Add support for Yash (Yet another shell)

[suprstarrd]

Code of Conduct

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What would you like to see added?

Requesting support for Yash (acronym for "Yet another shell") - website here. This is a shell I regularly use.

From their project README:

Yash, yet another shell, is a POSIX-compliant command line shell
written in C99 (ISO/IEC 9899:1999). Yash is intended to be the most
POSIX-compliant shell in the world while supporting features for daily
interactive and scripting use. Notable features are:

• Global aliases
• Arrays
• Socket redirection, pipeline redirection, and process redirection
• Brace expansion and extended globbing
• Fractional numbers in arithmetic expansion
• Prompt command and command-not-found handler
• Command line completion with predefined completion scripts for more
  than 100 commands
• Command line prediction based on command history

This could also be used to rework the Bash implementation for further compatibility with even more shells.

Something I have independently discovered is that files ending in .sh are automatically placed in a "POSIXly-correct" mode which removes some of these additional Yash features. (As an aside, that functionality could be helpful if you want to implement support for Bash's POSIX mode too. See Bash's POSIX mode limitations.)

Remainder of the Yash README (with things like implementation details and other limitations)

Yash now fully supports POSIX.1-2024 (IEEE Std 1003.1)
except for the limitations listed below.

Yash is stable. A maintenance update is released every three months or
so.

• In C, a null character represents the end of a string. If input to
  the shell itself contains a null character, characters following
  the null character will be ignored.
• The GCC extension keyword __attribute__ is used in the source
  code. When not compiled with GCC or Clang, this keyword is removed
  by the preprocessor, so generally there is no harm. But if your
  compiler uses this keyword for any other purpose, compilation may
  fail. Additionally, some other identifiers starting with _ may
  cause compilation errors on some rare environments.
• Some signals are assumed to have the specific numbers:
  SIGHUP=1 SIGINT=2 SIGQUIT=3 SIGABRT=6
  SIGKILL=9 SIGALRM=14 SIGTERM=15
• POSIX disallows non-interactive shells to ignore or catch SIGTTIN,
  SIGTTOU, and SIGTSTP by default. Yash, however, ignores these
  signals if job-control is enabled, even if non-interactive.
• File permission flags are assumed to have the specific values:

  0400=user read    0200=user write   0100=user execute
  0040=group read   0020=group write  0010=group execute
  0004=other read   0002=other write  0001=other execute
  

• The character categorization in locales other than the POSIX locale
  is assumed upward compatible with the POSIX locale.
• The -o nolog option is not supported: it is silently ignored.
• According to POSIX, the command printf %c foo should print the
  first byte of string foo. Yash prints the first character of
  foo, which may be more than one byte.
• The return built-in, if executed in a trap, can operate only on a
  function, script, or loop that has been executed within the trap.
  This limitation is not strictly POSIX-compliant, but needed for
  consistent and predictable behavior of the shell.
• Results of pathname expansion is sorted only by collating sequence
  of the current locale. If the collating sequence does not have a
  total ordering of characters, order of uncomparable results are
  unstable. This limitation is not strictly POSIX-compliant, but
  inevitable due to use of wide characters in the whole shell.
• Most part of the shell cannot handle bytes that do not represent
  valid characters, because string operations are written in terms of
  wide character strings. This design choice was made before POSIX
  added requirements for the shell to accept arbitrary bytes in some
  operations, and it is too late to fully implement them.
• The shell needs the setrlimit function to implement the behavior
  of propagating the wait status of a signaled child process to the
  parent process. This behavior is disabled if the setrlimit
  function is not available, in which case the shell will not be fully
  POSIX-compliant.