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/sys$common/syshlp/HELPLIB.HLB PIPE, Description *Conan The Librarian |
The PIPE command allows you to perform UNIX style command
processing by executing multiple DCL commands in a single command
line. You can use the PIPE command to execute DCL commands in a
number of ways:
o Multiple command execution
Multiple DCL commands are specified in a single PIPE command
and executed sequentially. The syntax for multiple command
execution is as follows:
PIPE command-sequence ; command-sequence [; command-sequences]...
o Conditional command execution
A command sequence is executed conditionally depending on the
execution result of the preceding command sequence.
Using the following form, command-sequence2 executes if, and
only if, command-sequence1 succeeds:
PIPE command-sequence1 && command-sequence2
Using the following form, command-sequence2 executes if, and
only if, command-sequence1 fails:
PIPE command-sequence1 || command-sequence2
o Pipeline command execution
A pipeline is formed by connecting DCL commands with pipes as
follows:
PIPE pipeline-segment-command | pipeline-segment-command [|...]
Each pipeline-segment command runs in a separate subprocess
with its SYS$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 (explode) :
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