just recently, I saw a real-life example of the problem of unreliability. Program A called Program B to change the library list. Program B failed, the library list did not get changed, but Program B never bothered to tell Program A about it. Program A continued running with the wrong library list. As I wrote in Proper CL Error Handling, it's imperative to catch every error and make sure that it is dealt with.

The same sort of thing can happen in Qshell scripts. If a Change Directory command fails, Qshell generates an error message, but does not halt the script. Just as Program A continued running with the wrong library list, a Qshell script can continue on its way with the wrong current directory. Here's an example.

#!/bin/qsh
cd br549
rm can*

If the current directory contains a directory named br549, everything is hunky dory, but if there is no directory named br549 in the current directory, the cd command fails, sending error message 001-0008. (Error found changing to directory br549.) The rm command removes the files whose names begin with "can" from the wrong directory.

To abort the script, use the or construction (two vertical lines) to send a non-zero return code to the caller.

cd br549 || return 5

If the cd command is successful, the return is ignored. If the cd fails, the return kicks in and sends a return code of five to the caller. It is up to the caller to check for the return code and take appropriate action.

If the error message that cd generates presents a problem, redirect it to /dev/null. Redirecting the message to /dev/null sends it to never-never land.

cd br549 2> /dev/null || return 5

You can't be too careful. If something can go wrong, assume it will, and plan for the failure.

--Ted

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