How does bash process user input from the command line

Victor43 · 05-11-2020, 12:41 PM

Running Ubuntu 19.10 have a question about Bash

How does bash process user input such as a command like "ls -a" ? How is this actually done in detail as possible ?

shruggy · 05-11-2020, 12:49 PM

From here:

Quote:

After a command is entered, the following things are done:

Command is entered and if length is non-null, keep it in history.
Parsing : Parsing is the breaking up of commands into individual words and strings
Checking for special characters like pipes, etc is done
Checking if built-in commands are asked for.
If pipes are present, handling pipes.
Executing system commands and libraries by forking a child and calling execvp.
Printing current directory name and asking for next input.

Here is another explanation.

And here are all the gory details. In particular:

Quote:

...
Now, thought experiment: What happens when you type ls into a shell? You know the fork(), exec() and wait() cycle that occurs, along with tty. But, even before this happens, ls is just another utility function, right? Which means there’s a program file somewhere which has the C code that does fork() and everything else.
...

and so on.

teckk · 05-11-2020, 01:47 PM

Quote:

How does bash process user input such as a command like "ls -a"

http://ftp.gnu.org/pub/gnu/coreutils/
https://github.com/coreutils/coreuti...aster/src/ls.c
http://www.gnu.org/software/coreutils/
http://git.savannah.gnu.org/cgit/cor.../tree/src/ls.c

MadeInGermany · 05-11-2020, 04:22 PM

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

Victor43 · 05-12-2020, 04:30 PM

Quote:

Originally Posted by teckk

http://ftp.gnu.org/pub/gnu/coreutils/
https://github.com/coreutils/coreuti...aster/src/ls.c
http://www.gnu.org/software/coreutils/
http://git.savannah.gnu.org/cgit/cor.../tree/src/ls.c

Thanks for the response. Can you tell me whether the above links are for the source code for bash ?

shruggy · 05-13-2020, 02:02 AM

No, they are for the source code of ls.

Turbocapitalist · 05-13-2020, 02:51 AM

Quote:

Originally Posted by Victor43

Can you tell me whether the above links are for the source code for bash ?

You can use the package manager to get the actual source, with modifications, used by your distro.
If you are on Ubuntu or other dpkg-based distro, you can fetch the source package with apt-src or apt-get like this:

Code:

cd /tmp/
apt-get source bash
ls -ld bash*
cd bash-5.0

You will find all the C files there that make up Bash.

pan64 · 05-13-2020, 03:01 AM

Quote:

Originally Posted by Victor43

How does bash process user input such as a command like "ls -a" ? How is this actually done in detail as possible ?

That is written on the man page of bash. Look for command execution. I cannot explain better (do not need to read the source code)

Victor43 · 05-13-2020, 10:43 AM

Quote:

Originally Posted by shruggy

From here:

Here is another explanation.

And here are all the gory details. In particular:

and so on.

Thank you shruggy. I have a question to ask. How did you figure out what system calls are being executed ? Did you use ltrace and strace or even gdb ?

pan64 · 05-13-2020, 10:46 AM

yes, strace will tell you.
But in general those are the fork and exec (to start another process).

Victor43 · 05-31-2020, 02:14 PM

Quote:

Originally Posted by pan64

yes, strace will tell you.
But in general those are the fork and exec (to start another process).

Many thanks pan64.

MadeInGermany · 06-03-2020, 01:53 AM

Actually on a recent Linux system strace will report clone() rather than fork().

Victor43 · 07-21-2020, 02:57 PM

Quote:

Originally Posted by pan64

yes, strace will tell you.
But in general those are the fork and exec (to start another process).

Thanks again but I could not find the system call fork() (when running strace ls -l) maybe I've missed something ? Also when I execute strace ls -l within a terminal window I see execve at the top of the list not exec.

Please advise. Thanks.

pan64 · 07-22-2020, 01:29 AM

what to advice? there was no fork that's why you can't find it.
ls is a simple binary, it does not fork any other process.
about exec: see the man page: https://man7.org/linux/man-pages/man3/exec.3.html (and the description)

Victor43 · 07-08-2021, 10:08 AM

Quote:

Originally Posted by MadeInGermany

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

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