Description

   The PIPE command allows you to perform UNIX style command
   processing by executing multiple DCL commands in a single command
   line. You can use the PIPE command to execute DCL commands in a
   number of ways:

   o  Multiple command execution

      Multiple DCL commands are specified in a single PIPE command
      and executed sequentially. The syntax for multiple command
      execution is as follows:

      PIPE   command-sequence ; command-sequence [; command-sequences]...

   o  Conditional command execution

      A command sequence is executed conditionally depending on the
      execution result of the preceding command sequence.

      Using the following form, command-sequence2 executes if, and
      only if, command-sequence1 succeeds:

      PIPE   command-sequence1   &&   command-sequence2

      Using the following form, command-sequence2 executes if, and
      only if, command-sequence1 fails:

      PIPE   command-sequence1   ||   command-sequence2

   o  Pipeline command execution

      A pipeline is formed by connecting DCL commands with pipes 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 finishes execution when the last pipeline-segment
      command is done.

      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.

   o  Subshell execution

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

      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.

   o  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.

   o  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:

      To redirect SYS$INPUT:

         PIPE    command-sequence < redirected-input-file

      To redirect SYS$OUTPUT:

         PIPE    command-sequence > redirected-output-file

      To redirect SYS$ERROR:

         PIPE    command-sequence 2> redirected-error-file

      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.

   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 the OpenVMS User's
   Manual for more information on the Ctrl/Y interrupt.

   Each command sequence sets the global symbol $STATUS with a
   returned value after it finishes execution. The return status
   of the PIPE command is the return status of the last command
   performed in the last segment. If all segments fail with some
   kind of error and the last segment returns with success, the
   status returned to DCL is the success.

   When a PIPE command is executed in a command procedure with the
   ON condition processing, the conditional execution of command
   sequences (&&, ||) takes precedence over the action previously
   specified by the ON condition statement.

Additional information available:

DCL_Command_Restrictions        Improving_Subprocess_Performance
Input_Output_Redirection        Pipelines_and_TEEs