Jethro's Braindump

What happens when we type a simple command on shell?

tags
Operating Systems

-source :: https://unix.stackexchange.com/questions/123457/what-happens-when-we-type-a-simple-command-on-shell

The shell is a program, that uses 2 important native C system calls: fork() and exec(). These 2 system calls are used in the creation of subprocesses.

Fork()

Fork creates a copy of the calling process as its child. This is how most processes (except for init) begins.

Exec()

Exec refers to a family of functions that replace the current image with a new process image. It replaces defining parts of the current process’s memory stack with new parts loaded from an executable (e.g. /bin/ls).

fork() is usually a necessary first step, otherwise the calling process will be replaced and no new process will be created.

Isn’t this inefficient?

No. The copy produced by the fork() is an abstraction, as the kernel uses a copy-on-write system. fork() creates a virtual memory map. If the copy is immediately followed by exec(), parts that are overwritten need not be copied. In addition, many parts of the child process (e.g. its environment) need not be duplicated.

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