I know that external commands are run in the shell by creating a separate process, but what exactly happens when a built-in command is run in a shell?
Are they executed as a function, or does the shell create a new thread to execute them?
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That's going to be shell-specific. And it probably varies depending on the command (and possibly how its used in a pipe). – derobert Jun 18 '13 at 20:22
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okay for example if the shell is bash, and command is 'cd', so how is it implemented? – corneliusfelix Jun 18 '13 at 20:25
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For your concrete example, there is a function cd_builtin, which is defined in builtins/cd.def (in the bash source code). It normally does a cd by calling that function. But it may fork first if you use it in a pipeline—for example, cd / | echo forks and calls cd_builtin in the child. You can also notice this by how the directory doesn't actually change:
anthony@Zia:~$ cd /tmp/
anthony@Zia:/tmp$ cd / | echo -n
anthony@Zia:/tmp$ cd /
anthony@Zia:/$
Notice how the directory only changes when I don't pipe from cd.
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Built-in commands, by definition, are executed inside the main executable, as opposed to in a different program.
All shell commands are synchronous: the shell waits for the command to complete before it executes the next one. When running an external command, the shell has to create a separate process to run that command, and wait for it to exit. When running a built-in command, there is no need to create a new thread of execution: the command is executed inside the main thread.
I don't think any of the common shells use threads in their internal design. Shell capabilities are pretty tied with the traditional single-thread-per-process Unix model, there's no problem in shell design that threads would solve.
It's highly likely that every builtin is implemented by a function at some level in the shell's source code.
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I would think they run as a function within the space of the bash executable. There is a comment in the Advanced Bash Scripting Guide that states it as follows:
excerpt
A builtin is a command contained within the Bash tool set, literally built in. This is either for performance reasons -- builtins execute faster than external commands, which usually require forking off 1 a separate process -- or because a particular builtin needs direct access to the shell internals.
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but here we aren't creating a new process, we are creating a new thread whose execution is faster than creating a process. i agree that function implementation is faster than thread creation. But the scenario here is this for the execution time: process creation>thread creation>function calling. We can't be sure whether it is thread or function – corneliusfelix Jun 18 '13 at 20:21
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1I think you're getting hung up on semantics. I'm pretty sure these are serviced by function calls by bash. My reasoning is that if I watch my bash process using a tool such as htop configured to show threads, there aren't any when I run the different built-in commands to bash. – slm♦ Jun 18 '13 at 20:27
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