How does bash process user input from the command line

Victor43 · 07-19-2021, 12:22 PM

Quote:

Originally Posted by MadeInGermany

In short:
it forks a subshell that execs into an "ls -a".

Thank you for the response. When you say "execs into an "ls -a" do you mean to say that the subshell calls the function execve() in order to execute the parameter "ls -a" ?

computersavvy · 07-19-2021, 12:44 PM

the return from strace tells you the sequence that happens, simply analyze what it tells you and each step leads to the next.

For example, the first line returned from your "strace ls -a" (on my system) is

Code:

execve("/usr/bin/ls", ["ls", "-a"], 0x7ffd9a4e7948 /* 99 vars */) = 0

which says that the shell calls execve to create the subshell and execute "/usr/bin/ls" (within that sub-shell) with the parameters "ls" and "-a" as $0 and $1, the 0x.... is the memory address to use, the /* ... */ is a comment, and the final "= 0" says it was successful
The remainder of the return is what is done inside the ls command, including the multiple write(....) statements that return the results back to the original shell (your terminal window).

MadeInGermany · 07-22-2021, 02:40 PM

Quote:

Originally Posted by Victor43

Thanks for the response but what does the above mean in layman's terms "forks a subshell" ? Please correct me if I'm wrong but a fork() system call is called which in turn creates a child process that in turn calls system function call execve() which processes the "ls -a" command. Does this sound correct ?

Thanks

Yes, but the execve() is special because the kernel replaces the caller by the called program.
And that is why the shell must fork before - so there is something that can wait and continue.
BTW in Linux the fork() is realized by clone() - in
strace -f
you'll see clone() then execve()

igadoter · 07-22-2021, 04:33 PM

It is wrong end of your problem. Why do you need such things? Just compile hello world program in C. Run it. You obtain all needed information. Besides have some fun. Say

Code:

$ echo 'void main(void) { ; }' > foo.c
$ cc foo.c 
$ ./a.out 
$ echo $?
242

ever hear about error code 242?

Code:

$ strace ./a.out 
execve("./a.out", ["./a.out"], 0x7ffcb9967fd0 /* 59 vars */) = 0
brk(NULL)                               = 0x837000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
...........