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UNIX下SH的实现源码

Some examples of ready-to-dynamic-load builtins.  Most of the
examples given are reimplementations of standard commands whose
execution time is dominated by process startup time.  The
exceptions are sleep, which allows you to sleep for fractions
of a second, finfo, which provides access to the rest of the
elements of the `stat' structure that `test' doesn't let you
see, and pushd/popd/dirs, which allows you to compile them out
of the shell.

All of the new builtins in ksh93 that bash didn't already have
are included here, as is the ksh `print' builtin.

The configure script in the top-level source directory uses the
support/shobj-conf script to set the right values in the Makefile,
so you should not need to change the Makefile.  If your system
is not supported by support/shobj-conf, and it has the necessary
facilities for building shared objects and support for the
dlopen/dlsyn/dlclose/dlerror family of functions, please make
the necessary changes to support/shobj-conf and send the changes
to [e-mail].

Loadable builtins are loaded into a running shell with

	enable -f filename builtin-name

enable uses a simple reference-counting scheme to avoid unloading a
shared object that implements more than one loadable builtin before
all loadable builtins implemented in the object are removed.

Many of the details needed by builtin writers are found in hello.c,
the canonical example.  There is no real `builtin writers' programming
guide'.  The file template.c provides a template to use for creating
new loadable builtins.