PIPE, Description
*Conan The Librarian (sorry for the slow response - running on an old VAX)
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VMS Help PIPE, Description *Conan The Librarian (sorry for the slow response - running on an old VAX) |
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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.
Additional Information (explode) :
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