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<p>Note: Links marked like <a href="irc://irc.mirbsd.org/!/bin/mksh">this
 one to the mksh IRC channel</a> connect to external resources.</p>
<p>⚠ <b>Notice:</b> the website will have <a
 href="http://www.mirbsd.org/mksh-faq.htm">the latest version of the
 mksh FAQ</a> online.</p>
<h1>Table of Contents</h1>
<ul>
<li><a href="#spelling">How do you spell <tt>mksh</tt>? How do you pronounce it?</a></li>
<li><a href="#sowhatismksh">I’m a $OS (<i>Android, OS/2, 
</i>) user, so what’s mksh?</a></li>
<li><a href="#os2">I’m an OS/2 user, what else do I need to know?</a></li>
<li><a href="#kornshell">How does this relate to ksh or the Korn Shell?</a></li>
<li><a href="#packaging">How should I package mksh? (common cases)</a></li>
<li><a href="#mkshrc">How does mksh load configuration files?</a></li>
<li><a href="#testsuite-fails">The testsuite fails!</a></li>
<li><a href="#selinux-androidiocy">I forbid stat(2) in my SELinux policy, and some things do not work!</a></li>
<li><a href="#makefile">Why doesn’t this use a Makefile to build?</a></li>
<li><a href="#oldbsd">Why do other BSDs and QNX still use pdksh instead of mksh?</a></li>
<li><a href="#openbsd">Why is there no mksh in OpenBSD’s ports tree?</a></li>
<li><a href="#book">I’d like an introduction.</a></li>
<li><a href="#ps1conv">My prompt from &lt;<i>some other shell</i>&gt; does not work!</a></li>
<li><a href="#ps1weird">My prompt is weird!</a></li>
<li><a href="#env">On startup files and <tt>$ENV</tt> across and detecting various shells</a></li>
<li><a href="#ctrl-x-e">Multiline command editing</a></li>
<li><a href="#escaping">Some characters don’t display right</a></li>
<li><a href="#ctrl-l-cls">^L (Ctrl-L) does not clear the screen</a></li>
<li><a href="#ctrl-u-pico">^U (Ctrl-U) clears the entire line</a></li>
<li><a href="#ctrl-w-bash">^W (Ctrl-W) deletes a word, not a bigword</a></li>
<li><a href="#cur-up-zsh">Cursor Up behaves differently from zsh</a></li>
<li><a href="#current">Can mksh set the title of the window according to the command running?</a></li>
<li><a href="#other-tty">How do I start mksh on a specific terminal?</a></li>
<li><a href="#completion">What about programmable tab completion?</a></li>
<li><a href="#posix-mode">How POSIX compliant is mksh? Also, UTF-8 vs. locales?</a></li>
<li><a href="#function-local-scopes">What differences in function-local scopes are there?</a></li>
<li><a href="#regex-comparison">I get an error in this regex comparison</a></li>
<li><a href="#trim-vector">${@?}: bad substitution</a></li>
<li><a href="#extensions-to-avoid">Are there any extensions to avoid?</a></li>
<li><a href="#while-read-pipe">Something is going wrong with my while...read loop</a></li>
<li><a href="#command-alias">“command” doesn’t expand aliases as in ksh93</a></li>
<li><a href="#builtin-cat">Didn’t there used to be a cat(1) builtin?</a></li>
<li><a href="#builtin-rename">“rename” doesn’t work as expected!</a></li>
<li><a href="#builtin-sleep">Didn’t there used to be a sleep(1) builtin?</a></li>
<li><a href="#arith-import">Some integer variables are 0?</a></li>
<li><a href="#string-concat">“+=” behaves differently from other shells</a></li>
<li><a href="#set-e">I use “set -e” and my code unexpectedly errors out</a></li>
<li><a href="#set-eo-pipefail">I use “set -eo pipefail” and my code unexpectedly errors out</a></li>
<li><a href="#faq">My question is not answered here!</a></li>
<li><a href="#contact">How do I contact you (to say thanks, for bugreports and questions)?</a></li>
</ul>

<h1>Frequently Asked Questions</h1>
<h2 id="spelling"><a href="#spelling">How do you spell <tt>mksh</tt>? How do you pronounce it?</a></h2>
<div><p>This <a href="http://www.mirbsd.org/mksh.htm">shell</a> is spelt either
 “<tt>mksh</tt>” (with, even at the beginning of a sentence, <a
 href="https://en.wikipedia.org/wiki/Wikipedia:Manual_of_Style/Capital_letters#Items_that_require_initial_lower_case">an
 initial lowercase letter</a>; this is important) or “MirBSD Korn Shell”,
 possibly with “the”.</p>
<p>I usually pronounce it as “<span xml:lang="de-DE-1901">em-ka-es-ha</span>”,
 that is, the letters individually in my native German, emphasis on the
 first syllable, or say “MirBSD Korn Shell”, although it is manageable,
 mostly for Slavic speakers, to actually say “mksh” as if it were a word â˜ș</p>
<p>Oh
 I’ve run into this one, didn’t I? “MirBSD” is pronounced “<span
 xml:lang="de-DE-1901">Mir-Be-Es-De</span>” germanically, for anglophones
 “Mir-beas’tie” is fine.</p>
<p>This translates well into other languages, such as <span
 xml:lang="es">eme-ka-ese-ache</span> in Spanish, although English
 speakers may still find “Mir-beastie korn shell” more palatable.</p></div>
<h2 id="sowhatismksh"><a href="#sowhatismksh">I’m a $OS (<i>Android, OS/2, 
</i>) user, so what’s mksh?</a></h2>
<div><p>mksh is a so-called (Unix) “shell” or “command interpreter”, similar to
 <tt>COMMAND.COM</tt>, <tt>CMD.EXE</tt> or PowerShell on other operating
 systems you might know. Basically, it runs in a terminal (“console” or
 “DOS box”) window, taking user input and running that as commands. It’s
 also used to write so-called (shell) “script”s, short programs made by
 putting several of those commands into a “batch file”.</p>
<p>On Android, mksh is used as the system shell — basically, the one
 running commands at system startup, in the background, and on user
 behalf (but never of its own). Any privilege pop-ups you might <a
 href="https://forum.xda-developers.com/showthread.php?t=1963976">be
 encountering</a> are therefore <a
 href="https://forum.xda-developers.com/showpost.php?p=33550523&amp;postcount=1553">not
 caused by mksh</a> but by some other code invoking mksh to do something
 on its behalf.</p></div>
<h2 id="os2"><a href="#os2">I’m an OS/2 user, what else do I need to know?</a></h2>
<div><p>Unlike the native command prompt, the current working directory is,
 for security reasons common on Unix systems which the shell is designed
 for, not in the search path at all; if you really need this, run the
 command <tt>PATH=.$PATHSEP$PATH</tt> or add that to a suitable
 initialisation file (<tt>~/.mkshrc</tt>).</p>
<p>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
 <tt>-T</tt> 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.</p></div>
<h2 id="kornshell"><a href="#kornshell">How does this relate to ksh or the Korn Shell?</a></h2>
<div><p>The Korn Shell (AT&amp;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.</p>
<p>There’s now <tt>ksh2020</tt>, a project having restarted development
 around November 2017 forking the last <tt>ksh93 v-</tt> (beta) snapshot
 and continuing to develop it, presented at FOSDEM.</p>
<p>AT&amp;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 <em>not</em>
 “Korn Shell” or “ksh”. However, mksh got blessed by David Korn, as long
 as it cannot be confused with the original Korn Shell.</p>
<p>The POSIX shell standard, while lacking most Korn Shell features, was
 largely based on AT&amp;T ksh88, with some from the Bourne shell.</p>
<p>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&amp;T’s was proprietary,
 closed-source code for a very long time. pdksh’s development ended in
 1999, with some projects like Debian and NetBSDÂź 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, NetBSDÂź and OpenBSD (who continue to
 maintain their variant on “low flame”).</p>
<p>dtksh is the “Desktop Korn Shell”, a build of AT&amp;T ksh93 with some
 additional built-in utilities for graphics programming (windows, menu
 bars, dialogue boxes, etc.) utilising Motif bindings.</p>
<p>MKS ksh is a proprietary reimplemention aiming for, but not quite
 getting close to, ksh88 compatibility.</p>
<p>SKsh is an AmigaOS-specific Korn Shell-lookalike by Steve Koren.</p>
<p>The <a href="http://www.mirbsd.org/ksh-chan.htm">Homepage of the <tt>#ksh</tt>
 channel on IRC</a> contains more information about the Korn Shell in
 general and its flavours.</p></div>
<h2 id="packaging"><a href="#packaging">How should I package mksh? (common cases)</a></h2>
<div><p>Export a few environment variables, namely <tt>CC</tt> (the C compiler),
 <tt>CPPFLAGS</tt> (all C prĂŠprocessor definitions), <tt>CFLAGS</tt> (only
 compiler flags, <em>no</em> <tt>-Dfoo</tt> or anything!), <tt>LDFLAGS</tt>
 (for anything to pass to the C compiler while linking) and <tt>LIBS</tt>
 (appended to the linking command line after everything else. You might
 wish to <tt>export LDSTATIC=-static</tt> for a static build as well.</p>
<p>When cross-compiling, <tt>CC</tt> is the <em>cross</em> compiler (mksh
 currently does not require a compiler targetting the build system), but
 you <em>must</em> also export <tt>TARGET_OS</tt> 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 <tt>TARGET_OSREV</tt>;
 consult the source code of <tt>Build.sh</tt> for details.</p>
<p>Create two subdirectories, say <tt>build-mksh</tt> and <tt>build-lksh</tt>.
 In each of them, start a compilation by issuing <tt>sh ../Build.sh -r</tt>
 followed by running the testsuite<a href="#packaging-fn1">Âč</a> via
 <tt>./test.sh</tt>. For lksh(1) add <tt>-DMKSH_BINSHPOSIX</tt> to
 <tt>CPPFLAGS</tt> and use <tt>sh ../Build.sh -r -L</tt> to compile.</p>
<p>See <a href="#testsuite-fails">below</a> if the testsuite fails.</p>
<p>Install <tt>build-mksh/mksh</tt> as <tt>/bin/mksh</tt> (or similar),
 <tt>build-lksh/lksh</tt> as <tt>/bin/lksh</tt> with a symlink(7) to it
 from <tt>/bin/sh</tt> (if desred), and <tt>mksh.1</tt> and <tt>lksh.1</tt>
 as manpages (mdoc macropackage required). Install <tt>dot.mkshrc</tt>
 either as <tt>/etc/skel/.mkshrc</tt> (meaning your users will have to
 manually resynchronise their home directories’ copies after every package
 upgrade) or as <tt>/etc/mkshrc</tt>, in which case you install a <a
 href="https://evolvis.org/plugins/scmgit/cgi-bin/gitweb.cgi?p=alioth/mksh.git;a=blob;f=debian/.mkshrc;hb=HEAD">redirection
 script like Debian’s</a> into <tt>/etc/skel/.mkshrc</tt>. You may need a <a
 href="http://www.mirbsd.org/TaC-mksh.txt">summary of the licence information</a>.</p>
<p>At runtime, the presence of <tt>/bin/ed</tt> as default history editor
 is recommended, as well as a manpage formatter; you can also install
 preformatted manpages from <tt>build-*ksh/*ksh.cat1</tt> if nroff(1) (or
 <tt>$NROFF</tt>) is available at build time by removing the <tt>-r</tt>
 flag from either <tt>Build.sh</tt> invocation.</p>
<p>Some shell features require the ability to create temporary files and
 FIFOS (cf. mkfifo(2))/pipes at runtime. Set <tt>TMPDIR</tt> to a suitable
 location if <tt>/tmp</tt> isn’t it; if this is known ahead of time, you
 can add <tt>-DMKSH_DEFAULT_TMPDIR=\"/path/to/tmp\"</tt> 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.</p>
<p id="packaging-fn1">① To run the testsuite, ed(1) must be available as
 <tt>/bin/ed</tt>, and perl(1) is needed. When cross-compiling, the version
 of the first <tt>ed</tt> binary on the <tt>PATH</tt> <em>must</em> 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.</p></div>
<h2 id="mkshrc"><a href="#mkshrc">How does mksh load configuration files?</a></h2>
<div><p>The shell loads first <tt>/etc/profile</tt> then <tt>~/.profile</tt>
 if called as login shell or with the <tt>-l</tt> flag, then loads the file
 <tt>$ENV</tt> points to (defaulting to <tt>~/.mkshrc</tt>) for interactive
 shells (that includes login shells).</p>
<p>Distributors should take care to either install the <tt>dot.mkshrc</tt>
 example provided into <tt>/etc/skel/.mkshrc</tt> (so that it’s available
 for newly created user accounts) and ensure it can propagate to existing
 accounts or, if upgrading these is difficult, install the shipped file
 as, for example, <tt>/etc/mkshrc</tt> and install a skeleton file, such
 as the one in Debian, that sources the file in <tt>/etc</tt>.</p>
<p>It’s vital that users can change the configuration, so do not force a
 root-provided config file onto them; the shipped file, after all, is just
 an example.</p>
<p>If you need central user and configuration management and cannot use
 something that installs skeleton files upon home directory creation
 (like pam_mkhomedir), you can <tt>export ENV</tt> in <tt>/etc/profile</tt>
 to a file (say <tt>/etc/shellrc</tt>) that sources the per-shell file.
 Users can, this way, still override it by setting a different <tt>$ENV</tt>
 in their <tt>~/.profile</tt>.</p></div>
<h2 id="testsuite-fails"><a href="#testsuite-fails">The testsuite fails!</a></h2>
<div><p>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 <tt>Build.sh</tt> option
 <tt>-c lto</tt>) try to disable it first — especially GCC is a repeat
 offender breaking LTO and its antecessor <tt>-fwhole-program --combine</tt>
 and tends to do wrong code generation quite a bit. Otherwise, try
 lowering the optimisation levels, bisecting, etc.</p></div>
<h2 id="selinux-androidiocy"><a href="#selinux-androidiocy">I forbid stat(2) in my SELinux policy, and some things do not work!</a></h2>
<div>Don’t break Unix. Read up on the GIGO principle. Duh.</div>
<h2 id="makefile"><a href="#makefile">Why doesn’t this use a Makefile to build?</a></h2>
<div><p>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.</p>
<p>You can, however, add the <tt>-M</tt> flag to your <tt>Build.sh</tt>
 invocations to let it produce a <tt>Makefrag.inc</tt> file <em>tailored
 for this specific build</em> which you can then include in a Makefile,
 such as with the BSD make(1) “.include” command or <a
 href="https://www.gnu.org/software/make/manual/make.html#Include">GNU
 make</a> 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.</p></div>
<h2 id="oldbsd"><a href="#oldbsd">Why do other BSDs and QNX still use pdksh instead of mksh?</a></h2>
<div><p>Some systems are resistant to change, mostly due to bikeshedding
 (some people would, for example, rather see all shells banned to
 ports/pkgsrcŸ) and hysterial raisins (historical reasons ☻). Most
 BSDs have mksh packages available, and it works on all of them and
 QNX just fine.</p>
<p>In fact, on all of these systems, you can replace their 1999-era
 <tt>/bin/ksh</tt> (which is a pdksh) with mksh. On at least NetBSDÂź
 1.6 and up (not 1.5) and OpenBSD, even <tt>/bin/sh</tt> is fair game.</p>
<p>MidnightBSD notably has adopted mksh as system shell (thanks laffer1).</p></div>
<h2 id="openbsd"><a href="#openbsd">Why is there no mksh in OpenBSD’s ports tree?</a></h2>
<div>OpenBSD don’t like people who fork off their project at all; heck,
they don’t even like the people they themselves forked off (NetBSD¼).
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 <a href="#oldbsd">the
“other BSDs” FAQ entry</a>.</div>
<h2 id="book"><a href="#book">I’d like an introduction.</a></h2>
<div>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 <tt>#ksh</tt> channel
homepage (see the top of this document) has many examples) and
trying to understand them and the mksh specifics from the manpage.</div>
<h2 id="ps1conv"><a href="#ps1conv">My prompt from &lt;<i>some other shell</i>&gt; does not work!</a></h2>
<div><a href="#contact">Contact</a> 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
<tt>/</tt> in less(1), to spot it quickly).</div>
<h2 id="ps1weird"><a href="#ps1weird">My prompt is weird!</a></h2>
<div><p>There are several reasons why your <tt>PS1</tt> might be not
 what you’d expect:</p><ul>
<li><tt>$PS1</tt> is <tt>export</tt>ed. <strong>Do not export PS1!</strong>
 (This was agreed upon as suggestion in a discussion between bash, zsh and
 Korn shell developers.) The feature set of different shells vastly differs
 and each shell should use its default PS1 or from its startup files.</li>
<li><tt>$ENV</tt> <a href="#env">is set and probably <tt>export</tt>ed</a>.</li>
<li>Your prompt is just “<tt># </tt>”: you’re entering a root shell, and
 <tt>$PS1</tt> does not contain the ‘#’ character, in which case the shell
 forces this prompt, making extra privileges obvious.</li>
<li>Your prompt is just “<tt>$ </tt>”: perhaps your system administrator
 did not install the shipped <tt>dot.mkshrc</tt> file, or you did not copy
 <tt>/etc/skel/.mkshrc</tt> into your home directory (perhaps it was created
 before <tt>mksh</tt> was installed?). Without another idea for a fix, get <a
 href="http://www.mirbsd.org/cvs.cgi/~checkout~/src/bin/mksh/dot.mkshrc?rev=HEAD;content-type=text%2Fplain">this
 file</a> and store it as <tt>~/.mkshrc</tt> then run <tt>mksh</tt>; this
 will at the very least install our sample (“user@host:path $ ”) prompt.</li>
<li>Your prompt contains things like “\u” or “\w”: it is for another shell
 and <a href="#ps1conv">needs converting</a>.</li>
<li>Your prompt contains colours, and when the command line is long the
 cursor position or screen contents, especially using the history, is off:
 terminal escapes must be escaped from the shell; check the PS1 section in
 the manpage: search for “otherwise unused char” (see above).</li>
<li>If the prompt doesn’t leave enough space on the right, the shell inserts
 a line break after it when rendering.</li>
</ul></div>
<h2 id="env"><a href="#env">On startup files and <tt>$ENV</tt> across and detecting various shells</a></h2>
<div><p>Interactive shells look at <tt>~/.mkshrc</tt> (or <tt>/system/etc/mkshrc</tt>
on Android and <tt>/etc/mkshrc</tt> on FreeWRT and OpenWrt) by default. This
location can, however, be overridden by setting the <tt>ENV</tt> environment
variable. (FreeBSD is rumoured to set it in their system profile.) It’s better
to not set <tt>$ENV</tt> if possible and let every shell user their native
startup files; otherwise, you must ensure that it runs under all shells. Check
<tt>$BASH_VERSION</tt> (GNU bash), <tt>$KSH_VERSION</tt> (contains “LEGACY KSH”
or “MIRBSD KSH” for mksh, “PD KSH” for ancient mirbsdksh/oksh/pdksh, “Version”
for ksh93); <tt>$NETBSD_SHELL</tt> (NetBSD ash); <tt>POSH_VERSION</tt> (posh, a
pdksh derivative); <tt>$SH_VERSION</tt> (“PD KSH” as sh), <tt>$YASH_VERSION</tt>
(yash), <tt>$ZSH_VERSION</tt> (or if <tt>$VERSION</tt> begins with “zsh”); a <a
href="http://www.mirbsd.org/ksh-chan.htm#which-shell">list of more approaches</a> exists.</p>

<p>Note that, in some scenarios, it might be very useful to actually set
 <tt>$ENV</tt>: the regular interactive shell startup file lies in the
 user’s home directory, relying on being copied from <tt>/etc/skel/</tt>
 which normally is only done at user creation time. If mksh was installed
 later, the user often won’t get it at all, and delivering updates is
 challenging. One way of partially working around this is to ship an
 <tt>/etc/skel/.mkshrc</tt> that reads <tt>/etc/mkshrc</tt> by default
 (but the user can change it of course) and ship the <tt>dot.mkshrc</tt>
 file as <tt>/etc/mkshrc</tt>, but that won’t fully help. This is where
 <tt>$ENV</tt> comes into play:</p><ul>
  <li>In <tt>/etc/profile</tt>, set <tt>ENV</tt> to a, say, <tt>shrc</tt>
   file shipped in <tt>/etc/</tt> and export it.</li>
  <li>In that new file, which must use only constructs compatible with
   all shells, usually a subset of POSIX, read the various rc files
   (<tt>.mkshrc</tt> for mksh, <tt>.kshrc</tt> for AT&amp;T ksh93, etc.)
   from the user’s home if they exist, from <tt>/etc/skel/</tt> otherwise.</li>
</ul><p>This may very well be <em>required</em> if the alternative would
 be <a href="#ps1weird">to <del><tt>export PS1</tt></del>[sic!]</a>. <a
  href="https://gitlab.alpinelinux.org/alpine/aports/-/issues/12398#note_146574"
 >alpine Linux</a> encountered this very problem, and the linked post is
 a (draft) solution using the <tt>$ENV</tt> method and looks at various
 other shells’ startup file situation as well.</p></div>
<h2 id="ctrl-x-e"><a href="#ctrl-x-e">Multiline command editing</a></h2>
<div><p>mksh is very independent of the terminal and external libraries and
 databases, such as termcap, and therefore is conservative in which ANSI
 control codes are sent to the terminal.</p>
<p>For this reason, mksh’s input line editing uses a “windowed one-line”
 concept: the line the cursor is on is a “window” into the whole input,
 horizontally scrolled. Some other shells (that are much larger and have
 more dependencies on external tooling) use a “multi-line” editing mode,
 and users occasionally wish for this. It is on the long-term TODO, but
 (due to the aforementioned implications) this is not trivial.</p>
<p>One way to achieve multi-line editing is to <em>dis</em>able input
 line editing: <tt>set +o emacs +o vi</tt><br />This will, however, lose
 you all editing features: tab completion, cursor keys, history, etc.</p>
<p>Another way, if you don’t need it all the time, is to use a function
 that spawns your editor on the input line: press <tt>^Xe</tt> in the
 default emacs mode or <tt>Esc + v</tt> in vi mode. Once you exit the
 editor, whatever was written there is run; this includes the original
 command line if you quit without saving, so request the editor to exit
 nƍn-zero (e.g. using jupp’s “abendjoe” command) to prevent execution.
 This is <em>really</em> useful to write ad-hƍc scripts as well.</p></div>
<h2 id="escaping"><a href="#escaping">Some characters don’t display right</a></h2>
<div><p>First, make sure that either you’re using a UTF-8 terminal and system
 and the shell’s UTF-8 mode is on (<tt>set -U</tt>) or that you’re using
 an 8-bit codepage/CCSID and the UTF-8 mode is off (<tt>set +U</tt>). If
 you’re on an EBCDIC system ensure to pick a codepage that has a bijective
 mapping to (Extended) ASCII and in which all necessary characters are
 present, for example 1047. Furthermore ensure the compile-time and runtime
 codepages match. (Other encoding schemes, e.g. DBCS or ISO-2022-JP, are
 not supported.) This should already fix most relevant issues.</p>
<p>If using an 8-bit coding system that (unlike e.g. ISO 8859 or EBCDIC)
 does not assign control characters to “Extended ASCII” codepoints 0x80 0x9F,
 such as codepages 437, 850, 1252, 
 (usually on OS/2 or DOS-based systems),
 enable the option <tt>set -o asis</tt> (new in R60); otherwise, they will
 be escaped to avoid accidentally setting off terminal control sequences.</p>
<p>Note that escaping of characters is, at runtime, dependent on whether the
 shell was compiled for EBCDIC and/or <tt>utf8-mode</tt> and/or <tt>asis</tt>
 are enabled, the latter being ignored if either of the former two are true
 (in UTF-8 mode, UCS C1 codepoints are always escaped).</p></div>
<h2 id="ctrl-l-cls"><a href="#ctrl-l-cls">^L (Ctrl-L) does not clear the screen</a></h2>
<div>Use ^[^L (Escape+Ctrl-L) or rebind it:<br />
<tt>bind '^L=clear-screen'</tt></div>
<h2 id="ctrl-u-pico"><a href="#ctrl-u-pico">^U (Ctrl-U) clears the entire line</a></h2>
<div>If you want it to only delete the line up to the cursor, use:<br />
<tt>bind -m ^U='^[0^K'</tt></div>
<h2 id="ctrl-w-bash"><a href="#ctrl-w-bash">^W (Ctrl-W) deletes a word, not a bigword</a></h2>
<div>If you want it to delete more, with R60 you can use:<br />
<tt>bind '^W=delete-bigword-backward'</tt></div>
<h2 id="cur-up-zsh"><a href="#cur-up-zsh">Cursor Up behaves differently from zsh</a></h2>
<div>Some shells make Cursor Up search in the history only for commands
starting with what was already entered. mksh separates the shortcuts:
Cursor Up goes up one command and PgUp searches the history as described
above. You can, of course, rebind:<br />
<tt>bind '^XA=search-history-up'</tt></div>
<h2 id="current"><a href="#current">Can mksh set the title of the window according to the command running?</a></h2>
<div>There’s no such thing as “the command currently running”; consider
pipelines and delays (<tt>cmd1 | (cmd2; sleep 3; cmd3) | cmd4</tt>).
There is, however, a way to make the shell display the command <em>line</em>
during the time it is executed; for testing, you will need to download <a
href="https://evolvis.org/plugins/scmgit/cgi-bin/gitweb.cgi?p=shellsnippets/shellsnippets.git;a=blob;f=mksh/terminal-title;hb=HEAD">this
script</a> and <tt>source</tt> it. For merging into your <tt>~/.mkshrc</tt>
you should first understand how it works: lines 4–18 set a <tt>PS1</tt>
(prompt) equivalent to lines 84–96 of the stock <tt>dot.mkshrc</tt>, with
one change: line 15 (<tt>print &gt;/dev/tty 
</tt>) is new, inserted just
before the <tt>return</tt> command of the function substitution in the
default prompt; this is what you’ll need to merge into your own, custom,
prompt (if you have one; otherwise pull this adaption to the default
one). Line 19 is the only other thing in this script rebinding the Ctrl-M
key (which is normally produced by the Enter/Return key) to code that

does <em>something crazy</em>. This trick however <em>does funny things with
multiline commands</em>, so if you type something out in multiple lines,
for example <strong>here documents</strong> or <strong>loops</strong> press
<strong>Ctrl-J instead of Enter/Return</strong> after <em>each</em> line
including the first (at PS1) and final (at PS2) one.</div>
<h2 id="other-tty"><a href="#other-tty">How do I start mksh on a specific terminal?</a></h2>
<div><p>Normally: <tt>mksh -T<i>/dev/tty2</i></tt></p>
<p>However, if you want for it to return (e.g. for an embedded system rescue
 shell), use this on your real console device instead:
 <tt>mksh -T!<i>/dev/ttyACM0</i></tt></p>
<p>mksh can also daemonise (send to the background):
 <tt>mksh -T- -c 'exec cdio lock'</tt></p></div>
<h2 id="completion"><a href="#completion">What about programmable tab completion?</a></h2>
<div>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 <tt>evaluate-region</tt>
editor command together with <tt>quote-region</tt> 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.</div>
<h2 id="posix-mode"><a href="#posix-mode">How POSIX compliant is mksh? Also, UTF-8 vs. locales?</a></h2>
<div><p>You’ll need to use the <tt>lksh</tt> binary, unless your C <tt>long</tt>
 type is 32 bits wide, for POSIX-compliant arithmetic in the shell. This is
 because <tt>mksh</tt> provides consistent, wraparound-defined, 32-bit
 arithmetics on all platforms normally. You’ll also need to enable POSIX mode
 (<tt>set -o posix</tt>) explicitly, which also disables brace expansion upon
 being enabled (use <tt>set -o braceexpand</tt> to reenable if needed).</p>
<p>For the purpose of POSIX, mksh supports only the <tt>C</tt> locale. mksh’s
 <tt>utf8-mode</tt> (which only supports the BMP (Basic Multilingual Plane) of
 UCS and maps raw octets into the U+EF80 U+EFFF wide character range; see
 <tt>Arithmetic expressions</tt> in mksh(1) for details) <em>must</em> stay
 disabled in POSIX mode (it is disabled upon enabling POSIX mode in R56+ but
 don’t depend on this to stay once locale tracking will be implemented; the
 disabling code is not present in this build).</p>
<p><strong>Future compatibility note:</strong> there’s work underway to use
 full 21-bit UTF-8 in mksh R60 or so. Raw octet mapping will almost certainly
 be moved out of the PUA and to some range outside of UCS.</p>
<p class="boxhead">The following POSIX sh-compatible code toggles the
 <tt>utf8-mode</tt> 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:</p>
<div class="boxtext">
 <pre>
<span style="display:none;">	</span>case ${KSH_VERSION:-} in
<span style="display:none;">	</span>*MIRBSD KSH*|*LEGACY KSH*)
<span style="display:none;">	</span>	case ${LC_ALL:-${LC_CTYPE:-${LANG:-}}} in
<span style="display:none;">	</span>	*[Uu][Tt][Ff]8*|*[Uu][Tt][Ff]-8*) set -U ;;
<span style="display:none;">	</span>	*) set +U ;;
<span style="display:none;">	</span>	esac ;;
<span style="display:none;">	</span>esac
 </pre>
</div><p class="boxfoot">In near future, (UTF-8) locale tracking will
 be implemented, though. This build of mksh already enables it.</p>
<p>The shell is pretty close to POSIX, when run as <tt>lksh -o posix</tt>
 under the "C" locale it is intended to match. It does not do everything
 like other POSIX-compatible or ‑compliant shells, though.</p></div>
<h2 id="function-local-scopes"><a href="#function-local-scopes">What differences in function-local scopes are there?</a></h2>
<div><p><tt>mksh</tt> has a different scope model from AT&amp;T <tt>ksh</tt>,
 which leads to subtle differences in semantics for identical builtins.
 This can cause issues with a <tt>nameref</tt> to suddenly point to a
 local variable by accident. (Other common shells share mksh’s scoping
 model.)</p>
<p class="boxhead">GNU <tt>bash</tt> allows unsetting local variables; in
 <tt>mksh</tt>, 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 <tt>unset</tt> to behave like other shells (the alias can be removed
 after the definitions):</p>
<div class="boxtext">
 <pre>
<span style="display:none;">	</span>case ${KSH_VERSION:-} in
<span style="display:none;">	</span>*MIRBSD KSH*|*LEGACY KSH*)
<span style="display:none;">	</span>	function unset_compat {
<span style="display:none;">	</span>		\\builtin typeset unset_compat_x

<span style="display:none;">	</span>		for unset_compat_x in "$@"; do
<span style="display:none;">	</span>			eval "\\\\builtin unset $unset_compat_x[*]"
<span style="display:none;">	</span>		done
<span style="display:none;">	</span>	}
<span style="display:none;">	</span>	\\builtin alias unset=unset_compat
<span style="display:none;">	</span>	;;
<span style="display:none;">	</span>esac
 </pre>
</div><p class="boxfoot">When a local variable is created (e.g. using
 <tt>local</tt>, <tt>typeset</tt>, <tt>integer</tt> or
 <tt>\\builtin typeset</tt>) 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).</p></div>
<h2 id="regex-comparison"><a href="#regex-comparison">I get an error in this regex comparison</a></h2>
<div><p>Use extglobs instead of regexes:<br />
 <tt>[[ foo =~ (foo|bar).*baz ]]</tt><br />
 
 becomes
<br />
 <tt>[[ foo = *@(foo|bar)*baz* ]]</tt></p></div>
<h2 id="trim-vector"><a href="#trim-vector">${@?}: bad substitution</a></h2>
<div><p>In mksh, you cannot assign to or trim a vector (yet). For most
 cases it is possible to write the affected code in a way avoiding
 this extension; for example, trimming <tt>${@#foo}</tt> could be
 applied to <tt>$1</tt> only and <tt>${@?}</tt> can be replaced
 with a test whether <tt>$# -eq 0</tt>.</p></div>
<h2 id="extensions-to-avoid"><a href="#extensions-to-avoid">Are there any extensions to avoid?</a></h2>
<div><p>GNU <tt>bash</tt> supports “<tt>&amp;&gt;</tt>” (and “|&amp;”) to redirect
 both stdout and stderr in one go, but this breaks POSIX and Korn Shell syntax;
 use POSIX redirections instead:</p>
<table border="1" cellpadding="3">
 <tr><td>GNU bash</td><td>
  <tt>foo |&amp; bar |&amp; baz &amp;&gt;log</tt>
 </td></tr>
 <tr><td>POSIX</td><td>
  <tt>foo 2&gt;&amp;1 | bar 2&gt;&amp;1 | baz &gt;log 2&gt;&amp;1</tt>
 </td></tr>
</table></div>
<h2 id="while-read-pipe"><a href="#while-read-pipe">Something is going wrong with my while...read loop</a></h2>
<div><p class="boxhead">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:</p>
<div class="boxtext">
 <pre>
<span style="display:none;">	</span>bar | baz | while read foo; do ...; done
 </pre>
</div><p class="boxfoot">Note that <tt>exit</tt> in the inner loop will
 also only exit the subshell and not the original shell. Likewise, if the
 code is inside a function, <tt>return</tt> in the inner loop will only
 exit the subshell and won’t terminate the function.</p>
<p class="boxhead">Use co-processes instead:</p>
<div class="boxtext">
 <pre>
<span style="display:none;">	</span>bar | baz |&amp;
<span style="display:none;">	</span>while read -p foo; do ...; done
<span style="display:none;">	</span>exec 3&gt;&amp;p; exec 3&gt;&amp;-
 </pre>
</div><p class="boxfoot">If <tt>read</tt> is run in a way such as
 <tt>while read foo; do ...; done</tt> then leading whitespace will be
 removed (IFS) and backslashes processed. You might want to use
 <tt>while IFS= read -r foo; do ...; done</tt> for pristine I/O.</p>
<p class="boxhead">Similarly, when using the <tt>-a</tt> option, use of the
 <tt>-r</tt> option might be prudent (<tt>read -raN-1 arr &lt;file</tt>);
 the same applies for NUL-terminated lines:</p>
<div class="boxtext">
 <pre>
<span style="display:none;">	</span>find . -type f -print0 |&amp; \
<span style="display:none;">	</span>    while IFS= read -d '' -pr filename; do
<span style="display:none;">	</span>	print -r -- "found &lt;${filename#./}&gt;"
<span style="display:none;">	</span>done
 </pre>
</div></div>
<h2 id="command-alias"><a href="#command-alias">“command” doesn’t expand aliases as in ksh93</a></h2>
<div>This is because AT&amp;T ksh93 ships a predefined alias enabling this:<br />
<tt>alias command='command '</tt><br />
put this into your <tt>~/.mkshrc</tt>
(note the space before the closing single quote)</div>
<h2 id="builtin-cat"><a href="#builtin-cat">Didn’t there used to be a cat(1) builtin?</a></h2>
<div><p>Up to and including mksh R59c, we indeed shipped a built-in cat(1)
 inside mksh; this was added originally because Android did not have
 one <em>at all</em> (but they have since imported a BSD cat). While
 it could speed up some sh scripts correct signal handling is hard to
 get right, so (with regret) it was removed in 2021. 🙀</p></div>
<h2 id="builtin-rename"><a href="#builtin-rename">“rename” doesn’t work as expected!</a></h2>
<div><p>There’s a <tt>rename</tt> 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.</p>
<p>The GNU package <tt>util-linux</tt> has a different <tt>rename</tt>
 command. If you wish to invoke an external utility (in favour over a
 builtin), you can use <tt>dot.mkshrc</tt>’s function <tt>enable</tt>
 or put the following into your <tt>~/.mkshrc</tt>:</p>
<pre>alias rename="$(whence -p rename)"</pre></div>
<h2 id="builtin-sleep"><a href="#builtin-sleep">Didn’t there used to be a sleep(1) builtin?</a></h2>
<div><p>Up to and including mksh R59c, we indeed shipped a subsecond-capable
 select(2)-based built-in sleep(1). This got originally added because
 too many platforms do not support sub-second sleep, which nowadays is
 of less concern. It also led to users complaining about lack for system
 *ahem* GNU extensions, but the cause of its demise is that getting signal
 handling right, in a portable way and without too many syscalls (there’s
 a threshold over which fork+exec is cheaper!), isn’t feasible if even at
 all possible.</p>
<p>The MirOS Project now ships <a href="http://www.mirbsd.org/subprj.htm#sleep">a
 portable sleep</a> which similarily is select(2)-based and capable of
 subsecond sleep but in addition supports all GNU extensions related to
 specifying the amount of time to sleep. It will work on <em>at least</em>
 all platforms on which mksh had a builtin before. Please install this
 if your operating system lacks a good enough sleep(1) utility.</p>
<p>Note that, if your OS lacks select(2), you’ll lose out either way.
 In that case, GNU coreutils’ sleep, which is built on older syscalls,
 may work if the copyleft licence isn’t a showstopper for you.</p></div>
<h2 id="arith-import"><a href="#arith-import">Some integer variables are 0?</a></h2>
<div><p class="boxhead">To mitigate potential exploits, variables imported
 from the environment are not trusted in arithmetic context; that is
</p>
<div class="boxtext">
 <pre>
<span style="display:none;">	</span>foo=1+1 mksh -c 'integer foo; print $foo'
<span style="display:none;">	</span>foo=1+1 mksh -c 'integer foo=$foo; print $foo'
 </pre>
</div><p class="boxfoot">
 will lose the value in the first line,
 while the second line explicitly “untaints”, to use a Perl term,
 the content. Purely numeric values will pass, though.</p></div>
<h2 id="string-concat"><a href="#string-concat">“+=” behaves differently from other shells</a></h2>
<div><p>In POSIX shell, “=” in code like <tt>var=content</tt> is a string
 assignment, always. You can use <tt>var=$((content))</tt> for an
 arithmetic assignment that mostly uses C language rules.</p>
<p>It stands to consider that the common shell extension “+=” as in
 <tt>var+=content</tt> would always do string concatenation; it does
 in mksh, but not in some other shells, in which, when <tt>var</tt> has
 been declared integer, addition is done instead.</p>
<p>You can make the code portable by using “((
))” (a.k.a. <tt>let</tt>)
 instead: <tt>(( var += content ))</tt> does arithmetic addition in
 all shells involved.</p></div>
<h2 id="set-e"><a href="#set-e">I use “set -e” and my code unexpectedly errors out</a></h2>
<div><p>I personally recommend people to not use “<tt>set -e</tt>”, 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 <tt>$?</tt> is always 0 ASAP even after
a command that doesn’t check it.</p>
<pre>istwo() {
        for i in "$@"; do
                test x"$i" = x"2" &amp;&amp; echo two
        done
}
set -e
istwo 1
echo END</pre>
<p>This can be fixed by either adding an explicit “<tt>:</tt>” (or
“<tt>true</tt>”) after the comparison, or even
</p>
<pre>test x"$i" = x"2" &amp;&amp; echo two || :</pre>
<p>
 or right after the <tt>done</tt> inside the function, but
</p>
<pre>test x"$i" != x"2" || echo two</pre>
<p>
 negating the condition and using “<tt>||</tt>” is preferable.</p>

<p>Remember that Korn shell-style functions (with <tt>function</tt>
 keyword and <strong>without</strong> parenthesēs) in AT&amp;T ksh93
 and mksh R51 and up have their own shell option scope, but while
</p>
<pre>function istwo {
        set +e
        

}</pre>
<p>
 might help in error handling, the return status of a function is
 still the last errorlevel inside, so an explicit true (“<tt>:</tt>”)
 or, more explicitly, “<tt>return 0</tt>” at its end is still needed
 if the <em>caller</em> runs under <tt>set -e</tt>.</p></div>
<h2 id="set-eo-pipefail"><a href="#set-eo-pipefail">I use “set -eo pipefail” and my code unexpectedly errors out</a></h2>
<div><p class="boxhead">Related to the above FAQ entry, using
 <tt>set -o pipefail</tt> makes the following construct error out:</p>
<div class="boxtext">
 <pre>
<span style="display:none;">	</span>set -e
<span style="display:none;">	</span>for x in 1 2; do
<span style="display:none;">	</span>	false &amp;&amp; echo $x
<span style="display:none;">	</span>done | cat
 </pre>
</div><p class="boxfoot">This is because, while the <tt>&amp;&amp;</tt>
 ensures that the inner command’s failure is not taken, it sets the entire
 <tt>for</tt> <tt>done</tt> loop’s errorlevel, which is passed on by
 <tt>-o pipefail</tt>.</p>
<p>Invert the inner command:<br />
 <tt>true || echo $x</tt></p></div>
<h2 id="faq"><a href="#faq">My question is not answered here!</a></h2>
<div>Do read the mksh(1) and lksh(1) manual page. You might also wish to read the <a
 href="http://www.mirbsd.org/ksh-chan.htm">homepage of the <tt>#ksh</tt> IRC channel</a>
which lists several resources for Korn or POSIX-compatible shells in general.
Or, <a href="#contact">contact</a> us (developer and users).</div>
<h2 id="contact"><a href="#contact">How do I contact you (to say thanks, for bugreports and questions)?</a></h2>
<div><p>You can say hi in the <tt>#!/bin/mksh</tt> channel on <a
 href="http://www.mirbsd.org/irc.htm">IRC</a> (OFTC, for now), although
 a
 <a href="http://www.mirbsd.org/danke.htm">donation</a> wouldn’t be amiss ☻
<br />The <a href="http://www.mirbsd.org/rss.htm#lists">mailing list</a> can also
 be used for this. The <a href="#faq">extra resources</a> from the FAQ
 entry just one above should also be considered ;-)</p>
<p>If you insist on sending a bugreport, IRC and the mailing list are
 great places for that; <a href="https://launchpad.net/mksh">Launchpad</a>,
 an external gratis service provided by a company, can also be used if you
 like web-based issue trackers better.</p></div>
<h1>Imprint</h1>
<p>This offline HTML page for mksh 59c-9+b2 was automatically generated
 from the sources.</p>
</body></html>
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