Monday, April 13, 2009


Got somewhat interested with the korn shell and looked up some faq's regarding it...

credits to


Q1. What is KornShell?
A1. KornShell is a command and scripting language
that is a superset of the System V UNIX shell,
aka, BourneShell (or 'sh').

Q2. What is ksh?
A2. ksh is the name of the program that implements the
KornShell language.

Q3. What is the history of ksh?
A3. ksh was written by David Korn at Bell Telephone Laboratories.
David Korn is currently at AT&T Laboratories.
The first version of ksh was in 1983. It was the first
shell to have command line editing with both emacs
and vi style interaction. The 1986 version was the first
to offer multibyte support. The 1988 version of ksh
is the version that was adopted by System V Release 4 UNIX
and was a source document for the IEEE POSIX and ISO
shell standards. The 1993 version is a major rewrite
of the 1988 version and focuses more on scripting.

Q4. Where is the official description of the KornShell language?
A4. The Bolsky and Korn book, "The KornShell Command and Programming
Language", published by Prentice Hall, defines the 1988
version. The newer Bolsky and Korn book, "The New KornShell Command
and Programming Language", also published by Prentice Hall,
describes the 1993 version.

Q5. What are the major new features of KornShell 1993?
A5. The only major new interactive feature is key binding.
Major new language features are floating point arithmetic,
associative arrays, complete ANSI-C printf, name reference
variables, new expansion operators, dynamic loading of
built-in commands, active variables, and compound variables.
Active and compound variables allow shell variables to
behave like objects. In addition, ksh93 has been written to be
extensible with an C language API for programming extensions.

Q6. Are any further releases of ksh planned?
A6. Yes, we are in the process of planning for a newer version,
ksh200X. We are interested in suggestions for new features.
Again, most of the focus will be on scripting and reusability.

Q7. What new features are planned for ksh200X?
A7. We are in the early stage of planning but the likely additions
are namespaces, an inheritance mechanism for objects,
and support for binary objects. Support for multi-threading
is also being considered.

Q8. Is KornShell public domain?
A8. Yes, the language description is public domain and
can be reimplemented. Some of the KornShell language
features have been reimplemented in the GNU shell, bash
and in pdksh, a public domain implementation.

Q9. Is ksh public domain?
A9. No, earlier versions were owned by both AT&T and Novell.
The 1993 version is owned by both Lucent and AT&T.

Q10. Is source code available?
A10. Starting in March 2000, the ksh93 source is available
as part of a larger collection of software called
the ast-open software package which can be downloaded
from the site

Q11. What are the licensing terms?
A11. The exact license terms can be found in

Q12. Does the license allow binaries to be freely redistributed?
A12. Yes, provided you make the license terms available to
everyone you distribute binaries to.

Q13. If I make changes to the code, do I have to make them public?
A13. No, you do not have to make them public. However, if you
distribute the changes, you must allow us to be able
to get these changes and distribute them along with the source.

Q14. Why do most vendors ship ksh88, not ksh93?
A14. Since ksh88 was included in System V release 4, most vendors
have just included this version. A separate license was
required for ksh93 from either Lucent or AT&T. We hope this
situation changes with the release of ksh93 in open source form.

Q15. Do you provide support for ksh?
A15. No, we will try to fix any bugs we hear about in future
releases, but we do not provide any official support.

Q16. Is ksh supported commercially?
A16. Global Technologies, Ltd,, distributes
and supports the "Common Operating Environment" which includes
ksh all of the other ast-open tools. They support it across
a broad range of systems. Software vendors that supply ksh with
their systems typically support it for that system.

Q17. What is pdksh and is it related to ksh or KornShell?
A17. pdksh is a public domain version of a UNIX shell that is
unrelated to ksh. It supports most of the 1988
KornShell language features and some of the 1993 features.
Some KornShell scripts will not run with pdksh.

Q18. How is the MKS Toolkit Kornshell related to KornShell?
A18. MKS Tookit Kornshell is a completely independent implementation
that supports a subset of the 1988 KornShell language.

Q19. What systems does ksh run on?
A19. ksh has been written to be portable. It has been ported
to virtually every known UNIX system. In addition it runs
on non-UNIX systems such as IBM's MVS using OpenEdition, and
Microsoft's Windows 9X, Windows NT and Windows 2000.
ksh is part of the UWIN (Unix for Windows) software,

Q20. Does ksh conform to the IEEE POSIX and ISO shell standard?
A20. The 1993 version should conform to the 1992 standard. At one
point it had passed the test suite created by X/OPEN.

Q21. Will KornShell 88 scripts run with KornShell 93?
A21. In almost all cases, the answer is yes. However,
the IEEE POSIX and ISO standards required a few
changes that could cause scripts to fail. There is a
separate document that lists all known incompatibilities.

Q22. Can ksh run as /bin/sh?
A22. We have installed ksh as /bin/sh on several systems without
encountering any problems. Our Linux systems use this
instead of bash.


Q1. How do I get separate history files for shell?
A1. ksh uses a shared history file for all shells that
use the same history file name. This means
that commands entered in one window will be seen by
shells in other windows. To get separate windows,
the HISTFILE variable needs to be set to different name
before the first history command is created.

Q2. Why does the screen width not function correctly when non-printing characters are in my prompt?
A2. The shell computes the screen width by subtracting the width of
the prompt from the screen width. To account for non-printing
characters, for example escape sequences that display in the title
bar, follow these characters with a carriage return. The shell
starts recomputing the width after each carriage return.

Q3. What is the PS4 prompt and how is it used?
A3. The PS4 prompt is evaluated and displayed before each line when
running an execution trace. If unset, a + and a will
be output before each line in the trace. Putting '$LINENO'
inside PS4 will cause the line number to be displayed. Putting
'$SECONDS' in the PS4 prompt will cause the elapsed time
to be displayed before each line. Note that single quotes
are used to prevent the expansion from happening when PS4
is defined.

Q4. How is keybinding done?
A4. ksh93 provides a KEYBD trap that gets executed whenever a key
is entered from the keyboard. Using this trap, and the associate
array feature of ksh93, a keybind function can easily be written
which will map any entered key sequence to another key sequence.

Q5. How do I get the arrow keys to work?
A5. Starting with the 'h' point release, on most keyboards you
do not have to do anything to get the arrow keys to work.
However, if they do not generate standard escape sequences,
then you will have to use a keybinding function to get them
to work.

Q6. Does ksh support file name completion?
A6. Yes, it does. The default key binding is
however, starting with the 'g' point release, also works
for completion. Note, the vi users need to set -o viraw
in order to get completion to work.

Q7. Does ksh support command completion?
A7. If you perform completion on the first word of a command,
ksh will do completion using aliases, functions, and commands.

Q8. Is completion programmable?
A8. Yes, using the key binding mechanism, you can script the behavior
of any key and therefore cause the current contents of any
line to be replaced by any other line.

Q9. Is there any way to get the command-line editor to go to more than a single line?
A9. In vi-mode, if you hit 'v' while in control mode, it will bring
up a full screen version of vi on the current command. The command
will execute when you exit vi.

Q10. Can I use the shell line editor on other commands?
A10. The command ie, that comes along with shell, can be used
to run line input oriented commands with command line editing.

Q11. When I do echo 0, I am getting 267. What does this mean?
A11. ksh93 reports process that terminate with a signal as 256+signo.
Earlier versions used 128+signo but this makes it impossible
to distinguish from a command exit with that value. If you run
kill -l $?
on this signal number, it will give the the name of the signal
that caused this exit.

Q12. When I type builtin, I notice that some of these are full pathnames. What does this mean?
A12. Builtins that are not bound to pathnames are always searched
for before doing a path search. Builtins that are bound
to pathnames are only executed when the path search would
bind to this pathname.

Q13. What is a self generating man page?
A13. A self generating man page is one that is generated by the
option parser within that command using an extended version
of the getopts function. The man page can be generated in html,
troff, or directly for the terminal. Most builtin commands
in the shell have self generating man pages so that you
can run for example, kill --man or kill --html to get
the description of kill to the screen or as an html file.
This same method can also be used for shell scripts. Run
getopts --man for more details.

Q14. What is autoloading?
A14. Autoloading was a method used in ksh88, and still permitted in ksh93
to declare that a name corresponded to a function. The function
would be loaded and executed when first referenced. This was
necessary since FPATH was always searched after PATH with ksh88
and therefore if you defined a function whose name was the same
as that of a program on your path, the program on your path
would have been executed. With ksh93, when a pathname is
encountered that is on PATH, but also is in FPATH, this directory
is assumed to be a function directory. Thus, you can have
function directories searched before program directories so
that autoloading is no longer needed.


Q1. What is the difference between * and @, for example, and ?
A1. When used outside of "", they are equivalent. However, within
double quotes, "$@" produces one argument for each positional
parameter, and "$* produces a single argument. Note that "$@"
preserves arguments lists, whereas $* may not unless both
word splitting and pathname expansion are disabled.

Q2. Why do I need spaces around { and } but not around ( and )?
A2. The characters ( and ) are shell metacharacters and are always
treated specially. For historical reasons, { and } were
treated as reserved words and are only special as separate
words at locations in which a command can begin.

Q3. How do I get read to maintain the \ characters?
A3. Use read -r instead.

Q4. How can a write a ksh script that responds directly to each
character so that you user just has to enter y, not y?
A4. There are two ways to do this. The easiest is to use
read -n1 x
Alternatively, you could do
function keytrap
trap keytrap KEYBD
and then
read x

Q5. What is the purpose of $'...'?
A5. The $'...' option was added to ksh93 to solve the problem
of entering special characters in scripts. It uses
ANSI-C rules to translate the string between the '...'.
It would have been cleaner to have all "..." strings handle
ANSI-C escapes, but that would not be backwards compatible.

Q6. What is the -n option used for?
A6. You should always run ksh -n on each script you write. The -n
option will check for syntax errors on paths that might not
even be checked when you run the script. It also produces
a number of warning messages.

Q7. Why are both `...` and $(...) used for command substitution?
A7. The `...` method has some rather strange quoting rules
and does not nest easily. $(...) was added to ksh88 to
make command substitution easy to use. `...` is provided
for backwords compatibility only.

Q8. How can I tell if all the commands of a pipeline have succeeded?
A8. The pipefail option was added to the 'g' point release of ksh93.
With pipefail set, a pipeline will fail if any element of the
pipeline fails. The exit status will be that of the first
command that has failed.

Q9. How do I connect to a socket from a shell script?
A9. exec 3<> /dev/tcp/hostname/portnum
will open a tcp connection to portnum on hostname for
reading and writing on file descriptor 3. You can then
use read and print statements with file descriptor 3,
or redirection operators <&3 or >&3 to use these connections.

Q10. What is the difference between [...] and [[...]]?
A10. The [[...]] is processed as part of the shell grammar
whereas [...] is processed like any other command.
Operators and operands are detected when the command is
read, not after expansions are performed. The shell does not
do word splitting or pathname generation inside [[...]].
This allows patterns to be specified for string matching

Q11. How come [[ $foo == $bar ]] is true and [[ $bar == $foo ]] is false?
A11. The == operator is not symmetrical. It takes a string on the left
and a pattern on the right. However, if you double quote the right
hand side, which removes the special meaning of pattern match
characters, then this becomes a string comparison so that
[[ "$foo" == "bar" ]] and [[ "$bar" == "$foo" ]] are equivalent.

Q12. Why does have print since echo already exists is is widely used?
A12. The behavior of echo varies from system to system.
The POSIX standard does not define the behavior of echo when
the first argument beings with a - or when any argument
contains a \ character. This makes echo pretty useless for
use in portable scripts.

Q13. What is $bar after, echo foo | read bar?
A13. The is foo. ksh runs the last component of a pipeline
in the current process. Some shells run it as a subshell
as if you had invoked it as echo foo | (read bar).

Q14. What is the difference between ((expr)) and $((expr))?
A14. ((expr)) is a command that evaluates an arithmetic expression.
The exit status of this command is 0 if the expression
evaluates to non-zero and is 1 if it evaluates to 0.
0 is an string expansion that expands to a string
representation of the value of this arithmetic expression.
It can be used anywhere a variable substitution is premitted.

Q15. What is the difference between $((x*y)) and $(($x*$y))?
A15. In the first case the value of x and the value of y are multiplied
together, and then their result is converted to a string. In the
second case variables $x, *, and $y are concatenated to form
an arithmetic expression which is then evaluated. This can
yield different results, for example,
x=2+3 y=4+5
print $((x*y)) \$(($x*$y))
45 19

Q16. How do I handle filenames with spaces in them?
A16. To be POSIX conforming, ksh has to do word splitting and
pathname expansion the results of substitutions. You can
enclose variable substitutions in "..." to prevent both
word splitting and pathname expansion. Alternatively,
you can disable word splitting by setting IFS='' and
pathname generation with set -o noglob.

Q17. What are active variables?
A17. By default shell variables are passive. They hold values
given to them on assignment, and return values on reference.
Active variables allow the assignment and reference (and
other actions) be controlled by functions specific to that
variable. At the shell level, a 'get', 'set', or 'unset'
shell function can be defined for any variable to make them
active, so that the function foo.set will be invoked whenever
the variable foo is assigned a value. At the C interface
level, several functions can be stacked together for an
active variable.

Q18. What is the difference between function name and name()?
A18. In ksh88 these were the same. However, the POSIX standard
choose foo() for functions and defined System V Release 2
semantics to them so that there are no local variables
and so that traps are not scoped. ksh93 keeps the ksh88
semantics for functions defined as function name, and
has changed the name() semantics to match the POSIX
semantics. Clearly, function name is more useful.

Q19. What are name reference variables and how are they used?
A19. Reference variables are variables in which all references
and assignments refer to the variable that they reference.
For example,
typeset -n name=$1
is equivalent to
eval \$1='value'
References are most useful for passing arguments such as
arrays to functions.

Q20. If i=1 and var1=some value, how do I print vartt to get its value?
A20. Either use
eval print var\$i
typeset -n x=var$i
print $x

Q21. How can I shift the elements of an array?
A21. The shift special builtin-command only works for positional
parameters. However, noting that array subscripts start at 0,
you can use
set -A name "${name[@]:1}"
to shift the array.

Q22. Why are the braces required with array references, e. g. ${x[1]}?
A22. It would be nice to do $x[1], but the POSIX shell would expand $x
and then search for the file pattern resulting by concatenating [1].
ksh is POSIX compatible.

Q23. How do I get the list of subscript names for an associative array?
A23. The prefix operator ! in variable expansions can be used to
get names. To get the names of subscripts for an array, associative
or indexed, use ${!var[@]}.

Q24. How do I do global substitutions on the contents of shell variables?
A24. Use // instead of / for global substitution, ${var//aa/bb} will
expand to the value of with each "aa" replace by "bb".

Q25. How can I convert %XX values to ascii?
A25. You can convert this to a sequence of ANSI C strings and then eval that
string, for example suppose the variable 'foo' contains %XX strings,
eval print -r -- "\$'${foo//'%'@(??)/'\x\1"'\$'"}'"
will print out the string in ascii.

Q26. I want to use exec to open a file. How do I prevent the script from exiting if the exec fails?
A26. If you run
command exec ... || error ...
then error will be executed if the exec fails, but the script
will not terminate. The command builtin will prevent the shell
from exiting when special built-ins fail.

Q27. How do I execute a builtin inside a function of the same name?
A27. You use the command builtin for this. For example,
function cd
command cd "$@" && title "$PWD"
will run the builtin command cd from within the function cd
rather than calling the function cd recursively.

Q28. How are variables scoped in ksh?
A28. The scoping of variables was not defined for ksh88 but in ksh93
static scoping was specified. For example the output from
function f1
print foo=$foo
function f2
typeset foo=local
will be "global". To get f2 to cause f1 to print the local
value of foo, f2 can run "foo=$foo f1" instead.

Q29. Can you write a self reproducing program in KornShell?
A29. Yes, the following program is self reproducing. Any shorter ones?
" q="'" x="cat <<-!" y=! z='n="$n" q="$q" x="$x" y=$y z=$q$z$q$n$x$n$z$n$y'
cat <<-!
n="$n" q="$q" x="$x" y=$y z=$q$z$q$n$x$n$z$n$y


Q1. Is there a shell compiler?
A1. There is a separate command named shcomp that will convert
a script into an intermediate machine independent form. The shell
will detect this format whenever it runs a script and execute
directly from this intermediate format.

Q2. What is the advantage of making commands built-in?
A2. The startup time is reduced by a couple of orders of
magnitude. In addition, built-in commands can access
ksh internals.

Q3. What is the disadvantage of making commands built-in?
A3. Errors in these built-ins can cause the shell to crash.

Q4. How do I add built-in commands?
A4. There are two ways to do this. One is write a shared library
with functions whose names are b_xxxx where xxxx is the name of
the builtin. The function b_xxxx takes three argument. The first
two are the same as a mail program. The third parameter is
a pointer argument which will point to the current shell context.
The second way is to write a shared library with a function named
lib_init(). This function will be called with an argument of 0
after the library is loaded. This function can add built-ins
with the sh_addbuiltin() API function. In both cases, the
library is loaded into the shell with the "builtin" utility.

Q5. Can ksh93 be embedded?
A5. Yes, ksh93 can be compiled as a shared or dynamically linked
library which can be embedded into applications. There is
an API for interfacing to shell variables and to several of
the internal shell functions.

Q6. Can I write GUI applications with ksh?
A6. There are two extensions to ksh that can be used to write
GUI applications as shell script. One is dtksh which
was written by Steve Pendergrast at Novell and is
included with the Common Desktop Environment, CDE. The other is
tksh which was written by Jeff Korn. tksh combines the tk graphics
package with ksh93 and reimplements the tcl language
as an extension so that both tcl and ksh scripts
can run in the same address space. The source for tksh
is included in the ast-open package.

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