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.