what are shell built-in commands in linux?

Question

I have just started using Linux and I am curious how shell built-in commands such as cd are defined.

Also, I'd appreciate if someone could explain how they are implemented and executed.

Answer 1

If you want to see how bash builtins are defined then you just need to look at Section 4 of The Bash Man Page.

If, however, you want to know how bash bultins are implemented, you'll need to look at the Bash source code because these commands are compiled into the bash executable.

One fast and easy way to see whether or not a command is a bash builtin is to use the help command. Example, help cd will show you how the bash builtin of 'cd' is defined. Similarly for help echo.

Answer 2

The actual set of built-ins varies from shell to shell. There are:

• Special built-in utilities, which must be built-in, because they have some special properties
• Regular built-in utilities, which are almost always built-in, because of the performance or other considerations
• Any standard utility can be also built-in if a shell implementer wishes.

You can find out whether the utility is built in using the type command, which is supported by most shells (although its output is not standardized). An example from dash:

$ type ls
ls is /bin/ls
$ type cd
cd is a shell builtin
$ type exit
exit is a special shell builtin

Re cd utility, theoretically there's nothing preventing a shell implementer to implement it as external command. cd cannot change the shell's current directory directly, but, for instance, cd could communicate new directory to the shell process via a socket. But nobody does so because there's no point. Except very old shells (where there was not a notion of built-ins), where cd used some dirty system hack to do its job.

How is cd implemented inside the shell? The basic algorithm is described here. It can also do some work to support shell's extra features.