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Building a Shell

healeycodes.com

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Re: Building a Shell

#31

Post by giancarlostoro »

doe88 wrote: post #30

Is there a (real) shell whose code is relatively short and self contained and would be valuable to read? This was always something I wanted to do but never quite spent time to look for a good one to explore.

Although not the same... Destroy All Software has videos on building your own shell using Ruby. I watched it to learn and it was a lot of fun to watch him basically building a shell, I'm not really a Ruby guy, but it was easy to grasp. It's not free, you would need a subscription, but its worth the watch otherwise.

https://www.destroyallsoftware.com/screencasts/catalog/shell...

Re: Building a Shell

#33

Post by epr »

doe88 wrote: post #30

Is there a (real) shell whose code is relatively short and self contained and would be valuable to read? This was always something I wanted to do but never quite spent time to look for a good one to explore.

I think there's a good one if you search around for "xv6 sh.c". Hard to tell immediately from a google search just now since there are many implementations (people do it in school) and github's currently blocking requests from my phone.

Also helpful may be running strace on your shell, then reviewing the output line by line to make sure you understand each. This is a VERY instructive exercise to do in general.

Re: Building a Shell

#34

Post by lasgawe »

Great article. There are many things every developer should do when starting to learn programming or when trying to improve their skills. This is one of them. I once built a shell-like programming language (not an interpreter). If anyone reading this wants to improve their skills, I strongly suggest building your own shell from scratch.

Re: Building a Shell

#35

Post by zokier »

healeycodes wrote: post #18
austy69 wrote: post #16

Fun read. Wonder if you are able to edit text in the shell, or if you need to implement a gap buffer to allow it?

Editing the current line works because I brought in https://man7.org/linux/man-pages/man3/readline.3.html towards the end so I could support editing, tab completion, and history. IIRC readline uses a `char *` internally since the length of a user-edited line is fairly bounded.

worth noting that you get basic line editing for "free" from kernels tty subsystem even if you don't use readline.

Re: Building a Shell

#37

Post by themafia »

zokier wrote: post #2

Bit of pedantry but I don't think traditional unix shell (like this) follows repl model; the shell is not usually doing printing of the result of evaluation . Instead the printing happens more as a side effect of the commands.

It prints a prompt.

Re: Building a Shell

#39

Post by tame3902 »

doe88 wrote: post #30

Is there a (real) shell whose code is relatively short and self contained and would be valuable to read? This was always something I wanted to do but never quite spent time to look for a good one to explore.

It depends on what you are looking for. My recommendation for learning "how is X done in a shell" is the OpenBSD ksh: https://github.com/ibara/oksh

It's what they use for /bin/sh, it has everything that a complete shell needs (including a mechanism for providing command completions) and has code that is much easier to read than bash or zsh.

Something that I also would recommend is the design document for the plan9 rc shell; it is a worthwhile read for anybody interested in shells: https://doc.cat-v.org/plan_9/4th_edition/papers/rc

An implementation is also available if one wants to look at how it could be done: https://github.com/rakitzis/rc

Re: Building a Shell

#40

Post by emmelaich »

doe88 wrote: post #30

Is there a (real) shell whose code is relatively short and self contained and would be valuable to read? This was always something I wanted to do but never quite spent time to look for a good one to explore.

I don't know how real you want -- those criteria are probably self contradictory :-)

Marc Rochkind's book Avanced UNIX Programming implemented a basic shell, through iterations. You can see the first at e.g. here https://github.com/gmarler/AUPv2/blob/master/c5/sh0.c

It might be a bit old too. The book is very good but again, quite old. There seem to be free copies of it on the net.

BTW, does anyone know if Marc Rochkind is alive? His site basepath.com seems to be for sale :-(

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