Lately I've been missing MS-DOS. Not because DOS was amazing, but because I know where things are and how to get things done. The PATH command is a good example. In DOS, you could use the PATH command to help DOS find programs you wanted to run. It created a list of directories to search through before it gave up and said something like BAD COMMAND OR FILENAME. You could put this command inside a file called AUTOEXEC.BAT, which was the name of a program that would get run every time the computer started. This way, you could save time as you didn't have to type out or go to the directory in which the program you wanted to execute resided.
In mainframe land, however, things are a bit more complicated.
In mainframe land, the equivalent of a BAT file (short for BATCH) is something called a CLIST. CLIST stands for Command Listing, and is a lot like a DOS batch file in that it provides a user the means to execute several commands at once. In other words, instead of issuing ten commands separately, you could make a list and all you had to do was type in the name of the list. So how do we save ourselves time like we did on our old DOS system?
There are two commands that you can issue on the mainframe that are roughly equivalent to the DOS PATH command. They are TSOLIB and ALTLIB.
TSOLIB is used for load modules, which are programs that have been compiled and link-edited. We issue this command against load modules to have it added to the STEPLIB data set. STEPLIB is a library that will be at the head of a load module search. So, when we want to run our "hello world!" program we lovingly wrote in C, we add it to the STEPLIB so the mainframe knows where to find it, thus saving us the trouble of pecking out it's location in the file system.
ALTLIB will do basically the same thing, but is used for CLISTs and uncompiled REXX programs. These are scripting languages and thus don't have load modules. By default, the mainframe will look in a dataset called SYSPROC for CLISTs. ALTLIB will add other data sets to that search, thus saving us time.
Now, it's important to remember that, like the PATH command in DOS, these changes all go away the minute you log off (or in the case of DOS, reboot the system). So, we need to find a way to have the system re-implement these changes every time we log in. We need a mainframe equivalent to AUTOEXEC.BAT.
On the logon screen , there is a field labeled COMMAND. From there you can issue any TSO command you want, including a CLIST that contains all your ALTLIB and TSOLIB statements in it.
There ya go. Hope it helps :-)