HP 3000 Manuals

MPE V to MPE XL: Getting Started

Lesson 3:  Understanding Search Priorities 

What if your command file or program has the same name as a UDC or system
command?  Which will execute?  To answer this, first consider how MPE XL
interprets what is typed at the prompt.

In brief, MPE XL interprets everything entered at the prompt to be a
command and prioritizes its search for that command in the following
order:

 *  UDCs first

 *  MPE commands next

 *  File names last (including command files and programs)

This hierarchy is called the search priority.

The detailed diagram on the next page illustrates the process of MPE XL's
search for a command called TRYIT. Then, on subsequent pages, the diagram
is broken down to allow you to look at each step of the process.


UDC Directory 

The system first searches for TRYIT in the UDC directory.  User created
UDCs are searched first, account UDCs next, and finally system UDCs.  If
TRYIT is a UDC, the search ends and TRYIT is executed.


Exercise 

7.   Which UDC in each of the following pairs would execute?

     a.   The user UDC ST or the system UDC ST?

     b.   The system UDC LF or the account UDC LF?

     c.   The account UDC RE or the user UDC RE?

Command Directory 

If TRYIT is not found in the UDC directory, the search continues to see
if TRYIT is an MPE XL system command.  If TRYIT is an MPE XL system
command, it executes.


Exercise 

8.   Which would execute, a UDC named LISTF or the MPE LISTF command?

File Directory 

If TRYIT is not found in the command directory, the search continues on
to file names.  If TRYIT were a qualified file name of a valid,
executable file (TRYIT.PUB.SYS or TRYIT.MYGROUP, for example), it would
execute.


Exercise 

9.   If you entered the full program file name SORT.PUB.SYS, would the
     system still begin its search in the UDC directory?

Search Path 

Since the filename, TRYIT, is not a qualified file name (that is, has
neither group nor account specified), the system follows a designated
search path looking for a matching name.  Following the default search
path, the system first looks in your current group, next in your PUB
group, and finally in PUB.SYS. (The search path for any given session is
user modifiable; the procedure is described in chapter 5, "Variables.")


If TRYIT is not found in the search path or is found to be a
nonexecutable file, the system issues an error message and returns you to
the MPE prompt.


Exercise 

10.  Indicate in the space provided what would execute in each of the
     following situations.

     a.   The LISTF system command or a command file named LISTF?

     b.   A command file or a UDC with the same name?

     c.   A program called PRINT or the MPE XL command PRINT?

     d.   A command file in your PUB group or a program with the same
          name in your current group?

MPE/iX 5.0 Documentation