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1 PIPE

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]... 2 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, since 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 ; 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. 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 form: PIPE command-sequence1 && command-sequence2 Command-sequence2 executes if and only if command-sequence1 succeeds. Using the form: PIPE command-sequence1 || command-sequence2 command-sequence2 executes if and only if command-sequence1 fails. 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. 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. 3 DCL_Command_Restrictions The PIPE command creates a special execution context for its command sequences. Some DCL commands either do not work or exhibit new behavior in this context. These commands are: 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 since 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. 3 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$ERROR. Such redirection can be created as either the user mode (using the /USER_MODE qualifier) or supervisor mode (using the /SUPERVISOR_MODE qualifier) redirection. A user-mode redirection will only affect the environment of the next user-mode image. In a PIPE command, redirection can be achieved by using the redirection syntax. A PIPE command redirection is quite different from that created by the DEFINE or ASSIGN command, as follows: o Redirections are created in supervisor mode. This means that both user-mode applications and DCL commands are affected by the redirections. o The redirected environment only applies to the command sequence or the pipeline-segment command that specifies the redirection syntax. After the execution of the command sequence or pipeline-segment command, the original process input/output environment (i.e. SYS$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$ERROR in supervisor mode.) 3 Pipelines Some aspects of DCL function differently in the context of a pipeline. 4 Using_SYS$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$INPUT of the pipeline-segment command). 4 Using_SYS$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 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 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. 2 Qualifiers /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. /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. /SYMBOLS /SYMBOLS (default) /NOSYMBOLS Determines whether global and local symbols (except $RESTART, $SEVERITY, and $STATUS) are passed to the subprocess. $RESTART, $SEVERITY, and $STATUS symbols are never passed to the subprocess. /TRUSTED /TRUSTED /NOTRUSTED Indicates that the PIPE command input originates in a trusted command procedure. PIPE commands are not allowed in CAPTIVE accounts. The /TRUSTED qualifier provides a way for properly written captive command procedures to perform PIPE operations when the command input originates in the captive command procedure where it can be trusted. For more information about trusted command procedures, see the OpenVMS Guide to System Security. 2 Examples 1.$ 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. 2.$ PIPE cc foo.c && link foo, sys$library:vaxcrtl.olb/lib If the compilation does not generate any error, the object file is linked to produce an executable image. If the program compilation generates an error, the linking step is skipped. 3.$ $ 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. 4.$ PIPE COPY LARGE_FILE.DAT REMOTE"user password"::[DESTINATION]*.* & This PIPE command creates a background process to handle the copying of the large file. 5.$ 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) won't 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. 6.$ PIPE SHOW SYSTEM | SEARCH SYS$INPUT HIB This example uses the pipeline function to identify all hibernating processes on the system in one command. 7.$ PIPE RUN TEST | SORT/SPECIFICATION=TEST.SRT SYS$INPUT SYS$OUTPUT - | DIFF SYS$INPUT TEST.BENCHMARK This example uses the pipeline function to run a test, sort the result, and compare the result to the benchmark file in a single command without generating unnecessary intermediate files. 8.$ 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. 9.$ more :== TYPE/PAGE=SAVE SYS$INPUT $ PIPE ANA/RMS PAGE.TXT | more Check RMS File Integrity 26-JAN-1996 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-1996 16:08:50.05 Revision Date: 26-NOV-1996 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.

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