LFS201 - Clarification Questions 1
syntapy
Posts: 8
It says when runnig a command in the command shell, it first forks a process from the parent, then a system call puts the parent to sleep.
Q: Where does that system call come from? Is it the command shell that forked the process or the child process?
Q: And is the fork initiated by the
It is really interesting, and I'm trying to understand it
It says:
A new process is created (forked from the user's login shell).
Ok, fork is a system call initiated by said login shell, correct?
A wait system call puts the parent shell process to sleep.
Is the wait system call initiated by the child or the parent process?
The command is loaded onto the child process's space via the exec system call. In other words, the code for the command replaces the bash program in the child process's memory space.
which process (child or parent) makes the exec system call?
The command completes executing, and the child process dies via the exit system call.
Is the exit system call something that is triggered in the child process code?
The parent shell is re-awakened by the death of the child process and proceeds to issue a new shell prompt.
The parent shell then waits for the next command request from the user, at which time the cycle will be repeated.
Next question: It says builtins are built into the shell code directly and do not involve loading programs, but when I run which echo or which kill I get that they are both in /usr/bin
Does that mean my specific system its not builtin? Or does builtin mean that the binaries for those are loaded directly into the process data or code or whatnot, i.e. that running builtins basically involve an exec call as opposed to a fork and exec call from normal shell calls
Comments
On your last point, I don't know a way to show the actual command is being run by a bash built-in using either which or whereis. However, if you type help echo (or any other command) you get the relevant part of the man page for bash. If you are asking about something not built in (try help cat) you will get an error, and if you type just help you will get a list of all builtins.
As for your forking questions the work is done by bash, as the parent and child process can be run in many different ways as so cannot be smart enough to tell someone to sleep or wait etc.
From your comment I found out that turns out none of the other builtins show a directory from which, so if I remove /usr/bin/echo then I can still run echo. which was showing a non-builtin binary of echo that was shadowing the builtin version
Interesting
Thank you for the help
There are many more utilities with both versions. For example "test" or "ulimit" etc. (i.e., you can do which ulimit and you can do help ulimit) Now there are times which ones you use makes a difference as the ones built into help sometimes have different options or output. I know this is different for "echo" and "time". Try something as simple as "time ls" vs "/usr/bin/time ls"
Oh weird, yeah the '/usr/bin/time ls' is different