Yash: yet another shell

https://magicant.github.io/yash/ This README is also available in 日本語.

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

Yash can be modified/redistributed under the terms of the GNU General Public License (Version 2) but the use of this program is without any warranty. See the COPYING file for the full text of GPL.

Yash is maintained on GitHub, but was formerly on OSDN.

Current Development Status

Yash now fully supports POSIX.1-2008 (IEEE Std 1003.1, 2016 Edition) except for the limitations listed below.

Yash is stable. A maintenance update is released every three months or so. For the change history, see the NEWS file.

Requirements

Yash is supposed to build and run on any POSIX.1-2001 environment with the Software Development Utilities and the C-Language Development Utilities options.

Currently, yash is mainly tested on Fedora, macOS, and Cygwin.

Installation

See the INSTALL file to see how to build and install yash.

After installation, the manual can be viewed by

$ man yash

The manual is also available online at https://magicant.github.io/yash/doc/.

Basic Configuration

Below is a description of basic configuration that you might want to see after installation to get started with yash. For configuration details, see the manual.

Initialization scripts

When yash is started as a login shell, it reads $XDG_CONFIG_HOME/yash/profile or ~/.yash_profile as a fallback. This file is a shell script in which you define environment variables using the export command.

When yash is started for an interactive use, it reads $XDG_CONFIG_HOME/yash/rc or ~/.yashrc (after reading $XDG_CONFIG_HOME/yash/profile if it is a login shell also). In this file, you make other configurations such as aliases, prompt strings, key bindings for command line editing, and command-not-found handler. Use the share/initialization/sample file as a template for your $XDG_CONFIG_HOME/yash/rc.

Making yash your login shell

In many Unix-like OSes, a shell must be listed in /etc/shells to be set as a login shell. Edit this file and ensure that the path to yash is written in the file.

Then, run the chsh command in the terminal and follow instructions from the command. Depending on your system, you may have to use another command to change the login shell. See documentation on your system.

Implementation Notes

• 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 value of variable PS1 is subject to parameter expansion. Yash performs command substitution and arithmetic expansion as well on the PS1 value.
• 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.

Known Issues

• Line number ($LINENO) may not be counted correctly in and after a complex expansion containing a line continuation.
• Non-ASCII characters may not be correctly handled in some locales on Solaris. This may be worked around by undefining the HAVE_WCSNRTOMBS macro in the config.h header file.

Contributions

Comments, suggestions, and bug reports are welcome at:

• Issue tracking system
• Discussion forum

If you are interested in translation, please refer to TRANSLATING.md.

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Watanabe, Yuki magicant@wonderwand.net