HP 3000 Manuals

Command Interpreter Access and Variables Programmer's Guide

Things to Remember about Redirection Constructions 

When creating redirection constructions it is important to remember that
the redirection specification is stripped from the command line after 
string substitution (variable and expression substitution) but before the
command is actually executed.

For example, suppose you entered:

     editor <myfile >$null newfile 

With CIOR, this command works without problems because both redirection
specifications, <myfile and $null, are removed from the command line
before it is invoked.  This leaves:

     editor newfile 

Input, however, would be read from myfile and command output would be
sent to $null.  An offline listing would go to newfile however, because
that's the function of this parameter in the EDITOR subsystem.

To further illustrate how string substitution works, let's revisit an
example already used in this chapter which has a command file with the
following contents:

     parm showdest="$stdlist"
     showme
     showout >!showdest
     showvar
     showjob >>!showdest ;job=!hpuser.!hpaccount

The default value for the showdest parameter is $stdlist in this example.
If the user does not enter a parameter the showout command becomes

     showout >$stdlist

The redirection specification is stripped from the command line and no
redirection is done because none is necessary.  $STDIN behaves similarly.
(In fact the capability to specify >$STDLIST and <$STDIN as redirection
specifications is provided to handle this type of defaulting).

But this example also illustrates why the scan for redirection
specifications is done after string substitution.  If the redirection
scan were done before string substitution, or at the same time, the user
couldn't specify an input or output redirection file using parameters or
variables as in the last example.  This ordering always allows the
following:

     setvar dest ">abc" 
     listf !dest 

Because string substitution occurs first listf becomes listf >abc and
then redirection sends the output to file abc.

Expression substitution is also done in the string substitution pass and
would, therefore, also be performed before the redirection scan.  For
example, the LISTF command might be invoked as follows:

     listsf ![input(`ENTER THE FILESET TO BE DISPLAYED:')],6 &
     >![input(`ENTER THE FILE NAME FOR THE OUTPUT OF THE LISTF:')]

During the string substitution scan the user is prompted first for the
file set, and then for the output file name.  If the user entered abc@ef
and then outfile the command line would be the following after the string
substitution pass:

     listf abc@ef,6 > outfile

Next, the output would be redirected and then the listf would be invoked.

MPE/iX 5.0 Documentation