*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.
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