bash 2.0 introduced a new feature that increased the flexibility of the shell: dynamically loadable built-ins. On systems that support dynamic loading, you can write your own built-ins in C, compile them into shared objects, and load them at any time from within the shell with the enable built-in (see Chapter 7 for details on all of the enable options).
This appendix will discuss briefly how to go about writing a built-in and loading it in bash. The discussion assumes that you have experience with writing, compiling, and linking C programs.
The bash archive contains a number of pre-written built-ins in the directory examples/loadables/. You can build them by uncommenting the lines in the file Makefile that are relevant to your system, and typing make. We’ll take one of these built-ins, tty, and use it as a “case study” for built-ins in general.
tty will mimic the standard UNIX command tty. It will print the name of the terminal that is connected to standard input. The built-in will, like the command, return true if the device is a TTY and false if it isn’t. In addition, it will take an option, -s, which specifies that it should work silently, i.e., print nothing and just return a result.
The C code for a built-in can be divided into three distinct sections: the code that implements the functionality of the built-in, a help text message definition, and a structure describing the built-in so that bash can access it.
The description structure is quite ...