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.