PIPE
*Conan The Librarian (sorry for the slow response - running on an old VAX)
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VMS Help PIPE *Conan The Librarian (sorry for the slow response - running on an old VAX) |
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Executes one or more DCL command strings from the same command
line. The PIPE command enables you to perform UNIX style command
processing, such as command pipelining, input/output redirection,
and conditional and background execution.
Format
PIPE command-sequence [separator command-sequence]...
| 1 - Parameter |
command-sequence
A DCL command, a pipeline, or a subshell:
o DCL command
A DCL command string, which can include qualifiers,
parameters, keywords, and values.
o Pipeline
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:
pipeline-segment-command | pipeline-segment-command [|...]
o Subshell
A subshell is one or more command sequences separated by
separators and enclosed in parentheses. The format of a
subshell is as follows:
(command-sequence [separator command-sequence]...)
Input/output redirection is allowed in a command sequence. The
command before an angle bracket (> or <) redefines its SYS$INPUT,
SYS$OUTPUT, or SYS$ERROR during execution. You cannot use angle
brackets (<>) to represent a directory specification in a PIPE
command because the PIPE command interprets angle brackets as
input/output redirection syntax.
separator
Determines the processing action of the command sequences
specified in a PIPE command. The valid PIPE separators are
described in the following table.
Separator Action
| Key pipe separator. The pipe connects the SYS$OUTPUT of
one pipeline-segment command to the SYS$INPUT of the
next command.
; Sequential execution. The command sequence following
the semicolon (;) is executed after the preceding
command sequence is completed. You must precede
this separator with a blank space; otherwise, it is
parsed as the Record Management System (RMS) file
specification version number delimiter.
&& Conditional execution (upon success). The command
sequence following the double ampersand (&&) is
executed only if the preceding command sequence
succeeds.
|| Conditional execution (upon failure). The command
sequence following the double vertical bar (||) is
executed only if the preceding command sequence fails.
& Background execution. All command sequences that
precede the ampersand (&) are executed asynchronously
in a subprocess environment. The & separator is similar
to the SPAWN/NOWAIT command.
Note: Any ampersand that precedes a character string
without spaces in between is parsed as a conventional
DCL symbol substitution expression rather than the
background execution syntax.
@TEE Command file, TEE.COM. Used for redirecting output to
two targets (for example, one output is directed to
the next stage in pipeline, and the other to a file).
See the Examples section for an example of how to use
TEE.COM.
In a PIPE command line, the "&" has the highest precedence,
followed by "|", ";", "&&", and "||", which have equal
precedence.
| 2 - 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. Refer to the OpenVMS
User's Manual for more information on the Ctrl/Y interrupt.
The return status of the PIPE command is the return status of
the last executed command sequence. Each command sequence sets
the global symbol $STATUS with a returned value after it finishes
execution.
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.
2.1 - 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.
2.2 - 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.
2.3 - 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$ERROR in supervisor
mode.)
2.4 - Pipelines and TEEs
This section describes aspects of DCL that function differently
in the context of a pipeline.
Some of the following constructs are used in the implementation
of a TEE.
2. 4.1 - Using SYS$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$INPUT of the pipeline-segment command).
2. 4.2 - Using TEEs and SYS$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.
2. 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 ; ...) | ...
2. 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.
| 3 - Qualifiers |
3.1 - /LOGICAL_NAMES
/LOGICAL_NAMES (default)
/NOLOGICAL_NAMES
Copies process logical names and logical name tables to the
subprocess of a command sequence. By default, all process logical
names and logical name tables are copied to the subprocess except
those explicitly marked CONFINE or created in executive or kernel
mode.
3.2 - /PRIVILEGES
/PRIVILEGES={CURRENT|AUTHORIZED}
Determines whether the subprocess inherits the current
process's current or authorized privileges as its authorized
privileges. By default, the authorized privilege mask for
the subprocess is taken from the current privileges of its
creator. (This corresponds to /PRIVILEGES=CURRENT.) If the
/PRIVILEGES=AUTHORIZED qualifier is specified, the subprocess's
authorized privileges are taken from the creator's authorized
privileges.
3.3 - /SYMBOLS
/SYMBOLS (default)
/NOSYMBOLS
Determines whether global and local symbols (except $RESTART,
$SEVERITY, and $STATUS) are passed to the subprocess. $RESTART,
$SEVERITY, and $STATUS symbols are never passed to the
subprocess.
3.4 - /TRUSTED
/TRUSTED
/NOTRUSTED
Indicates that the PIPE command input originates in a trusted
command procedure. PIPE commands are not allowed in CAPTIVE
accounts. The /TRUSTED qualifier provides a way for properly
written captive command procedures to perform PIPE operations
when the command input originates in the captive command
procedure where it can be trusted. For more information about
trusted command procedures, refer to the OpenVMS Guide to System
Security.
| 4 - Examples |
1.$ PIPE SHOW SYSTEM | SEARCH SYS$INPUT HIB
This example uses the pipeline function to identify all
hibernating processes on the system in one command.
2.$ PIPE RUN TEST | SORT/SPECIFICATION=TEST.SRT SYS$INPUT SYS$OUTPUT -
| DIFF SYS$INPUT TEST.BENCHMARK
This example uses the pipeline function to run a test, sort
the result, and compare the result to the benchmark file in
a single command without generating unnecessary intermediate
files.
3.$ PIPE ( SET DEF WRK$:[WORK] ; RUN REPORT ) | MAIL SYS$INPUT SMITH
This example shows one way a subshell can be specified as a
pipe segment command in a pipeline.
4.$ more :== TYPE/PAGE=SAVE SYS$INPUT
$ PIPE ANA/RMS PAGE.TXT | more
Check RMS File Integrity 26-DEC-2001 16:12:00.06 Page 1
SYS$SYSDEVICE:[TEST]PAGE.TXT;2
FILE HEADER
File Spec: SYS$SYSDEVICE:[TEST]PAGE.TXT;2
File ID: (4135,58220,0)
Owner UIC: [PIPE]
Protection: System: RWED, Owner: RWED, Group: RE, World:
Creation Date: 26-NOV-2001 16:08:50.05
Revision Date: 26-NOV-2001 16:09:09.06, Number: 1
Expiration Date: none specified
Backup Date: none posted
Contiguity Options: none
Performance Options: none
Reliability Options: none
Journaling Enabled: none
RMS FILE ATTRIBUTES
RETURN/SPACE=More, PREV/NEXT=Scroll, INS/REM=Pan, SELECT=80/132, Q=Quit
This example shows the use of the /PAGE qualifier within a
pipeline. The /PAGE function exists in a number of other DCL
commands as well, and can be used similarly in conjunction with
the PIPE command to form other useful tools.
5.$ ! 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 next stage of the pipeline
$ WRITE tee_file LINE ! Log output to the log file
$ GOTO LOOP
$ EXIT:
$ CLOSE tee_file
$ EXIT
This is an example of a pipeline DCL application TEE.COM.
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$INPUT.
6.$ CD_WORK :== PIPE SAVE_DIR=F$DIRECTORY() ; SET DEFAULT FOO:[WORK]
$ BACK :== SET DEF 'SAVE_DIR'
$
$ CD_WORK ! Switch to working directory
$ :
$ :
$ BACK ! Switch back to home directory
$ GET_RECORD :== PIPE READ/END_OF_FILE=CLEANUP IN RECORD ; -
F$EDIT(RECORD, "COMPRESS, TRIM")
$
$ OPEN IN EMPLOYEE.DAT
$ LOOP:
$ GET_RECORD
$ :
$ :
$ GOTO LOOP
$
$ CLEAN_UP:
$ :
This example shows two simple uses of multiple commands with
symbol definitions to build useful tools in command procedures.
7.$ PIPE cc foo.c && link foo, sys$library:vaxcrtl.olb/lib
If the compilation does not generate any error, the object
file is linked to produce an executable image. If the program
compilation generates an error, the linking step is skipped.
8.$
$ PIPE RUN COLLECT_DATA.EXE || GOTO CLEAN_UP
$ :
$ :
$ EXIT
$
$ CLEAN_UP:
$ :
$ :
Using conditional command execution, it is easy to set up
simple error handling control flow in a command procedure.
If the image COLLECT_DATA fails, control is directed to CLEAN_
UP.
9.$ PIPE COPY LARGE_FILE.DAT REMOTE"user password"::[DESTINATION]*.* &
This PIPE command creates a background process to handle the
copying of the large file.
10$ PIPE (SET DEF [.DATA_DIR] ; BACKUP DATA.SAV/SAV [...]) ; RUN FOO
The subshell command sequence is done in a subprocess. This
means that changing a process-specific characteristic (for
example, the default directory) will not affect the current
process after the subshell is finished. In this example, the
save set is restored in a subdirectory to provide the necessary
data to run the program FOO.
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