Guidelines for writing ksh-93 built-in commands

David G. Korn


One of the features of ksh93, the latest version of ksh, is the ability to add built-in commands at run time. This feature only works on operating systems that have the ability to load and link code into the current process at run time. Some examples of the systems that have this feature are Linux, System V Release 4, Solaris, Sun OS, HP-UX Release 8 and above, AIX 3.2 and above, and Microsoft Windows systems.

This memo describes how to write and compile programs that can be loaded into ksh at run time as built-in commands.


A built-in command is executed without creating a separate process. Instead, the command is invoked as a C function by ksh. If this function has no side effects in the shell process, then the behavior of this built-in is identical to that of the equivalent stand-alone command. The primary difference in this case is performance. The overhead of process creation is eliminated. For commands of short duration, the effect can be dramatic. For example, on SUN OS 4.1, the time to run wc on a small file of about 1000 bytes, runs about 50 times faster as a built-in command.

In addition, built-in commands may have side effects on the shell environment. This is usually done to extend the application domain for shell programming. For example, there is a group of X-windows extension built-ins that make heavy use of the shell variable namespace. These built-ins are added at run time and result in a windowing shell that can be used to write X-windows applications.

While there are definite advantages to adding built-in commands, there are some disadvantages as well. Since the built-in command and ksh share the same address space, a coding error in the built-in program may affect the behavior of ksh; perhaps causing it to core dump or hang. Debugging is also more complex since your code is now a part of a larger entity. The isolation provided by a separate process guarantees that all resources used by the command will be freed when the command completes. Resources used by a built-in must be meticulously maintained and freed. Also, since the address space of ksh will be larger when built-in are loaded, it may increase the time it takes ksh to fork() and exec() non-built-in commands. It makes no sense to add a built-in command that takes a long time to run or that is run only once, since the performance benefits will be negligible. Built-ins that have side effects in the current shell environment have the disadvantage of increasing the coupling between the built-in and ksh, making the overall system less modular and more monolithic.

Despite these drawbacks, in many cases extending ksh by adding built-in commands makes sense and allows reuse of the shell scripting ability in an application specific domain. This memo describes how to write ksh extensions.


There is a development kit available for writing ksh built-ins as part of the AST (AT&T Software Technology) Toolkit. The development kit has three directories, include, lib, and bin. It is best to set the value of the environment variable PACKAGE_ast to the pathname of the directory containing the development kit. The include directory contains a sub-directory named ast that contains interface prototypes for functions that you can call from built-ins. The lib directory contains the ast library and a library named cmd that contains a version of several of the standard POSIX[1] utilities that can be made run time built-ins. The lib/ksh directory contains shared libraries that implement other ksh built-ins. The bin directory contains build tools such as nmake[2]. To add built-ins at runtime, it is necessary to build a shared library containing one or more built-ins that you wish to add. The built-ins are then added by running builtin -f shared_lib. Since the procedure for building share libraries is system dependent, it is best to use nmake using the sample nmake makefile below as a prototype. The AST Toolkit also contains some examples of built-in libraries under the src/cmd/kshlib directory.

There are two ways to code adding built-ins. One method is to replace the function main with a function b_name, where name is the name of the built-in you wish to define. A built-in command has a calling convention similar to the main function of a program, int main(int argc, char *argv[]). except that it takes a third argument of type Shbltin_t* which can be passed as NULL if it is not used. The definition for Shbltin_t* is in <ast/shcmd.h>. Instead of exit, you need to use return to terminate your command. The return value will become the exit status of the command. The open built-in, installed in lib/ksh in the AST Toolkit, uses this method. The Shbltin_t structure contains a field named shp which is a pointer the the shell data that is needed for shell library callbacks. It also contains the fields, shrun, shtrap, shexit, and shbltin that are function pointers to the shell library functions sh_run, sh_trap sh_exit, and sh_addbuiltin, respectively. These functions can be invoked without the need for runtime symbol lookup when the shell is statically linked with libshell.

The alternative method is to create a function lib_init and use the Shbltin_t.shbltin() function to add one or more built-ins. The lib_init function will be called with two arguments. The first argument will be 0 when the library is loaded and the second argument will be of type Shbltin_t*. The dbm_t and dss shell built-ins use this method.

No matter which way you add built-ins you should add the line SHLIB(identifier) as the last line of one of the built-in source file, where identifier is any C identifier. This line provides version information to the shell builtin command that it uses to verify compatibility between the built-in and ksh implementation versions. builtin fails with a diagnostic on version mismatch. The diagnostic helps determine whether ksh is out of date and requires an upgrade or the built-in is out of date and requires recompilation.

The steps necessary to create and add a run time built-in are illustrated in the following simple example. Suppose you wish to add a built-in command named hello which requires one argument and prints the word hello followed by its argument. First, write the following program in the file hello.c:

#include     <stdio.h>
int b_hello(int argc, char *argv[], void *context)
        if(argc != 2)
                fprintf(stderr,"Usage: hello arg\n");
        printf("hello %s\n",argv[1]);

Next, the program needs to be compiled. If you are building with AT&T nmake use the following Makefile:

:PACKAGE: --shared ast
hello plugin=ksh :LIBRARY: hello.c
and run nmake install to compile, link, and install the built-in shared library in lib/ksh/ under PACKAGE_ast. If the built-in extension uses several .c files, list all of these on the :LIBRARY: line.

Otherwise you will have to compile hello.c with an option to pick up the AST include directory (since the AST <stdio.h> is required for ksh compatibility) and options required for generating shared libraries. For example, on Linux use this to compile:

cc -fpic -I$PACKAGE_ast/include/ast -c hello.c
and use the appropriate link line. It really is best to use nmake because the 2 line Makefile above will work on all systems that have ksh installed.

If you have several built-ins, it is desirable to build a shared library that contains them all.

The final step is using the built-in. This can be done with the ksh command builtin. To load the shared library from the current directory and add the built-in hello, invoke the command,

builtin -f ./ hello
The shared library prefix (lib here) and suffix (.so here) be omitted; the shell will add an appropriate suffix for the system that it is loading from. If you install the shared library in lib/ksh/, where ../lib/ksh/ is a directory on $PATH, the command
builtin -f hello hello
will automatically find, load and install the built-in on any system. Once this command has been invoked, you can invoke hello as you do any other command. If you are using lib_init method to add built-ins then no arguments follow the -f option.

It is often desirable to make a command built-in the first time that it is referenced. The first time hello is invoked, ksh should load and execute it, whereas for subsequent invocations ksh should just execute the built-in. This can be done by creating a file named hello with the following contents:

function hello
        unset -f hello
        builtin -f hello hello
        hello "$@"
This file hello needs to be placed in a directory that is in your FPATH variable, and the built-in shared library should be installed in lib/ksh/, as described above.


As mentioned above, the entry point for built-ins must either be of the form b_name or else be loaded from a function named lib_init. Your built-ins can call functions from the standard C library, the ast library, interface functions provided by ksh, and your own functions. You should avoid using any global symbols beginning with sh_, nv_, and ed_ since these are used by ksh itself. #define constants in ksh interface files use symbols beginning with SH_ and NV_, so avoid using names beginning with these too.

Header Files

The development kit provides a portable interface to the C library and to libast. The header files in the development kit are compatible with K&R C[3], ANSI-C[4], and C++[5].

The best thing to do is to include the header file <shell.h>. This header file causes the <ast.h> header, the <error.h> header and the <stak.h> header to be included as well as defining prototypes for functions that you can call to get shell services for your builtins. The header file <ast.h> provides prototypes for many libast functions and all the symbol and function definitions from the ANSI-C headers, <stddef.h>, <stdlib.h>, <stdarg.h>, <limits.h>, and <string.h>. It also provides all the symbols and definitions for the POSIX[6] headers <sys/types.h>, <fcntl.h>, and <unistd.h>. You should include <ast.h> instead of one or more of these headers. The <error.h> header provides the interface to the error and option parsing routines defined below. The <stak.h> header provides the interface to the memory allocation routines described below.

Programs that want to use the information in <sys/stat.h> should include the file <ls.h> instead. This provides the complete POSIX interface to stat() related functions even on non-POSIX systems.


ksh uses sfio, the Safe/Fast I/O library[7], to perform all I/O operations. The sfio library, which is part of libast, provides a superset of the functionality provided by the standard I/O library defined in ANSI-C. If none of the additional functionality is required, and if you are not familiar with sfio and you do not want to spend the time learning it, then you can use sfio via the stdio library interface. The development kit contains the header <stdio.h> which maps stdio calls to sfio calls. In most instances the mapping is done by macros or inline functions so that there is no overhead. The man page for the sfio library is in an Appendix.

However, there are some very nice extensions and performance improvements in sfio and if you plan any major extensions I recommend that you use it natively.

Error Handling

For error messages it is best to use the ast library function errormsg() rather that sending output to stderr or the equivalent sfstderr directly. Using errormsg() will make error message appear more uniform to the user. Furthermore, using errormsg() should make it easier to do error message translation for other locales in future versions of ksh.

The first argument to errormsg() specifies the dictionary in which the string will be searched for translation. The second argument to errormsg() contains that error type and value. The third argument is a printf style format and the remaining arguments are arguments to be printed as part of the message. A new-line is inserted at the end of each message and therefore, should not appear as part of the format string. The second argument should be one of the following:


If n is not-zero, the builtin will exit value n after printing the message.

Exit builtin with exit value n after printing the message. The message will display the message corresponding to errno enclosed within [ ] at the end of the message.

Will generate a usage message and exit. If n is non-zero, the exit value will be 2. Otherwise the exit value will be 0.

Will print a level n debugging message and will then continue.

Prints a warning message. n is ignored.

Option Parsing

The first thing that a built-in should do is to check the arguments for correctness and to print any usage messages on standard error. For consistency with the rest of ksh, it is best to use the libast functions optget() and optusage()for this purpose. The header <error.h> includes prototypes for these functions. The optget() function is similar to the System V C library function getopt(), but provides some additional capabilities. Built-ins that use optget() provide a more consistent user interface.

The optget() function is invoked as

int optget(char *argv[], const char *optstring)
where argv is the argument list and optstring is a string that specifies the allowable arguments and additional information that is used to format usage messages. In fact a complete man page in troff or html can be generated by passing a usage string as described by the getopts command. Like getopt(), single letter options are represented by the letter itself, and options that take a string argument are followed by the : character. Option strings have the following special characters:
Used after a letter option to indicate that the option takes an option argument. The variable opt_info.arg will point to this value after the given argument is encountered.
Used after a letter option to indicate that the option can only take a numerical value. The variable opt_info.num will contain this value after the given argument is encountered.
Used after a : or # (and after the optional ?) to indicate the the preceding option argument is not required.

After a : or #, the characters contained inside the brackets are used to identify the option argument when generating a usage message.

The remainder of the string will only be used when generating usage messages.

The optget() function returns the matching option letter if one of the legal option is matched. Otherwise, optget() returns

If there is an error. In this case the variable opt_info.arg contains the error string.
Indicates the end of options. The variable opt_info.index contains the number of arguments processed.
A usage message has been required. You normally call optusage() to generate and display the usage message.

The following is an example of the option parsing portion of the wc utility.

#include <shell.h>
while(1) switch(n=optget(argv,"xf:[file]"))
	case 'f':
		file = opt_info.arg;
	case ':':
		error(ERROR_exit(0), opt_info.arg);
	case '?':
		error(ERROR_usage(2), opt_info.arg);

Storage Management

It is important that any memory used by your built-in be returned. Otherwise, if your built-in is called frequently, ksh will eventually run out of memory. You should avoid using malloc() for memory that must be freed before returning from you built-in, because by default, ksh will terminate you built-in in the event of an interrupt and the memory will not be freed.

The best way to to allocate variable sized storage is through calls to the stak library which is included in libast and which is used extensively by ksh itself. Objects allocated with the stakalloc() function are freed when you function completes or aborts. The stak library provides a convenient way to build variable length strings and other objects dynamically. The man page for the stak library is contained in the Appendix.

Before ksh calls each built-in command, it saves the current stack location and restores it after it returns. It is not necessary to save and restore the stack location in the b_ entry function, but you may want to write functions that use this stack are restore it when leaving the function. The following coding convention will do this in an efficient manner:

        char	*savebase;
        int	saveoffset;
        	savebase = stakfreeze(0);


Some of the more interesting applications are those that extend the functionality of ksh in application specific directions. A prime example of this is the X-windows extension which adds builtins to create and delete widgets. The nval library is used to interface with the shell name space. The shell library is used to access other shell services.

The nval library

A great deal of power is derived from the ability to use portions of the hierarchal variable namespace provided by ksh-93 and turn these names into active objects.

The nval library is used to interface with shell variables. A man page for this file is provided in an Appendix. You need to include the header <nval.h> to access the functions defined in the nval library. All the functions provided by the nval library begin with the prefix nv_. Each shell variable is an object in an associative table that is referenced by name. The type Namval_t* is pointer to a shell variable. To operate on a shell variable, you first get a handle to the variable with the nv_open() function and then supply the handle returned as the first argument of the function that provides an operation on the variable. You must call nv_close() when you are finished using this handle so that the space can be freed once the value is unset. The two most frequent operations are to get the value of the variable, and to assign value to the variable. The nv_getval() returns a pointer the the value of the variable. In some cases the pointer returned is to a region that will be overwritten by the next nv_getval() call so that if the value isn't used immediately, it should be copied. Many variables can also generate a numeric value. The nv_getnum() function returns a numeric value for the given variable pointer, calling the arithmetic evaluator if necessary.

The nv_putval() function is used to assign a new value to a given variable. The second argument to putval() is the value to be assigned and the third argument is a flag which is used in interpreting the second argument.

Each shell variable can have one or more attributes. The nv_isattr() is used to test for the existence of one or more attributes. See the appendix for a complete list of attributes.

By default, each shell variable passively stores the string you give with with nv_putval(), and returns the value with getval(). However, it is possible to turn any node into an active entity by assigning functions to it that will be called whenever nv_putval() and/or nv_getval() is called. In fact there are up to five functions that can associated with each variable to override the default actions. The type Namfun_t is used to define these functions. Only those that are non-NULL override the default actions. To override the default actions, you must allocate an instance of Namfun_t, and then assign the functions that you wish to override. The putval() function is called by the nv_putval() function. A NULL for the value argument indicates a request to unset the variable. The type argument might contain the NV_INTEGER bit so you should be prepared to do a conversion if necessary. The getval() function is called by nv_getval() value and must return a string. The getnum() function is called by by the arithmetic evaluator and must return double. If omitted, then it will call nv_getval() and convert the result to a number.

The functionality of a variable can further be increased by adding discipline functions that can be associated with the variable. A discipline function allows a script that uses your variable to define functions whose name is varname.discname where varname is the name of the variable, and discname is the name of the discipline. When the user defines such a function, the settrap() function will be called with the name of the discipline and a pointer to the parse tree corresponding to the discipline function. The application determines when these functions are actually executed. By default, ksh defines get, set, and unset as discipline functions.

In addition, it is possible to provide a data area that will be passed as an argument to each of these functions whenever any of these functions are called. To have private data, you need to define and allocate a structure that looks like

struct yours
        Namfun_t	fun;

The shell library

There are several functions that are used by ksh itself that can also be called from built-in commands. The man page for these routines are in the Appendix.

The sh_addbuiltin() function can be used to add or delete builtin commands. It takes the name of the built-in, the address of the function that implements the built-in, and a void* pointer that will be passed to this function as the third agument whenever it is invoked. If the function address is NULL, the specified built-in will be deleted. However, special built-in functions cannot be deleted or modified.

The sh_fmtq() function takes a string and returns a string that is quoted as necessary so that it can be used as shell input. This function is used to implement the %q option of the shell built-in printf command.

The sh_parse() function returns a parse tree corresponding to a give file stream. The tree can be executed by supplying it as the first argument to the sh_trap() function and giving a value of 1 as the second argument. Alternatively, the sh_trap() function can parse and execute a string by passing the string as the first argument and giving 0 as the second argument.

The sh_isoption() function can be used to set to see whether one or more of the option settings is enabled.


POSIX - Part 2: Shell and Utilities, IEEE Std 1003.2-1992, ISO/IEC 9945-2:1993.
Glenn Fowler, A Case for make, Software - Practice and Experience, Vol. 20 No. S1, pp. 30-46, June 1990.
Brian W. Kernighan and Dennis M. Ritchie, The C Programming Language, Prentice Hall, 1978.
American National Standard for Information Systems - Programming Language - C, ANSI X3.159-1989.
Bjarne Stroustroup, C++, Addison Wesley, xxxx
POSIX - Part 1: System Application Program Interface, IEEE Std 1003.1-1990, ISO/IEC 9945-1:1990.
David Korn and Kiem-Phong Vo, SFIO - A Safe/Fast Input/Output library, Proceedings of the Summer Usenix, pp. , 1991.

March 13, 2012