This shell is spelt either “mksh” (with, even at the beginning of a sentence, an initial lowercase letter; this is important) or “MirBSD Korn Shell”, possibly with “the”.
I usually pronounce it as “em-ka-es-ha”, that is, the letters individually in my native German, or say “MirBSD Korn Shell”, although it is manageable, mostly for Slavic speakers, to actually say “mksh” as if it were a word ☺
Oh… I’ve run into this one, didn’t I? “MirBSD” is pronounced “Mir-Be-Es-De” germanically, for anglophones “Mir-beas’tie” is fine.
---- ToC: sowhatismksh Title: I’m a $OS (Android, OS/2, …) user, so what’s mksh?mksh is a so-called (Unix) “shell” or “command interpreter”, similar to COMMAND.COM, CMD.EXE or PowerShell on other operating systems you might know. Basically, it runs in a terminal (“console” or “DOS box”) window, taking user input and running that as commands. It’s also used to write so-called (shell) “script”s, short programs made by putting several of those commands into a “batch file”.
On Android, mksh is used as the system shell — basically, the one running commands at system startup, in the background, and on user behalf (but never of its own). Any privilege pop-ups you might be encountering are therefore not caused by mksh but by some other code invoking mksh to do something on its behalf.
---- ToC: os2 Title: I’m an OS/2 user, what else do I need to know?Unlike the native command prompt, the current working directory is, for security reasons common on Unix systems which the shell is designed for, not in the search path at all; if you really need this, run the command PATH=.$PATHSEP$PATH or add that to a suitable initialisation file (~/.mkshrc).
There are two different newline modes for mksh-os2: standard (Unix) mode, in which only LF (0A hex) is supported as line separator, and “textmode”, which also accepts ASCII newlines (CR+LF), like most other tools on OS/2, but creating an incompatibility with standard mksh. If you compiled mksh from source, you will get the standard Unix mode unless -T is added during compilation; however, you will most likely have gotten this shell through komh’s port on Hobbes, or from his OS/2 Factory on eComStation Korea, which uses “textmode”, though. Most OS/2 users will want to use “textmode” unless they need absolute compatibility with Unix mksh and other Unix shells and tools.
---- ToC: kornshell Title: How does this relate to ksh or the Korn Shell?The Korn Shell (AT&T ksh) was authored by David Korn; two major flavours exist (ksh88 and ksh93), the latter having been maintained until 2012 (last formal release) and 2014 (last beta snapshot, buggy). A ksh86 did exist.
There’s now ksh2020, a project having restarted development around November 2017 forking the last ksh93 v- (beta) snapshot and continuing to develop it, presented at FOSDEM.
AT&T ksh88 is “the (original) Korn Shell”. Other implementations, of varying quality (MKS Toolkit’s MKS ksh being named as an example of the lower end, MirBSD’s mksh at the upper end). They are all not “Korn Shell” or “ksh”. However, mksh got blessed by David Korn, as long as it cannot be confused with the original Korn Shell.
The POSIX shell standard, while lacking most Korn Shell features, was largely based on AT&T ksh88, with some from the Bourne shell.
mksh is the currently active development of what started as the Public Domain Bourne Shell in the mid-1980s with ksh88-compatibl-ish extensions having been added later, making the Public Domain Korn Shell (pdksh), which, while never officially blessed, was the only way for most to get a Korn Shell-like command interpreter for AT&T’s was proprietary, closed-source code for a very long time. pdksh’s development ended in 1999, with some projects like Debian and NetBSD® creating small bug fixes (which often introduced new bugs) as part of maintenance. Around 2003, OpenBSD started cleaning up their shipped version of pdksh, removing old and compatibility code and modernising it. In 2002, development of what is now mksh started as the system shell of MirBSD, which took over almost all of OpenBSD’s cleanup, adding compatibility to other operating systems back on top of it, and after 2004, independent, massive development of bugfixes including a complete reorganisation of the way the parser works, and of new features both independent and compatible with other shells (ksh93, GNU bash, zsh, BSD csh) started and was followed by working with the group behind POSIX to fix issues both in the standard and in mksh. mksh became the system shell in several other operating systems and Linux distributions and Android and thus is likely the Korn shell, if not Unix shell, flavour with the largest user base. It has replaced pdksh in all contemporary systems except QNX, NetBSD® and OpenBSD (who continue to maintain their variant on “low flame”).
dtksh is the “Desktop Korn Shell”, a build of AT&T ksh93 with some additional built-in utilities for graphics programming (windows, menu bars, dialogue boxes, etc.) utilising Motif bindings.
MKS ksh is a proprietary reimplemention aiming for, but not quite getting close to, ksh88 compatibility.
SKsh is an AmigaOS-specific Korn Shell-lookalike by Steve Koren.
The Homepage of the #ksh channel on Freenode IRC contains more information about the Korn Shell in general and its flavours.
---- ToC: packaging Title: How should I package mksh? (common cases)Export a few environment variables, namely CC (the C compiler), CPPFLAGS (all C præprocessor definitions), CFLAGS (only compiler flags, no -Dfoo or anything!), LDFLAGS (for anything to pass to the C compiler while linking) and LIBS (appended to the linking command line after everything else. You might wish to export LDSTATIC=-static for a static build as well.
When cross-compiling, CC is the cross compiler (mksh currently does not require a compiler targetting the build system), but you must also export TARGET_OS to whatever system you are compiling for, e.g. “Linux”. For most operating systems, that’s just the uname(1) output. Some very rare systems also need TARGET_OSREV; consult the source code of Build.sh for details.
Create two subdirectories, say build-mksh and build-lksh. In each of them, start a compilation by issuing sh ../Build.sh -r followed by running the testsuite¹ via ./test.sh. For lksh(1) add -DMKSH_BINSHPOSIX to CPPFLAGS and use sh ../Build.sh -r -L to compile.
See below if the testsuite fails.
Install build-mksh/mksh as /bin/mksh (or similar), build-lksh/lksh as /bin/lksh with a symlink(7) to it from /bin/sh (if desred), and mksh.1 and lksh.1 as manpages (mdoc macropackage required). Install dot.mkshrc either as /etc/skel/.mkshrc (meaning your users will have to manually resynchronise their home directories’ copies after every package upgrade) or as /etc/mkshrc, in which case you install a redirection script like Debian’s into /etc/skel/.mkshrc. You may need a summary of the licence information.
At runtime, the presence of /bin/ed as default history editor is recommended, as well as a manpage formatter; you can also install preformatted manpages from build-*ksh/*ksh.cat1 if nroff(1) (or $NROFF) is available at build time by removing the -r flag from either Build.sh invocation.
Some shell features require the ability to create temporary files and FIFOS (cf. mkfifo(2))/pipes at runtime. Set TMPDIR to a suitable location if /tmp isn’t it; if this is known ahead of time, you can add -DMKSH_DEFAULT_TMPDIR=\"/path/to/tmp\" to CPPFLAGS. We currently are unable to determine one on Android because its bionic libc does not expose any method suitable to do so in the generic case.
① To run the testsuite, ed(1) must be available as /bin/ed, and perl(1) is needed. When cross-compiling, the version of the first ed binary on the PATH must be the same as that in the target system on which the tests are to be run, in order to be able to detect which flavour of ed to adjust the tests for. Busybox ed is broken beyond repair, and all three ed-related tests will always fail with it.
---- ToC: mkshrc Title: How does mksh load configuration files?The shell loads first /etc/profile then ~/.profile if called as login shell or with the -l flag, then loads the file $ENV points to (defaulting to ~/.mkshrc) for interactive shells (that includes login shells).
Distributors should take care to either install the dot.mkshrc example provided into /etc/skel/.mkshrc (so that it’s available for newly created user accounts) and ensure it can propagate to existing accounts or, if upgrading these is difficult, install the shipped file as, for example, /etc/mkshrc and install a skeleton file, such as the one in Debian, that sources the file in /etc.
It’s vital that users can change the configuration, so do not force a root-provided config file onto them; the shipped file, after all, is just an example.
If you need central user and configuration management and cannot use something that installs skeleton files upon home directory creation (like pam_mkhomedir), you can export ENV in /etc/profile to a file (say /etc/shellrc) that sources the per-shell file. Users can, this way, still override it by setting a different $ENV in their ~/.profile.
---- ToC: testsuite-fails Title: The testsuite fails!The mksh testsuite has uncovered numerous bugs in operating systems (kernels, libraries), compilers and toolchains. It is likely that you just ran into one. If you’re using LTO (the Build.sh option -c lto) try to disable it first — especially GCC is a repeat offender breaking LTO and its antecessor -fwhole-program --combine and tends to do wrong code generation quite a bit. Otherwise, try lowering the optimisation levels, bisecting, etc.
---- ToC: selinux-androidiocy Title: I forbid stat(2) in my SELinux policy, and some things do not work! Don’t break Unix. Read up on the GIGO principle. Duh. ---- ToC: makefile Title: Why doesn’t this use a Makefile to build?Not all supported target operating environments have a make utility available, and shell was required for “mirtoconf” (like autoconf) already anyway, so it was chosen to run the make part as well.
You can, however, add the -M flag to your Build.sh invocations to let it produce a Makefrag.inc file tailored for this specific build which you can then include in a Makefile, such as with the BSD make(1) “.include” command or GNU make equivalent. It even contains, for the user to start out with, a commented-out example of how to do that in the most basic manner.
---- ToC: oldbsd Title: Why do other BSDs and QNX still use pdksh instead of mksh?Some systems are resistent to change, mostly due to bikeshedding (some people would, for example, rather see all shells banned to ports/pkgsrc®) and hysterial raisins (historical reasons ☻). Most BSDs have mksh packages available, and it works on all of them and QNX just fine.
In fact, on all of these systems, you can replace their 1999-era /bin/ksh (which is a pdksh) with mksh. On at least NetBSD® 1.6 and up (not 1.5) and OpenBSD, even /bin/sh is fair game.
MidnightBSD notably has adopted mksh as system shell (thanks laffer1).
---- ToC: openbsd Title: Why is there no mksh in OpenBSD’s ports tree? OpenBSD don’t like people who fork off their project at all; heck, they don’t even like the people they themselves forked off (NetBSD®). Several people tried over the years to get one committed, but nobody dared so as to not lose their commit bit. If you try, succeed, and survive Theo, however, kudos to you! See also the “other BSDs” FAQ entry. ---- ToC: book Title: I’d like an introduction. Unfortunately, nobody has written a book about mksh yet, although other shells have received (sometimes decent) attention from authors and publishers. This FAQ lists a subset of things packagers and generic people ask, and the mksh(1) manpage is more of a reference, so you are probably best off starting with a shell-agnostic, POSIX or ksh88 reference such as the first edition (the second one deals with ksh93 which differs far more from mksh than ksh88, as ancient as it is, does) of the O’Reilly book (⚠ disclaimer: only an example, not a recommendation) and going forward by reading scripts (the “shellsnippets” repository referenced in the #ksh channel homepage (see the top of this document) has many examples) and trying to understand them and the mksh specifics from the manpage. ---- ToC: ps1conv Title: My prompt from <some other shell> does not work! Contact us on the mailing list or on IRC, we’ll convert it for you. Also have a look at the PS1 section in the mksh(1) manpage (search for “otherwise unused char”, e.g. with / in less(1), to spot it quickly). ---- ToC: ps1weird Title: My prompt is weird!There are several reasons why your PS1 might be not what you’d expect:
Normally: mksh -T/dev/tty2
However, if you want for it to return (e.g. for an embedded system rescue shell), use this on your real console device instead: mksh -T!/dev/ttyACM0
mksh can also daemonise (send to the background): mksh -T- -c 'exec cdio lock'
---- ToC: completion Title: What about programmable tab completion? The shell itself provides static deterministic tab completion. However, you can use hooks like reprogramming the Tab key to a command line editor macro, and using the evaluate-region editor command (modulo a bugfix) together with quote-region and shell functions to implement a programmable completion engine. Multiple people have been considering doing so in our IRC channel; we’ll hyperlink to these engines when they are available. ---- ToC: posix-mode Title: How POSIX compliant is mksh? Also, UTF-8 vs. locales?You’ll need to use the lksh binary, unless your C long type is 32 bits wide, for POSIX-compliant arithmetic in the shell. This is because mksh provides consistent, wraparound-defined, 32-bit arithmetics on all platforms normally. You’ll also need to enable POSIX mode (set -o posix) explicitly, which also disables brace expansion upon being enabled (use set -o braceexpand to reenable if needed).
For the purpose of POSIX, mksh supports only the C locale. mksh’s utf8-mode (which only supports the BMP (Basic Multilingual Plane) of UCS and maps raw octets into the U+EF80‥U+EFFF wide character range; see Arithmetic expressions in mksh(1) for details) must stay disabled in POSIX mode (it is disabled upon enabling POSIX mode in R56+).
The following POSIX sh-compatible code toggles the utf8-mode option dependent on the current POSIX locale, for mksh to allow using the UTF-8 mode, within the constraints outlined above, in code portable across various shell implementations:
case ${KSH_VERSION:-} in
*MIRBSD KSH*|*LEGACY KSH*)
case ${LC_ALL:-${LC_CTYPE:-${LANG:-}}} in
*[Uu][Tt][Ff]8*|*[Uu][Tt][Ff]-8*) set -U ;;
*) set +U ;;
esac ;;
esac
In near future, (UTF-8) locale tracking will be implemented, though.
The shell is pretty close to POSIX, when run as lksh -o posix under the "C" locale it is intended to match. It does not do everything like other POSIX-compatible or ‑compliant shells, though.
---- ToC: function-local-scopes Title: What differences in function-local scopes are there?mksh has a different scope model from AT&T ksh, which leads to subtle differences in semantics for identical builtins. This can cause issues with a nameref to suddenly point to a local variable by accident. (Other common shells share mksh’s scoping model.)
GNU bash allows unsetting local variables; in mksh, doing so in a function allows back access to the global variable (actually the one in the next scope up) with the same name. The following code, when run before function definitions, changes the behaviour of unset to behave like other shells (the alias can be removed after the definitions):
case ${KSH_VERSION:-} in
*MIRBSD KSH*|*LEGACY KSH*)
function unset_compat {
\\builtin typeset unset_compat_x
for unset_compat_x in "$@"; do
eval "\\\\builtin unset $unset_compat_x[*]"
done
}
\\builtin alias unset=unset_compat
;;
esac
When a local variable is created (e.g. using local, typeset, integer or \\builtin typeset) it does not, like in other shells, inherit the value from the global (next scope up) variable with the same name; it is rather created without any value (unset but defined).
---- ToC: regex-comparison Title: I get an error in this regex comparisonUse extglobs instead of regexes:
[[ foo =~ (foo|bar).*baz ]]
… becomes…
[[ foo = *@(foo|bar)*baz* ]]
GNU bash supports “&>” (and “|&”) to redirect both stdout and stderr in one go, but this breaks POSIX and Korn Shell syntax; use POSIX redirections instead:
| GNU bash | foo |& bar |& baz &>log |
| POSIX | foo 2>&1 | bar 2>&1 | baz >log 2>&1 |
Most likely, you’ve encountered the problem in which the shell runs all parts of a pipeline as subshell. The inner loop will be executed in a subshell and variable changes cannot be propagated if run in a pipeline:
bar | baz | while read foo; do ...; done
Note that exit in the inner loop will also only exit the subshell and not the original shell. Likewise, if the code is inside a function, return in the inner loop will only exit the subshell and won’t terminate the function.
Use co-processes instead:
bar | baz |& while read -p foo; do ...; done exec 3>&p; exec 3>&-
If read is run in a way such as while read foo; do ...; done then leading whitespace will be removed (IFS) and backslashes processed. You might want to use while IFS= read -r foo; do ...; done for pristine I/O.
Similarly, when using the -a option, use of the -r option might be prudent (read -raN-1 arr <file); the same applies for NUL-terminated lines:
find . -type f -print0 |& \
while IFS= read -d '' -pr filename; do
print -r -- "found <${filename#./}>"
done
There’s a rename built-in utility in mksh, which is a very thin wrapper around the rename(2) syscall. It receives two pathnames, source and destination where the first is then atomically renamed to the latter. It does not move, i.e. fails for different filesystems.
The GNU package util-linux has a different rename command. If you wish to invoke an external utility (in favour over a builtin), you can use dot.mkshrc’s function enable or put the following into your ~/.mkshrc:
alias rename="$(whence -p rename)"---- ToC: builtin-sleep Title: “sleep” does not accept ‘m’ for minutes!
mksh contains a sleep built-in utility, in order to be able to offer sub-second sleep to shell scripts for most platforms. (It does not exist if the platform lacks select(2) — which should be rare.)
GNU coreutils contains a sleep implementation accepting suffixed numbers. If you wish to invoke an external utility (in favour over a builtin), you can use dot.mkshrc’s function enable or put something along the following lines into ~/.mkshrc:
alias sleep="$(whence -p sleep)"
timer() { sleep $(($1*60${2:++$2})); } # timer mins [secs]
timer() {
local arg=${1/m/'*60+'}
[[ $arg = *+ ]] && arg+=0
sleep $(($arg)
}
----
ToC: string-concat
Title: “+=” behaves differently from other shells
In POSIX shell, “=” in code like var=content is a string assignment, always. You can use var=$((content)) for an arithmetic assignment that mostly uses C language rules.
It stands to consider that the common shell extension “+=” as in var+=content would always do string concatenation; it does in mksh, but not in some other shells, in which, when var has been declared integer, addition is done instead.
You can make the code portable by using “((…))” (a.k.a. let) instead: (( var += content )) does arithmetic addition in all shells involved.
---- ToC: set-e Title: I use “set -e” and my code unexpectedly errors outI personally recommend people to not use “set -e”, as it makes error handling more difficult. However, some insist. There have been bugfixes (relative to e.g. oksh/loksh and posh) in this aspect, and the user has to make sure $? is always 0 ASAP even after a command that doesn’t check it.
istwo() {
for i in "$@"; do
test x"$i" = x"2" && echo two
done
}
set -e
istwo 1
echo END
This can be fixed by either adding an explicit “:” (or “true”) after the comparison, or even…
test x"$i" = x"2" && echo two || :
… or right after the done inside the function, but…
test x"$i" != x"2" || echo two
… negating the condition and using “||” is preferable.
Remember that Korn shell-style functions (with function keyword and without parenthesēs) in AT&T ksh93 and mksh R51 and up have their own shell option scope, but while…
function istwo {
set +e
…
}
… might help in error handling, the return status of a function is still the last errorlevel inside, so an explicit true (“:”) or, more explicitly, “return 0” at its end is still needed if the caller runs under set -e.
---- ToC: set-eo-pipefail Title: I use “set -eo pipefail” and my code unexpectedly errors outRelated to the above FAQ entry, using set -o pipefail makes the following construct error out:
set -e for x in 1 2; do false && echo $x done | cat
This is because, while the && ensures that the inner command’s failure is not taken, it sets the entire for‥done loop’s errorlevel, which is passed on by -o pipefail.
Invert the inner command:
true || echo $x