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This is an old revision of CoreSyntax made by DavidLee on 2009-05-13 11:49:36.

 

Core Syntax

The core syntax for xmlsh was derived from the open source Shell Command Language syntax document. xmlsh does not follow this syntax exactly, but rather it was used as a basis.

Syntax Synopsis

This is a synopsis of the supported syntax. Complete specifications (will be) provided in the future when the language is stabilized.

Commands

A simple-command is a sequence of non-blank words separated by blanks. The first word specifies the name of the command
to be executed. Except as specified below, the remaining words are passed as arguments to the invoked command.
The command name is used to locate the command (builtin, nternal,external) . The
value of a simple-command.

A pipeline is a sequence of one or more commands separated
by |. The standard output of each command but the last is
connected by a Pipe to the standard input of the next
command. Each command is run as a separate thread (if builtin or internal)
or as a seperate process (if external). The
shell waits for the last command to terminate. The exit
status of a pipeline is the exit status of the last command
in the pipeline.
A list is a sequence of one or more pipelines separated by
;, &, &&, or
, and optionally terminated by ; or &. Of
these four symbols, ; and & have equal precedence, which is
lower than that of && and
. The symbols && and also
have equal precedence. A semicolon (;) causes sequential
execution of the preceding pipeline, that is, the shell
waits for the pipeline to finish before executing any com-
mands following the semicolon. An ampersand (&) causes asyn-
chronous execution of the preceding pipeline, that is, the
shell does not wait for that pipeline to finish. The symbol
&& (
) causes the list following it to be executed only if
the preceding pipeline returns a zero (non-zero) exit
status. An arbitrary number of newlines may appear in a
list, instead of semicolons, to delimit commands.
A command is either a simple-command or one of the follow-
ing. Unless otherwise stated, the value returned by a com-
mand is that of the last simple-command executed in the com-
mand.
for name [ in word ... ] ; do list done
Each time a for command is executed, name is set to the
next word taken from the in word list. If word is an XML sequence
then it is expanded as seperate words. If in word ... is
omitted, then the for command executes the do list once
for each positional parameter that is set (see Parameter
Substitution section below). Execution ends when there
are no more words in the list.


case word in [ pattern [ | pattern ] ) list ;; ] ... esac
A case command executes the list associated with the
first pattern that matches word. The form of the pat-
terns is the same as that used for file-name generation
(see File Name Generation section), except that a slash,
a leading dot, or a dot immediately following a slash
need not be matched explicitly.


if list ; then list ; [ elif list ; then list ; ] ... [ else list ; ] fi
The list following if is executed and, if it returns a zero
exit status, the list following the first then is executed.
Otherwise, the list following elif is executed and, if its
value is zero, the list following the next then is executed.
Failing that, the else list is executed. If no else list or
then list is executed, then the if command returns a zero
exit status.

while list ; do list ; done
A while command repeatedly executes the while list and,
if the exit status of the last command in the list is
zero, executes the do list; otherwise the loop terminates.
If no commands in the do list are executed,
then the while command returns a zero exit status;
until may be used in place of while to negate the loop
termination test.

eval arg [args ...]
Converts each argument a string and joins them with a space. The resulting string
is passed to the parser and re-evaluated and executed in the current context.

(list)
Execute list in a sub-shell.
Hence changes to the environment made within this subshell do not affect the parent shell.
{ list;}
list is executed in the current (that is, parent)
shell. The { must be followed by a space.


name () { list;}
Define a function which is referenced by name. The body of the function is the
list of commands between { and }. The { must be followed by a space. \
Execution of functions is described below (see Execution section).
The { and } are unnecessary if the body of the function
is a command as defined above, under Commands.
break
terminates a for ,while, or until loop
The following words are only recognized as the first word of
a command and when not quoted:
if then else elif fi case esac for while until do eval
done { }

Comments Lines
A word beginning with # causes that word and all the follow-
ing characters up to a newline to be ignored.

Variables
Variables can be set using the assigment statement
var = value
This initializes or overwrites a variable within the current shell environment.
Variables can be either String or XML types. XML types can be atomic, node, or sequence values.
( any value which can be returned from an xquery expression).
XML Variables are implicitly converted to strings when used in a string context.

Variable Substitution
Words are scanned for $ which indicates variable substition.
$name or ${name} is substituted for the value of the variable named "name".
$0 $2 ... $nnnnn is substituted for the positional paramter (note this differs from sh where n may be > 9)
There are several builtin variables

$? the exit code of the last command
$@ All positional parameters as if they were quoted. equal to "$1" "$2" "$3"
$$ The thread id of the current shell
$! The thread id of the last background command executed with "cmd &"
$* All positional parameters unquoted. equal to $1 $2 $3 ...
$# The number of positional paramters
$(cmd) cmd is executed and its output is is substituted with newlines turned into word seperators
$<(cmd) cmd is executed and the output parsed as an XML document and the result is an XML expression

XML Substition
<[ xml ]> specifies explicit xml construction. The string between "<[" and "[>" is passed to xquery
and the result is interprested as an XML sequence. All variables in the shell environment are made
available to the xquery as declared external variables.


Command Substitution
The shell reads commands from the string between $( and )
and the standard output from these commands may
be used as all or part of a word. Trailing newlines from the
standard output are removed.
No interpretation is done on the string before the string is
read, except to remove backslashes (\) used to escape other
characters.

If the command is of the form $(<file) then the file is read, trailing newlines
stripped and the result is a string containing the entire file.
Variable expansion is done on "file" so that the following syntax works:
a=foo.txt
echo $(<$a)


XML Command Substitution
Similar to Command Substitution, $<( command ) indicates a command which
is executed and the output is parsed as an XML document.

Command is executed identically to the $(command) syntax except that the result
is then parsed as an XML document and the result is an XML expression.

If the command is of the form $<(<file) then the file is read, trailing newlines
stripped and the result is parsed as an XML document.
Variable expansion is done on "file" so that the following syntax works:
a=foo.xml
xecho -i $<(<$a)

XML Namespaces
Namespace support is manged with the declare namespaces command.

Command execution
Commands can be either builtin, internal, user defined or external commands.
Builtin commands are "special" in that they interact with the shell environment.
Internal commands implemented as a "convenience" in that they could be also implemented as
user defined commands. User defined commands are supported by creation of a jar containing
classes which implement the ICommand interface. External commands are any command (process)
outside of xmlsh. External commands are executed as seperate processes, all other commands are
executed with the JVM of xmlsh.



See Also BasicSyntax
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