POSIX sh EBNF grammar

Question

Is there an existing POSIX sh grammar available or do I have to figure it out from the specification directly?

Note I'm not so much interested in a pure sh; an extended but conformant sh is also more than fine for my purposes.

Answer 1

I've had multiple attempts at writing my own full blown Bash interpreters over the past year, and I've also reached at some point the same book appendix reference stated in the marked answer (#2), but it's not completely correct/updated (for example it doesn't define production rules using the 'coproc' reserved keyword and has a duplicate production rule definition for a redirection using '<&', might be more problems but those are the ones I've noticed).

1. The best way i've found was to go to http://ftp.gnu.org/gnu/bash/
2. Download the current bash version's sources
3. Open the parse.y file (which in this case is the YACC file that basically contains all the parsing logic that bash uses) and just copy paste the lines between '%%' in your favorite text editor, those define the grammar's production rules
4. Then, using a little bit of regex (which I'm terrible at btw) we can delete the extra code logic that are in between '{...}' to make the grammar look more BNF-like.

The regex i used was :

(\{(\s+.*?)+\})\s+([;|])

It matches any line non greedily .*? including spaces and new lines \s+ that are between curly braces, and specifically the last closing brace before a ; or | character. Then i just replaced the matched strings to \3 (e.g. the result of the third capturing group, being either ; or |).

Here's the grammar definition that I managed to extract at the time of posting https://pastebin.com/qpsK4TF6

Answer 2

The POSIX standard defines the grammar for the POSIX shell. The definition includes an annotated Yacc grammar. As such, it can be converted to EBNF more or less mechanically.

If you want a 'real' grammar, then you have to look harder. Choose your 'real shell' and find the source and work out what the grammar is from that.

Note that EBNF is not used widely. It is of limited practical value, not least because there are essentially no tools that support it. Therefore, you are unlikely to find an EBNF grammar (of almost anything) off-the-shelf.