Shell命令如何执行

Question

I am a newbee and looking for some info.

Thanks in advance.

1. What is difference between echo "Hello World!" and a c-program which prints "Hello World!" using printf.

2. How do shell commands get executed. For example if I give ls it lists all the files in the directory. Is there executable binary which is run when we enter ls in shell.

Please let me know if you guys have any links or source to get this clear.

Answer 1

There are two main types of "commands" that the shell can execute. Built-in commands are executed by the shell itself - no new program is started. Simply typing echo in a shell prompt is an example of such a built-in command.

On the other hand, other commands execute external programs (also called binaries) - and ls is an example of this kind of command.

So, if you run echo in a shell, it's executed by the shell itself, but if you write a C program that performs the same action, it wil be run as an external program. As a matter of fact, most Linux systems come with such a binary, located at /bin/echo .

Why does it sometimes make sense to have both a built-in command and a program to accomplish the same task? Built-in commands are faster to execute as there is some cost involved in running an external program. But built-ins have some drawbacks, too: they can't be too complex as this would make the shell big and slow; they can not be upgraded separately from the shell and from each other; finally, there are situations where an external program which is not your shell would like to run an application: it can run external programs but it can't execute shell built-ins directly since it's not the shell. So sometimes it makes sense to have it both ways. Apart from echo , time is another example of this double approach.

Answer 2

The shell is just a user level way of interacting with the operating system, or the kernel. That's one of the reasons it's called a shell . The shell itself ( sh , csh , tcsh , ksh , zsh , bash , etc...) is essentially just a binary the operating system executes to allow you to execute other binaries.

It generally gives a lot of other functionality though like built in functions ( echo , fg , jobs , etc...), an interpreted language ( for x in ... , if then , etc...), command history, and so on...

So, any text entered into the shell (like echo ), the binary (or process) interprets and runs the corresponding functions in its code. Built in functions (like echo ) don't need to create a new process, but if the text is interpreted as a request to execute a binary ( vim , emacs , gcc , test , true , false , etc...) the shell will create a new process for it (unless you prefix it with exec ), and execute it.

So, echo "Hello World! just runs code in the shell (process). A printf("Hello World!") would be in seperate binary that the shell would create a new process for ( fork ), and have the operating system execute ( exec ).