Hello
I wrote a blog post explaining the philosophy of the 's' shell better.
Here it is reformatted for email, I hope it'll be interesting to the
suckless group:
# { :; }
After the Shellshock bug happened they fixed the parsing bug in bash
that let people hijack webservers by simply including `{ :; }`.
<https://en.wikipedia.org/wiki/Shellshock_(software_bug)>
Fixing a bug after it's been found is good but it doesn't have to be
the end. Sometimes you think about the bigger picture and see if why
it was ever a bug in the first place.
I suspected that since bash syntax is very complex the parser is going
to be a lot of complicated code too. So I did some line counts of
various shells to what the situation is like:
.c .h
gnu bash: 138227, 13746
zsh: 135589, 5698
shivers scsh: 118475 (scheme)
templeos: 119115
mirbsd mksh: 29223, 2562
debian dash: 16503, 2084
freebsd sh: 15453, 1622
es shell: 9017, 1436
plan9 rc: 5989, 327
execline: 3794, 117
The line count for templeos isn't just counting its shell. It's the
linecount of the entire system: The operating system kernel, the
compiler, the shell, the drawing program, the graphic user interface
the 3D flight simulator. Bash, zsh and scsh have more code in them
than an entire operating system.
Shell syntax being overcomplex and ideosyncratic is a long standing
problem. Other than shellshock, annoyances happen on a daily basis.
Like the time a bug in a bash script in steam rm -rf'd peoples home
directory "Ran steam. It deleted everything". I've had makefiles break a
few times because there's a space in my path. It took these people four
years to fix that "Build failure when builddir contains spaces"
<https://github.com/nodejs/node-gyp/issues/65>
<https://github.com/valvesoftware/steam-for-linux/issues/3671>
My idea to improve things is to use an extremely minimal shell with
the simplest syntax possible.
# The new command interpreter
So I designed a new shell that aims to be extremely simple, and
implemened it in a short number of lines of code. Right now it's 780
lines plus 1200 for the linenoise library. (Technically it is a
"command interpreter" not a shell since shell implies it implements
the POSIX shell standard).
Here is the man page for *s*:
S(1) General Commands Manual
S(1)
NAME
s – command line interpreter
SYNOPSIS
s [script]
DESCRIPTION
s has two main modes of operation: interactive shell and batch
script
processor.
If the command line argument script is provided it will execute
that
file. This also allows you to use s as the shebang line in an
executable.
Alternatively you can provide commands via stdin.
Otherwise if you start s in a terminal you will be given an
interactive
shell prompt like this:
s$ echo hello world!
hello world!
SYNTAX
The lexical syntax of the shell language is very strictly tokenized
based
on spaces. Variables may occur inside tokens with ${FOO} syntax,
the {}'s
are optional.
The # character starts a comment until the end of the line.
A token always stays a single token there is no globs or bash style
expansion. You will need to master xargs to use this shell.
GRAMMAR
The grammar is line based. Sequences of tokens are treated as
commands
and the operators | , && and || are parsed in order of tightest
binding
first.
At the end of a line you can specify that job should be run in the
background with &
BUILTINS
cd [directory]
set variable value
unset variable
source filename
If there is an error the builtins will cause the script to exit
immediately, in interactive mode a warning is printed.
The important thing is what's missing: We don't have globs or
"splatting" where a variable `$FOO` turns into multiple command line
arguments. One token stays one token forever. This is a "no surprises"
straightforward approach.
We don't even have redirection operators `>` in the shell language.
They are added as extra programs. So you write `grep foo bar | >
out.txt`. `>` is just another unix command. `<` is essentially `cat`.
A `glob` program is also provided along with *s*, so you write
`glob rm *`.
The benefit of this extreme minimalism is that the implementation can
be done with a very logical pipeline of operations: tokenizing splits
the input based on spaces and expands the variables out, parsing looks
for the operators and builds an AST, interpreting is a simple recursive
walk down the AST implementing the various operators.
* tokenizer.c: https://github.com/rain-1/s/blob/master/tokenizer.c
* variables.c: https://github.com/rain-1/s/blob/master/variables.c)
* parser.c: https://github.com/rain-1/s/blob/master/parser.c
* interpreter.c: https://github.com/rain-1/s/blob/master/interpreter.c
* repo: https://github.com/rain-1/s
What do you think? I would be very interested in hearing peoples
opinions about *s* and the more general idea of stripping programs
down to their essential parts. Feel free to comment or make 'issues'
on the repo if you have any criticism, suggestions or just want to
discuss.
# See also
* https://skarnet.org/software/execline/
