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PROJECT BASH_BUILTIN

I wonder what you can do with a custom Bash builtin function written in C. Will it run in the same process as the main Bash thread? Can the custom builtin access other Bash builtins? And most importantly for my purposes, can the custom Bash builtin access script-based functions?

Using in Bash

Including a shared-object library for use in Bash entails the use of the enable function. Search the Bash manpage for *^ enable [. to jump to the documentation for the enable function.

Planning For Development

It's not so hard to setup a function to be available as a builtin, you define the function, an array of strings for a long description, then populate a struct builtin instance.

Reading the parameters is the first real hurdle I found. As I work it out, I'm recording my thoughts and discoveries.

Argument Handling

Assuming there are arguments to the builtin, there are a few argument types to consider

  1. Flag options have no values, but only set a flag. Take the -r grep options for recursive search

  2. Value options are followed by a value, that is, a value option usually consists of two arguments. (It can be a single argument if the space between the option and the value is omitted.) Think -e PATTERN in grep.

  3. Value arguments that are not options. For example, the filename after rm filename is not an option, but a value.

internal_getopt Concepts

This is not exhaustive, but is another record of questions and discoveries.

internal_getopt Arguments

internal_getopt() collects the command options out of the list of arguments. It only returns options, a separate pass through the WORD_LIST can retrieve string arguments.

  1. The first argument is a pointer to the WORD_LIST struct from which the arguments are read.

  2. The second argument is a list of recognized option letters. If the letter is followed by a colon, the option takes an argument, if the letter is followed by another letter instead, the option is a flag.

Collecting String Arguments

Look at walk_args.c for an example collecting string arguments.

References

I haven't written any code yet, but I am collecting what I hope is useful information. The links below are a record of places visited that I may want to find again.

  • This mirror of Bash source code is the best thing I've found (so far) from which I can see enough examples to figure out what's going on. I cloned it and looked in the bash/examples/loadables directory and poked around in the source files for some of the standard builtin functions.

    Under the cloned bash directory, the following files are of particular interest:

    • bash/builtins/common.h consolidates the contents of several C source files.

    • bash/variables.h and bash/variables.c will help the developer understand access to script variables.

    • bash/examples/loadables/asortr.c is an example of working with arrays, both regular and associative, particularly in function sort_index.

  • /usr/include/bash contains several header files that appear to be designed for custom Bash builtins.

  • Bash GitHub Page may include source code for the standard builtins that can demonstrate implementation details we might need for our own builtins.

  • This simple example my be the minimalist example needed for a boilerplate builtin project.