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.
1 – DCL Command Restrictions
The PIPE command creates a special execution context for its
command sequences. The following DCL commands either do not work
or exhibit new behavior in this context:
o PIPE - Nested PIPE commands in the same command procedure
level are not allowed. There can only be one PIPE command
context for each command procedure level; however, nested
PIPE commands at different procedure levels are allowed. For
example:
$ TYPE FOO.COM
$ ! FOO.COM
$ :
$ PIPE ...
$ :
$
$ PIPE @FOO.COM ; ...
In this example, the PIPE command inside FOO.COM is allowed
because it is executed at a different command procedure level.
o GOTO and EXIT - These two commands, when executed as PIPE
command sequences, delete the PIPE command context before
the GOTO or EXIT command is executed. Any command sequences
following these two commands in a PIPE command are flushed.
o STOP - The STOP command, when executed after a PIPE command is
interrupted by Ctrl/Y, deletes the PIPE command context.
o THEN, ELSE, ENDIF, SUBROUTINE, ENDSUBROUTINE, RETURN, and
DCL labels - These commands cannot execute as PIPE command
sequences because it is not possible to realize their
functions in a PIPE command context.
2 – 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.
3 – Input Output Redirection
DCL users can use the DEFINE or ASSIGN command to redirect
SYS$INPUT, SYS$OUTPUT, or SYS$ERROR. Such redirection can be
created as either the user-mode (using the /USER_MODE qualifier)
or supervisor-mode (using the /SUPERVISOR_MODE qualifier)
redirection. A user-mode redirection only affects the environment
of the next user-mode image.
In a PIPE command, redirection can be achieved by using the
redirection syntax. A PIPE command redirection is quite different
from that created by the DEFINE or ASSIGN command, as follows:
o Redirections are created in supervisor mode. This means that
both user-mode applications and DCL commands are affected by
the redirections.
o The redirected environment only applies to the command
sequence or the pipeline-segment command that specifies
the redirection syntax. After the execution of the command
sequence or pipeline-segment command, the original process
input/output environment (that is, SYS$INPUT, SYS$OUTPUT, and
SYS$ERROR) is restored before command execution continues.
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:
$ PIPE SEARCH TRANS.LOG "alpha" > TRANS.LOG
%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$ERROR in supervisor
mode.)
4 – Pipelines and TEEs
This section describes aspects of DCL that function differently
in the context of a pipeline.
Some of the following constructs are used in the implementation
of a TEE.
4.1 – Using SYS$COMMAND
The SYS$COMMAND of a subprocess is normally the same as its
SYS$INPUT (if no command procedures are involved). In a
pipeline, however, the SYS$COMMAND of a subprocess is set to
the SYS$COMMAND of the parent process instead of to the preceding
pipe (which is the SYS$INPUT of the pipeline-segment command).
4.2 – Using TEEs and SYS$PIPE
In most cases, input from the pipe can be obtained by reading
the data from SYS$INPUT; however, when a command procedure is
invoked as a pipeline segment command, SYS$INPUT is redirected to
the command procedure file. To obtain data from the pipe inside a
command procedure, the logical SYS$PIPE can be used.
The following is an example of a pipeline DCL application
TEE.COM:
$ ! TEE.COM - command procedure to display/log data flowing through
$ ! a pipeline
$ ! Usage: @TEE log-file
$
$ OPEN/WRITE tee_file 'P1'
$ LOOP:
$ READ/END_OF_FILE=EXIT SYS$PIPE LINE
$ WRITE SYS$OUTPUT LINE !Send it out to the next stage of the pipeline
$ WRITE tee_file LINE ! Log output to the log file
$ GOTO LOOP
$ EXIT:
$ CLOSE tee_file
$ EXIT
The PIPE command to use TEE.COM can be:
$ PIPE SHOW SYSTEM | @TEE showsys.log | SEARCH SYS$INPUT LEF
The command procedure TEE.COM is used to log the data flowing
through the pipeline. It reads in the data from SYS$PIPE instead
of SYS$INPUT.
4.3 – Image Verification in a Pipeline
In a pipeline, image verification is turned off by default, even
when the command SET VERIFY=IMAGE is executed before the PIPE
command is entered. This prevents duplication of data records
going through the pipeline.
To turn on image verification in a pipeline, an explicit SET
VERIFY=IMAGE command must precede the pipeline segment command.
You can use a subshell to do this, as follows:
$ PIPE ... | (SET VERIFY=IMAGE ; ...) | ...
4.4 – File Access Methods in a Pipeline
A pipeline segment command can only use the RMS sequential file
access method to read and write to the pipes. Certain OpenVMS
utilities may access their input and output files using methods
other than sequential access. These operations are not supported
in a pipeline, and will fail, as in the following example:
$ PIPE CC/NOOBJ/NOLIS TEST.C | SEARCH SYS$INPUT/WIND=(1,1) "%cc-w-"
%SEARCH-F-RFAERR, RMS error using RFA access
-RMS-F-RAC, invalid record access mode
In this example, the /WINDOW qualifier for the SEARCH command
requires the relative file access method.