UNIX Bourne Shell Programming

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7. VAX DCL TO UNIX SHELL SCRIPT CONVERSION.
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7.  VAX DCL TO UNIX SHELL SCRIPT CONVERSION

This chapter will describe the steps necessary to convert DCL
command files into Shell scripts. It is not a one to one conversion
and many features found in one operating system are not found in
the other. This requires you to write shell scripts that emulate
features of the other. This is especially true of VAX and UNIX. The
best way to accomplish this is to know exactly what it is that the
command file does in VMS and then write the equivalent function in
a shell script. There are few features that are the similar and
those will be examined.

There is really no "best" way to approach this subject. VMS and
UNIX are both unique operating systems. Much of the material
covered in this course up to this point will be used in the
conversion process. We will start with a list of standard UNIX
tools and their VMS equivalents. This will give a you a flavor of
the kinds of tools each operating system has to offer.
Major Tool                  VMS             UNIX               
                                                               
Editors                     EDT             ed                 
                            TECO            ex                 
                            TPU             vi                 
                                                               
Communications              MAIL            mail               
                            REPLY           write              
                            PHONE           talk               
                            DECnet          ftp                
                                            telnet             
                                                               
Compilers                   FORTRAN         f77                
                                            cc                 
                                                               
Text Processing             RUNOFF          troff              
                                            nroff              
                                            awk                
                                            lex                
                                                               
                                            sed                
                                                               
                            SORT            sort               
                            MERGE           merge              
                                                               
Program Development         LINK            link               
Tools                       DEBUG           adb/dbx            
                            LIBRARIAN       ar/ranlib          
                            DEC MMS         make               
                                            yacc               
                            DEC CMS         scs                
                                                              
Miscellaneous               DECalc          bc/dc             
                            DECspell        spell              
                                                               


As you can see from the above lists, both systems offer editors
capable of screen or line editing capabilities. Both have
interactive communications, electronic mail, networking, file
transfer, remote command execution, and remote logins. These
utilities and tools are standard to UNIX but require a license to
run under VMS. The Digital Command Language (DCL) and the UNIX
Shell (Bourne,C, or Korn) are command interpreters. That is, they
are both programs that parse the command line and then pass control
to other programs that are the kernel of the operating system.

7.1  Processes

When you login to either system the operating system will create
a unique process. This process is given access to memory and CPU
resources. The differences between the two operating systems with
respect to multi-tasking needs explanation. Multi-tasking is
concurrent processes initiated by a single user. When the user
starts a terminal session the system initiates a single process
called the PARENT process. It is possible to start multiple
processes from the parent process. This is called spawning in VMS
and forking in UNIX.  The new process is called a sub-process in
VMS and a child process in UNIX. This idea of "forking" a child
process occurs frequently in UNIX. 

When UNIX creates a child process or VMS creates a subprocess
different things occur. First, let's look at VMS. When the
subprocess is spawned the parent goes to "sleep" until the user
logs off from the subprocess. When the logoff occurs control
returns to the parent. The VMS ATTACH command gives control back
to the parent process and the subprocess goes to sleep. The point
is that only one process is active at a time. The exception is the
VMS RUN/PROCESS=name which will run a user-defined process at the
same time commands can be issued at the parent process.

In UNIX, you can run parent and children processes at the same
time. A child process can fork another process and thus a process
can be both a parent and child. Child processes are not restricted
to user-defined images but can be any valid UNIX operation. UNIX
processes that are running or stopped but not getting input from
a terminal are said to be in background. When you begin a UNIX
session the kernel gives you a copy of the shell. When you enter
a command the Shell forks a child. That child then processes the
command you entered.

 Note: some commands are executed by the Shell itself and no child
is forked.
          
This is a different concept from VMS, in which all commands are
executed  by the parent process. Once a subprocess is created the
parent remains dormant until the subprocess completes.

All the above parent and child processes use the standard default
for input and output devices, the terminal. Input and output
streams in UNIX are called standard input (stdin) and standard
output (stdout). Standard error (stderr) also uses the terminal as
it's default output.  In order to redirect these streams from a
terminal in VMS it is requires the assignment of the logical names
SYS$OUTPUT, SYS$INPUT, and SYS$ERROR to a file or device. UNIX has
a much nicer means of redirecting the input or output.

7.2  Pipes

A vertical bar (|) is used to redirect the output of a command to
the input of another command. This is the power of UNIX. For
example, we want to get a list of all currently active users,
sorted in alphabetical order, and sent to the printer, how could
that be done in VMS? Here's one solution:

VMS Sample Session:


$SHOW USERS/OUTPUT=A.TEMP                                      
$SORT/KEY=POSITION:40,SIZE:6)-                                 
  A.TEMP SYS$PRINT                                             


Notice the need for an intermediate file called A.TEMP.

Now how would this same requirement be met using UNIX. There is a
command that will list the users that are currently logged on to
the system. The command is who. Pipes allow the output of one
command to be the input into another command. The output of who
can be put into another UNIX command sort by a pipe. In a similar
manner, the output of sort can be redirected to another command
called lp. Thus the same problem can be solved using UNIX Shell in
this way:

UNIX Bourne Shell Sample Session:


$who | sort | lp -dmtlzr                                       


Notice that there is not a one-to-one command conversion that is
taking place here. The idea is to convert the process more than
individual commands from VMS DCL to UNIX Shell.

7.3  Input, Output, and Error Redirection

Just as the VMS logicals SYS$INPUT, SYS$OUTPUT, SYS$COMMAND, and
SYS$ERROR point at the terminal by default, so do the UNIX
equivalents stdin, stdout, and stderr. The equivalent of the
SYS$COMMAND in UNIX is the "Here is" document. UNIX uses a much
more simplified method of redirecting input and output to or from
a file. UNIX does not require the effect of an ASSIGN statement
ahead of the redirection. UNIX uses a metacharacter that is
included as part of the command line. Here is an example of both
VMS and UNIX.
 
VMS Sample Session:


$ASSIGN/USER A.LIS SYS$OUTPUT                                  
$ASSIGN/USER INPUT.DAT FOR005                                  
$RUN MYPROG                                                    

                                                                 
The equivalent function can be written in UNIX Shell like this:

UNIX Bourne Shell Sample:


$myprog < input.dat > a.lis                                    

                                                                 
The point here is to see that the metacharacters < and > act as
the input and output redirection symbols. Please don't get the idea 
that UNIX is much simpler than VMS DCL. That is not the case. They
each have strong points and weak points, they are not the same.
This is comparing apples and oranges. 

Here is a partial list of the metacharacters used by the UNIX
Shells and their meanings. These can be useful when you try to
redirect input and output.
Character      Meaning                                         
>        Redirect standard output (stdout)                     
                                                               
>>       Redirect and append standard output (stdout)          
                                                               
>&       Redirect standard output (stdout) and standard error  
         (stderr)                                              
                                                               
>>&      Redirect and append standard output (stdout) and      
         standard error (stderr)                               
                                                               
<        Redirect standard input (stdin)                       
                                                               
|        Redirect standard output (stdout) to another          
         command                                               
                                                               


These are the most commonly used redirection metacharacters.

Notes:

     UNIX redirection only affects the command line on which the
     redirection character occurs.

     Error messages are not redirected and will appear on the
     terminal.

     If a file already exists, VMS will create a new version of
     the file with a higher version number. By default, UNIX will
     overwrite the existing file.
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