How do I write a function of some commands of shell (UNIX) in C language? E.g what is the source code of the exit function? How can we write its function in C lanugage?
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For example; exit command: Implement the exit command that quits the shell. (how to do this/ from which link/book ; a concept can be taken?)
and similarly For pwd command: Implement the pwd command that prints the user’s current working directory.
If you want the source code, you can find it at the GNU Coreutils page.
If you're asking how to exit a shell from within a program, the exit command line command is a bash builtin, there's no /bin/exit. You'd have to find the appropriate PID and kill it. The exit builtin probably performs some cleanup and then uses exit(int).
If you want the source code for the shell builtins, look at the bash source.
There are many shells which are open-source or free software, so you can download them and study their source code. A simple example is sash, a more complex example is zsh
The exit function (called from the shell's exit builtin) is inside GNU libc, it runs the registered handlers (e.g. by atexit etc..) and then call the _exit system call.
system calls are mostly handled by the linux kernel whose source code is on kernel.org
I strongly recommend reading a good textbook on operating systems, like e.g. Modern Operating Systems by Andrew Tanenbaum, or Understanding Operating Systems by Ann McHoes &, Ida M. Flynn.
I also recommend reading a good Posix/Unix programming book like e.g. Advanced Unix Programming by Rochkind & the late Stevens
exitis necessarily a shell built-in.