I’ve been contributing where I can to Simon Ser’s mrsh project, a work-in-progress strictly POSIX shell implementation. I worked on some small mrsh features during my holiday travels and it’s in the forefront of my mind, so I’d like to share some of its design details with you.
There are two main components to a shell: parsing and execution. mrsh uses a simple recursive descent parser to generate an AST (Abstract Syntax Tree, or an in-memory model of the structure of the parsed source). This design was chosen to simplify the code and avoid dependencies like flex/bison, and is a good choice given that performance isn’t critical for parsing shell scripts. Here’s an example of the input source and output AST:
#!/bin/sh
say_hello() {
echo "hello $1!"
}
who=$(whoami)
say_hello "$who"
This script is parsed into this AST (this is the output of mrsh -n test.sh):
program
program
└─command_list ─ pipeline
└─function_definition say_hello ─ brace_group
└─command_list ─ pipeline
└─simple_command
├─name ─ word_string [3:2 → 3:6] echo
└─argument 1 ─ word_list (quoted)
├─word_string [3:8 → 3:14] hello
├─word_parameter
│ └─name 1
└─word_string [3:16 → 3:17] !
program
program
└─command_list ─ pipeline
└─simple_command
└─assignment
├─name who
└─value ─ word_command ─ program
└─command_list ─ pipeline
└─simple_command
└─name ─ word_string [6:7 → 6:13] whoami
program
└─command_list ─ pipeline
└─simple_command
├─name ─ word_string [7:1 → 7:10] say_hello
└─argument 1 ─ word_list (quoted)
└─word_parameter
└─name who
Most of these names come directly from the POSIX shell specification.
The parser and AST is made available as a standalone public interface of
libmrsh, which can be used for a variety of use-cases like syntax-aware text
editors, syntax highlighting (see highlight.c), linters, etc. The most
important use-case is, of course, task planning and execution.
Most of these AST nodes becomes a task. A task defines an implementation of the following interface:
struct task_interface {
/**
* Request a status update from the task. This starts or continues it.
* `poll` must return without blocking with the current task's status:
*
* - TASK_STATUS_WAIT in case the task is pending
* - TASK_STATUS_ERROR in case a fatal error occured
* - A positive (or null) code in case the task finished
*
* `poll` will be called over and over until the task goes out of the
* TASK_STATUS_WAIT state. Once the task is no longer in progress, the
* returned state is cached and `poll` won't be called anymore.
*/
int (*poll)(struct task *task, struct context *ctx);
void (*destroy)(struct task *task);
};
Most of the time the task will just do its thing. Many tasks will have sub-tasks
as well, such as a command list executing a list of commands, or each branch of
an if statement, which it can defer to with task_poll. Many tasks will wait on
an external process, in which case it can return TASK_STATUS_WAIT to have the
process waited on. Feel free to browse the full list of tasks to get
an idea.
One concern more specific to POSIX shells is built-in commands. Some commands
have to be built-in because they manipulate the shell’s state, such as . and
cd. Others, like true & false, are there for performance reasons, since
they’re simple and easily implemented internally. POSIX specifies a list of
special builtins which are necessary to implement in the shell
itself. There’s a second list that must be present for the shell
environment to be considered POSIX compatible (plus some reserved names like
local and pushd that invoke undefined behavior - mrsh aborts on these).
Here are some links to more interesting parts of the code so you can explore on your own:
I might write more articles in the future diving into specific concepts, feel free to shoot me an email if you have suggestions. Shoutout to Simon for building such a cool project! I’m looking forward to contributing more until we have a really nice strictly POSIX shell.
Have a comment on one of my posts? Start a discussion in my public inbox by sending an email to ~sircmpwn/public-inbox@lists.sr.ht [mailing list etiquette]
Articles from blogs I follow around the net
Working full-time on open-source software
I’ll soon be working full-time on open-source software! I’m pleased to announce that I’m joining Sourcehut. Huge thanks to Drew DeVault for making this possible. I also want to thank everyone supporting Sourcehut and allowing it to grow. Being able to do …
via emersion October 15, 2019Updates in September 2019
This post gives an overview of the recent updates to the Writing an OS in Rust blog and the used libraries and tools. I finished my master thesis and got my degree this month, so I only had limited time for my open source work. I still managed to perform a…
via Writing an OS in Rust October 6, 2019Generated by openring
