SH(2) SH(2)
NAME
Sh - module interface to the shell
SYNOPSIS
include "sh.m";
sh := load Sh Sh->PATH;
Context, Listnode: import sh;
system: fn(drawctxt: ref Draw->Context, cmd: string): string;
run: fn(drawctxt: ref Draw->Context, argv: list of string): string;
parse: fn(s: string): (ref Cmd, string);
cmd2string: fn(c: ref Cmd): string;
list2stringlist: fn(nl: list of ref Listnode): list of string;
stringlist2list: fn(sl: list of string): list of ref Listnode;
Context: adt {
new: fn(drawcontext: ref Draw->Context): ref Context;
get: fn(c: self ref Context,
name: string): list of ref Listnode;
set: fn(c: self ref Context,
name: string,
value: list of ref Listnode);
setlocal: fn(c: self ref Context,
name: string,
value: list of ref Listnode);
envlist: fn(c: self ref Context):
list of (string, list of ref Listnode);
push, pop: fn(c: self ref Context);
copy: fn(c: self ref Context, copyenv: int): ref Context;
run: fn(c: self ref Context,
args: list of ref Listnode,
last: int): string;
addmodule: fn(c: self ref Context, name: string,
mod: Shellbuiltin);
addbuiltin: fn(c: self ref Context, name: string,
mod: Shellbuiltin);
removebuiltin: fn(c: self ref Context, name: string,
mod: Shellbuiltin);
addsbuiltin: fn(c: self ref Context, name: string,
mod: Shellbuiltin);
removesbuiltin: fn(c: self ref Context, name: string,
mod: Shellbuiltin);
fail: fn(c: self ref Context, ename, msg: string);
options: fn(c: self ref Context): int;
setoptions: fn(c: self ref Context, flags, on: int): int;
};
Listnode: adt {
cmd: ref Cmd;
word: string;
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};
Cmd: adt {
# private data
};
Shellbuiltin: module {
initbuiltin: fn(ctxt: ref Context, sh: Sh): string;
whatis: fn(ctxt: ref Sh->Context, sh: Sh,
name: string, wtype: int): string;
runbuiltin: fn(ctxt: ref Context, sh: Sh,
cmd: list of ref Listnode,
last: int): string;
runsbuiltin: fn(ctxt: ref Context, sh: Sh,
cmd: list of ref Listnode): list of ref Listnode;
getself: fn(): Shellbuiltin;
};
DESCRIPTION
Sh is a command-line interpreter and a scripting language;
it also presents a module interface to allow Limbo modules
to access its functionality at a lower level. The Sh module
can be used in several different ways. At the simplest
level, it can be run as a command-line program; for details
of this, see sh(1). The simplest access at the Limbo level
is through the system function, which given a draw Context
(see draw-context(2)) and a string executes the sh command
contained in s and returns its result. It catches any excep-
tions raised by the command. Almost as simple is exec,
which runs argv as a command, taking the first word as the
command to be executed (it can be a braced block) and giving
the rest as arguments, catching any exceptions raised.
Although program arguments are passed to external programs
as lists of strings, at the Sh module level, an argument
list is held as a list of ref Listnode. A Listnode holds
either a simple string, or a braced block that has been
parsed by the shell. Sometimes it can hold both; in this
case the string and the block both represent the same thing.
Parse converts from a string to a Cmd (a braced block). It
returns a tuple (cmd, error) where cmd holds the parsed
block, and error is non-empty if an error has occurred doing
so. Cmd2string performs the opposite conversion; it returns
a string that when parsed will yield the same command block
it was passed. The utility functions List2stringlist and
stringlist2list convert from and to a list of ref Listnode
to or from a list of string respectively.
A Context holds all the state information needed by a cur-
rently running sh process; this adt holds current values of
environment variables and a list of currently loaded modules
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and builtin commands. It is specific to the process within
which it was created. If it is desired to run sh commands
in a newly spawned process, a new Context must be created,
or a copy of an existing Context made (making sure to syn-
chronise access until the copy has been made).
Context.new(drawcontext)
New creates a new context. Drawcontext represents
the current graphics context within which sh com-
mands will be run (see draw-context(2)).
ctxt.get(name)
Get retrieves the value of environment variable
name from ctxt. It is retrieved from the innermost
scope in which a value for name has been set.
ctxt.set(name, value)
Set sets the value of environment variable name in
ctxt to value. It is set in the innermost scope in
which a value for name has been set, or the outer-
most level if it has not been set.
ctxt.setlocal(name, value)
Similar to set() except that the value is set in
the innermost scope that has been pushed.
ctxt.envlist()
Envlist retrieves the list of all the environment
variables currently in scope, and their values.
It returns a list of (name, value) tuples, where
name is the name of the variable and value is its
value.
ctxt.push()
Push creates a new innermost environment variable
scope.
ctxt.pop()
Pop discards the current innermost scope, losing
the values of all variables that have been defined
there. It is an error to pop a context that has
not been pushed. Care must be taken to ensure
that a push is always matched by a pop. In par-
ticular, exceptions should be caught, the context
popped, and the exception re-raised.
ctxt.copy(copyenv)
The shell's Context is associated with a particu-
lar process; copy returns a copy of ctxt associ-
ated with the current process. If copyenv is non-
zero, the whole environment will be copied - this
should be set if the new process is to run
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asynchronously - i.e. if there is a chance that
there might be two processes accessing the context
in parallel. It is an error to copy a context if
not within a new process.
ctxt.run(args, last)
Run executes a sh command. Last should be non-
zero if this is the last time that run will be
called, so sh does not have to spawn a new process
in order to hide file redirection side-effects.
ctxt.addmodule(name, mod)
Addmodule adds the Shellbuiltin module mod to its
list of builtin modules. The module will be ini-
tialised as described in ``Builtin modules'',
below.
ctxt.addbuiltin(name, mod)
Addbuiltin may be called by a module that has pre-
viously been loaded by addmodule or by the load sh
command to add a new builtin command to the shell.
Any subsequent invocation of name within ctxt will
result in a call of runbuiltin() to mod. Any
attempt to redefine the command ``builtin'' will
be ignored.
ctxt.removebuiltin(name, mod)
Removebuiltin removes name from the list of buil-
tin commands in ctxt. If name had not previously
been defined by mod, or had subsequently been
replaced, then this function does nothing.
ctxt.addsbuiltin(name, mod)
Addsbuiltin may be called by a module that has
previously been loaded by addmodule or by the load
sh command to add a new builtin substitution oper-
ator to the shell. Any subsequent invocation of
${name} within ctxt will result in a call of
runsbuiltin() to mod.
ctxt.removesbuiltin(name, mod)
Removesbuiltin removes name from the list of buil-
tin substitution operators in ctxt. If name had
not previously been defined by mod, or had subse-
quently been replaced, then this function does
nothing.
ctxt.fail(ename, msg)
Fail prints msg to the standard error if message
printing is currently enabled, and raises the
exception fail:ename.
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ctxt.options()
Options returns a bitmask of the options currently
enabled in ctxt. The bits are defined by constants
declared within Context. They include:
ctxt.INTERACTIVE
Sh is currently being run from an interactive
command-line.
ctxt.VERBOSE
Message printing is currently enabled.
ctxt.EXECPRINT
Commands are printed to standard error as
they are executed.
ctxt.ERROREXIT
An exception will be raised when the first
simple command returns an error status.
Options are defined in the innermost scope of ctxt
and will be lost when it is popped.
ctxt.setoptions(flags, on)
Setoptions sets the specified flags within ctxt.
Flags is a bitmask of options, as described in
options, above. If on is non-zero, the specified
bits will be set; otherwise they will be reset.
Setoptions returns the previously set options bit-
mask.
Builtin modules
Shellbuiltin specifies the interface to a loadable sh buil-
tin module. Any Limbo module mod adhering to this interface
may be loaded into the shell.
mod->initbuiltin(ctxt, sh)
Initbuiltin is called when sh loads mod either via
the load command, or via the loadmodule() func-
tion. Ctxt is the context within which the buil-
tin has been loaded, and sh is the Sh module
itself. When initbuiltin is called, mod is
expected to call ctxt.addbuiltin and
ctxt.addsbuiltin to define any builtin commands
and builtin substitution operators that it wants.
If an error occurs on initialisation, initbuiltin
should return a non-nil value; this will cause the
load to fail.
mod->runbuiltin(ctxt, sh, cmd, last)
Runbuiltin is invoked when sh executes a command
that has previously been defined as a builtin
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command by mod. Ctxt is the current execution con-
text (which may not be the original context passed
to initbuiltin()), sh is the running Sh module,
and cmd is the command to be executed. Last is
true if this is the last command to be executed in
the current process; it can be passed to
ctxt.run() as appropriate. The name of the com-
mand can be found in (hd cmd).word. Runbuiltin
returns its exit status; by convention this is the
exit status of the last command executed. A non-
nil exit status is usually treated as false. By
convention, if an invalid set of arguments are
passed to a builtin command, a usage exception is
raised by calling ctxt.fail with usage and an
explanatory usage message as arguments.
mod->runsbuiltin(ctxt, sh, cmd)
Similar to runbuiltin, runsbuiltin is called when
sh encounters a builtin substitution operator that
has previously been defined by mod. It returns the
list of values that will be substituted in place
of the operator.
mod->getself()
Getself should return the Shellbuiltin module han-
dle for mod, usually obtained by invoking load
$self. N.B. it is important that the value
returned by getself is the same as that passed to
addbuiltin or addsbuiltin. As the Limbo load
operator returns a different value each time, the
value to be returned by getself() should be ini-
tialised once, during the call to initbuiltin().
mod->whatis(ctxt, sh, name, wtype)
Whatis is called by the shell's whatis command to
query the definition of a name. Wtype gives the
type of name that is being asked about; it can be
BUILTIN (conventional commands), SBUILTIN (substi-
tution builtins), or OTHER (any other names that
the module defines). Return nil to get the usual
default behaviour. The std module, for example,
uses this feature to display the definition of a
shell function correctly.
Exceptions
The exceptions used within sh are exactly the same as those
used within Limbo, except that all exceptions generated by
the shell are prefixed by the string ``fail:'', and any
exception caught with the prefix fail: has its first 5 char-
acters removed before being made available to the sh script.
This adheres to the convention defined by other shells
within Inferno that a process that raises an exception with
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a fail: prefix is just returning a non-zero exit status, and
should not be left in a Broken state. It also means that
the number of bytes available for the exception string is
reduced by 5 (to 59). Care must therefore be taken to avoid
generating an exception with a name that is too long; sh
takes the pragmatic approach of truncating any exception
string that is too long.
FILES
/prog/pid/wait
The file used by the shell to wait for dead child
processes.
SOURCE
/appl/cmd/sh/sh.y
SEE ALSO
sh(1), sh-std(1), sh-expr(1), sh-tk(1), sys-exception(2)
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