On UNIX Shells

A (not so) brief history of UNIX shells

In UNIX, the shell is the text-mode program that interfaces between the user and the kernel via a teletype interface — which is usually purely a software construct these day. It interprets commands, starts programs as necessary, and pipes data between programs.

Like a lot of things in UNIX the original shell, /bin/sh, was created by Ken Thompson. In 1976, with UNIX System 7, the Thompson Shell was replaced with a new /bin/sh created by another colleague at Bell Labs, Stephen Bourne. The Bourne Shell had all the key features we expect today like unlimited string size, command substitution, redirection, loops, case statements and by 1979 was pretty much done.

In 1978, Bill Joy created the C shell /bin/csh with the intention of being more friendly as an interactive environment. It turned out to be a bad scripting environment but was popular at Berkely and became the default interactive shell in Berkely UNIX and BSD.

David Korn created a new shell /bin/ksh in the 1980s based on Stephen Bourne’s source code. Korn Shell was used a lot on Solaris and with Oracle things and OpenBSD.

Kenneth Almquist reimplemented a clone of the Bourne Shell for BSD as a part of the catastrophic 1990s copyright dispute with AT&T. Debian has forked ash to the Debian Almquist Shell dash.

The Bourne Again Shell bash is the GNU project reimplementation of Bourne Shell. GNU did not stop at cloning the Bourne Shell features, they put in a whole ton of interactive and programming features.

Z (zed) Shell is a reimplementation of bash with a more liberal license. zsh aims to have full compatibility with all of the bash features and even more features of its own.

There are more shells, but I’m going to stop now.

Common system shells

Ever since AT&T Research UNIX System 7, the world has agreed that the default system interpreter is “Bourne Shell”. This is codified in the POSIX.2 standard and Single UNIX Specification. Since not everyone who wanted to create a UNIX-type operating system had legal access to the Bourne Shell source code from AT&T, fancy later shells ksh, bash, and zsh have a trick where they pretend to be the lowly old Bourne Shell if they are named sh. ash and dash pretty much are just the same as good old sh and don’t have to do a lot of pretending.

You might be surprised how deeply the system shell /bin/sh is embedded. It is used by init to run startup scripts. It’s used by web servers to connect a user request to a CGI program. It’s used by mail servers to connect bits together internally. There are tons of system and server things that are connected together with /bin/sh.

This seemed pretty smart until the shellshock family of vulnerabilities in bash were discovered in 2014 which allowed tricking servers into running arbitrary code through public services on the Internet. Now it seems like a good idea that the system shell should be as minimal and hardened as possible.

Here’s how things break down in the real world:

Red Hat uses bash as /bin/sh.

Debian and Ubuntu use dash as /bin/sh and /bin/bash as the default interactive interpreter.

NetBSD uses ash as /bin/sh and FreeBSD has their own /bin/sh.

OpenBSD uses ksh as /bin/sh.

Apple is a bit rudderless. If I recall, originally Apple was tied to its BSD roots from NeXT and used pdksh for /bin/sh and a version of the C Shell as the default for interactive users in OS X. They changed that bash for both in 10.3 Panther to be more similar to Red Hat, but kept the rest of the core system utilities BSD not GNU.

Today Apple macOS uses a really, really old forked version of bash 3.2 with security patches applied as /bin/sh. Apple stopped including bash updates in macOS (neé OS X) because the GNU project changed the license of bash to GPLv3. In macOS 10.15 Catalina bash is still the system shell /bin/sh but they changed the default shell for new users to /bin/zsh and have added /bin/dash.

In retrospect, Apple’s half-hearted attempt to include bash as a linuxism was a mistake. I hope that the arrival of dash is a sign that Apple is going to delete their decrepit old version of bash and make dash the system shell soon.

Cut to the chase or “what I use”

For interactive shell use, I use bash.

I have tried all the shells. I really tried to like zsh but I have found by the time you install all the plugins and whatnot, it is painfully slow. Today I use bash everywhere as my interactive shell. Mostly this is because it’s installed and the default on every version of Linux. This means that I install my own modern copy of bash on a Mac.

For shell scripts, I generally use the #!/bin/sh shebang but am careful to use the Bourne Shell features and not BASH features. If you are writing a script that uses #!/bin/sh as the shebang, it needs to work with dash because that’s what is on Debian and Ubuntu.

If you really want to use something other than the 1978 Bourne Shell language for a shell script, don’t hard code a path to /bin/sh. Use the /bin/env trick to allow the system to find the first match in the path. Instead of #!/bin/bash use #!/bin/env bash.