Pragmatic summary of shells

The ones in bold are the ones you might run into in day-to-day software. The others, you might notice occasionally in old documents. This is probably incorrect (I was not around when the majority of these were written), but maybe helpful.

I categorise the following as "POSIX-compatible shells". The subset of functionality that's possible in POSIX sh is fairly limited, but worth being familiar with, since they will run pretty much everywhere.

• POSIX sh: a standard written in the 80s or something that defines how shells "should" behave
  • Almquist Shell (ash): comes with busybox (a one-binary toolkit that runs on many embedded systems)
    • Debian Almquist Shell (dash): /bin/sh on Ubuntu and Debian systems
  • Bourne Shell: default on System 7 Unix
    • ksh: nobody uses this any more
    • Bourne Again Shell (bash): the default on most modern systems

The following are not compatible with POSIX sh, but have more user-friendly features or customisation. Typically people will use these as their "main interface", but continue to write scripts in POSIX sh or bash.

• C-shell (csh): nobody uses this any more
  • tcsh: nobody uses this any more
  • zsh: way too many features, a favourite of a lot of software engineers. Has a compatibility mode for POSIX sh.
• fish: friendly shell focusing on interactive mode, not scripts
• Powershell: object-oriented cross-platform shell created by Microsoft

The following are some wild new approaches to shells that haven't hit the mainstream yet.

• Oil Shell: aims to be a bash-compatible modern shell with nicer syntax and semantics.
• Nushell: data-oriented cross-platform shell.

Alternative reading:

• https://developer.ibm.com/tutorials/l-linux-shells/
• https://en.wikipedia.org/wiki/Unix_shell