Document revision date: 15 July 2002
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14.20.2 Using the PIPE Command for Pipeline Execution

A pipeline is a sequence of pipeline-segment commands connected by pipes, represented by the vertical-bar (|) separator. A pipeline-segment command is a DCL command that appears in a pipeline. The pipe connects the SYS$OUTPUT of one pipeline-segment command to the SYS$INPUT of the next command. The format of a pipeline is as follows:

PIPE pipeline-segment-command | pipeline-segment-command [|...]

Each pipeline-segment command runs in a separate subprocess with its SYS$OUTPUT connected to the SYS$INPUT of the next pipeline-segment command. These subprocesses execute in parallel; however, they are synchronized to the extent that each pipeline-segment command, except the first, reads the standard output of its predecessor as its standard input. A pipeline completes execution when the last pipeline-segment command is finished.

It is very common to use "filter applications" in a pipeline. A filter application is a program that takes data from SYS$INPUT, transforms it in a specific way, and writes it to SYS$OUTPUT.

Some aspects of DCL function differently in the context of a pipeline. For example:

14.20.3 Using the PIPE Command for Subshell Execution

A subshell is one or more command sequences separated by separators and enclosed in parentheses. The format of a subshell is as follows:

PIPE ( command-sequence [separator command-sequence]... )

The command sequences in a subshell are executed in a subprocess environment. DCL waits for the subshell to complete before executing the next command sequence. The ( ) separator is similar to the SPAWN/WAIT command.

When using the PIPE command in this format, handle symbol substitution carefully. After defining a symbol, precede subsequent references to that symbol with an ampersand (&) to delay symbol substitution. Otherwise symbol substitution takes place during phase 1 of command processing, at which time the symbol definition is unreliable.

14.20.4 Using the PIPE Command for Background Execution

Command sequences can be executed in a subprocess environment by using the following form:

PIPE command-sequence [ separator command-sequence]... &

DCL does not wait for the command sequences to finish. Control passes back to DCL once the background subprocess is created.

14.20.5 Using the PIPE Command for Input/Output Redirection

A command sequence can redirect its SYS$INPUT, SYS$OUTPUT, or SYS$ERROR to a file during execution of the command as follows:

A pipeline-segment command can also redirect its SYS$INPUT, SYS$OUTPUT or SYS$ERROR. However, SYS$OUTPUT redirection is allowed only for the last pipeline-segment command, and SYS$INPUT redirection is allowed only for the first pipeline-segment command.

Note that a PIPE command redirection is different from one created using the DEFINE or ASSIGN command. The differences are as follows:

When SYS$OUTPUT is redirected, the redirected output file is always created, whether or not the command sequence actually writes to SYS$OUTPUT. If a version of a file with the same name as the redirected output file already exists, a new version of that file is created. (This behavior is the same as using the DEFINE or ASSIGN command to redefine SYS$OUTPUT in supervisor mode.) Note that the redirected file is created before the command sequence is executed. If the redirected file is also used in the command sequence, the operation may fail, as in the following example:

%SEARCH-W-OPENIN, error opening TRANS.LOG;2 as input
-RMS-E-FLK, file currently locked by another user
In this example, a new version of TRANS.LOG is created and opened for write access; the SEARCH command then tries to get read access to the most recent version of TRANS.LOG instead of the expected previous version.

When SYS$ERROR is redirected, the redirected error file is only created when the command sequence actually writes to the SYS$ERROR during execution, and there is no existing file with the same name as the redirected error file. If a file with the same name as the redirected error file already exists, that file is opened as the redirected error file. The error output generated by this command sequence is then appended to the end of the redirected error file. (This behavior is the same as using the DEFINE or ASSIGN command to redefine SYS$ERROR in supervisor mode.)

14.20.6 Interrupting a PIPE Command

You can interrupt a PIPE command by pressing Ctrl/Y. If the PIPE command is executing in a pipeline or a subshell command sequence, the command sequence and the PIPE command are deleted. In this case, a CONTINUE command entered immediately after the interrupt will not resume the execution of the PIPE command.

If the PIPE command is executing a command sequence other than a subshell or a pipeline command sequence, DCL behaves as if the command sequence were entered as a DCL command without the PIPE command verb and interrupted by Ctrl/Y. See Section 13.11 for more information about the Ctrl/Y interrupt.

14.20.7 Improving Subprocess Performance

A PIPE command can generate a number of subprocesses during execution. Often, the applications invoked by command sequences do not depend on the process logical names and symbol names. In this case, the spawning of subprocesses can be accelerated by using the /NOLOGICAL_NAMES and /NOSYMBOLS qualifiers, which suppress the passing of process logical names and symbols to the subprocesses created by the PIPE command.

The following examples use the PIPE command:

Chapter 15
Using Lexical Functions to Obtain and Manipulate Information

Lexical functions return information to a command line or command procedure. The information returned can be about your process, the system, files and devices, logical names, strings, or data types. Lexical functions are identified by the prefix F$.

You can use lexical functions in any context in which you normally use symbols or expressions. In command procedures, you can use lexical functions to translate logical names, to perform character string manipulations, and to determine the current processing mode of the procedure. Many lexical functions return information that you can also get from DCL commands.

This chapter includes information about:

For additional information about lexical functions, refer to online help. For more information about the commands discussed in this chapter, refer to the OpenVMS DCL Dictionary.

15.1 Why Use Lexical Functions

You can manipulate information in a command procedure more easily if you obtain it from a lexical function rather than from a command. For example, you can use either the F$ENVIRONMENT function or the SHOW DEFAULT command to obtain the name of your current default directory. The differences are as follows:

15.2 Obtaining Information About Your Process

You often change process characteristics for the duration of a command procedure and then restore them. You can use the following lexical functions to obtain information about your process:
F$DIRECTORY Returns the current default directory string.
F$ENVIRONMENT Returns information about the command environment for your process.
F$GETJPI Returns accounting, status, and identification information about your process or about other processes on the system.
F$MODE Shows the mode in which your process is executing.
F$PRIVILEGE Indicates whether your process has the specified privileges.
F$PROCESS Returns the name of your process.
F$SETPRV Sets the specified privileges. This function also indicates whether the specified privileges were previously enabled before you used the F$SETPRV function.
F$USER Returns your user identification code (UIC).
F$VERIFY Indicates whether verification is on or off.

The following table shows process characteristics that are commonly changed in command procedures. It also gives the lexical functions that save these characteristics and the DCL commands that restore the original settings.

Characteristic Operation Command or Lexical Function
Control characters Save F$ENVIRONMENT("CONTROL")
  Restore SET PROMPT
Default protection Save F$ENVIRONMENT("PROTECTION")
  Restore SET KEY
Message format Save F$ENVIRONMENT("MESSAGE")
Privileges Save F$PRIVILEGE or F$SETPRV
Verification Save F$VERIFY or F$ENVIRONMENT

If you save process characteristics, you must ensure that an error or Ctrl/Y interruption does not cause the procedure to exit before you restore the original characteristics. (See Chapter 13 for more information on handling errors and Ctrl/Y interruptions.)

15.2.1 Changing Verification Settings

You can use the F$VERIFY lexical function to disable verification for the duration of a command procedure. This prevents users from displaying a procedure's contents while executing the procedure.

There are two types of verification:

By default, the SET [NO]VERIFY command and the F$VERIFY function turn both types of verification on or off. In general, the procedure and image verification settings in a procedure are the same (both on or both off). However, if you decide to change the settings, save each verification setting separately.

In the following example, the symbol TEMP is used to enable and disable verification:

$ ! Enable verification 
$ ! 
$ LOOP: 
$     INQUIRE FILE "File name" 
$     PRINT 'FILE' 
$     GOTO LOOP 
$ ! Disable verification 
$ ! 

In the following example, the verification settings are saved:

$ ! Save each verification state 
$ ! Turn both states off 
$ ! Restore original verification states 

The F$ENVIRONMENT function returns the current image verification setting and assigns this value to the symbol SAVE_VERIFY_IMAGE. Next, the F$VERIFY function returns the current procedure verification setting and assigns this value to the symbol SAVE_VERIFY_PROCEDURE. The F$VERIFY function disables both image and procedure verification. You can use the F$ENVIRONMENT function to obtain the procedure verification setting before you disable verification with F$VERIFY. However, it is shorter to use F$VERIFY to accomplish both tasks in one command line, as shown in the previous example.

At the end of this procedure, the F$VERIFY function restores the original settings (specified by the symbols SAVE_VERIFY_PROCEDURE and SAVE_VERIFY_IMAGE.)


If you are using time-stamping, remember that it applies only if verification is enabled. For more information on time-stamping and the SET PREFIX command, refer to the OpenVMS DCL Dictionary or DCL help.

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