lksh(1) Legacy Korn shell built on mksh


-words [-+abCefhiklmnprUuvXx [-+o opt ] ] [-c string | -s | file [args ... ] ]


is a command interpreter intended exclusively for running legacy shell scripts. It is built on mksh refer to its manual page for details on the scripting language. It is recommended to port scripts to mksh instead of relying on legacy or idiotic POSIX-mandated behaviour, since the MirBSD Korn Shell scripting language is much more consistent.

Note that it's strongly recommended to invoke with at least the -o posix option, if not both that and -o sh to fully enjoy better compatibility to the POSIX standard (which is probably why you use over mksh in the first place) or legacy scripts, respectively.


currently has the following differences from mksh

  • There is no explicit support for interactive use, nor any command line editing or history code. Hence, is not suitable as a user's login shell, either; use mksh instead.
  • The KSH_VERSION string identifies as ``LEGACY KSH'' instead of ``MIRBSD KSH'' Note that the rest of the version string is identical between the two shell flavours, and the behaviour and differences can change between versions; see the accompanying manual page mksh(1) for the versions this document applies to.
  • uses POSIX arithmetics, which has quite a few implications: The data type for arithmetics is the host ISO C Vt long data type. Signed integer wraparound is Undefined Behaviour; this means that...
    $ echo $((2147483647 + 1))

    ... is permitted to, e.g. delete all files on your system (the figure differs for non-32-bit systems, the rule doesn't). The sign of the result of a modulo operation with at least one negative operand is unspecified. Shift operations on negative numbers are unspecified. Division of the largest negative number by -1 is Undefined Behaviour. The compiler is permitted to delete all data and crash the system if Undefined Behaviour occurs (see above for an example).

  • only offers the traditional ten file descriptors to scripts.
  • The rotation arithmetic operators are not available.
  • The shift arithmetic operators take all bits of the second operand into account; if they exceed permitted precision, the result is unspecified.
  • The GNU bash extension &> to redirect stdout and stderr in one go is not parsed.
  • The mksh command line option -T is not available.
  • Unless set -o posix is active, always uses traditional mode for constructs like:
    $ set -- $(getopt ab:c "$@")
    $ echo $?

    POSIX mandates this to show 0, but traditional mode passes through the errorlevel from the getopt(1) command.

  • Unlike AT&T System ksh mksh in -o posix or -o sh mode and lksh do not keep file descriptors > 2 private from sub-processes.
  • Functions defined with the function reserved word share the shell options (set -o ) instead of locally scoping them.


The distinction between the shell variants (lksh / mksh ) and shell flags (-o posix / sh ) will be reworked for an upcoming release.

To use as /bin/sh compilation to enable set -o posix by default if called as sh is highly recommended for better standards compliance. For better compatibility with legacy scripts, such as many Debian maintainer scripts, Upstart and SYSV init scripts, and other unfixed scripts, using the compile-time options for enabling both set -o posix -o sh when the shell is run as sh is recommended.

tries to make a cross between a legacy bourne/posix compatibl-ish shell and a legacy pdksh-alike but ``legacy'' is not exactly specified.

The set built-in command does not currently have all options one would expect from a full-blown mksh or pdksh

Talk to the MirOS development team using the mailing list at Aq [email protected] or the #!/bin/mksh (or #ksh ) IRC channel at (Port 6697 SSL, 6667 unencrypted) if you need any further quirks or assistance, and consider migrating your legacy scripts to work with mksh instead of requiring .