What does sh -c “ps -l” mean in linux?

Question

My Command Interpreter is bash shell.After opening terminal on my OS(cent OS), I have executed following commands:

scenario 1

$sh -c "ps -l"
UID   PID   PPID TIME       CMD
3038  2577  2504 00:00:00   bash
3038  2992  2577 00:00:00    ps

scenario 2

$sh    
$ps -l
UID   PID   PPID TIME       CMD
3038  2577  2504 00:00:00   bash
3038  3005  2577 00:00:00    sh
3038  3006  3005 00:00:00    ps

Observe PID and PPID of ps .
In scenario 1, I am executing ps -l command on sh shell. So it's parent should be sh ie, it's PPID should be PID of sh . But ps -l command listing that it's parent is bash . I am not understanding what is happening exactly. I am understanding the difference between scenario 1 and scenario 2 . But when I am executing the same commands on another OS(ubuntu), I am getting same listing under ps -l in scenario 3 and scenario 4 , as below:

scenario 3

$sh    
$ps -l
UID   PID   PPID TIME       CMD
3038  2577  2504 00:00:00   bash
3038  2991  2577 00:00:00    sh
3038  2992  2991 00:00:00    ps

scenario 4

$sh    
$ps -l
UID   PID   PPID TIME       CMD
3038  2577  2504 00:00:00   bash
3038  3005  2577 00:00:00    sh
3038  3006  3005 00:00:00    ps

Here in both scenarios, I am getting PPID of ps CMD as PID of sh . What is happening exactly. Is my interpretation wrong?

Answer 1

When you type a command, bash just fork then execve the command. scenario 3 and scenario 4 are the case.

While sh -c 'ps -l' depends on shells.

On my linux distribution, the result of ls -l `which sh` is lrwxrwxrwx 1 root root 4 Oct 6 14:06 /usr/bin/sh -> bash .

When -c is present, bash execve the following command directly. While another shell like fish does not.

fish -c 'ps -l' FS UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD 0 S 1000 711 710 0 80 0 - 5707 wait pts/5 00:00:00 bash 0 S 1000 1519 711 0 80 0 - 7054 wait pts/5 00:00:00 fish 0 R 1000 1526 1519 0 80 0 - 9266 - pts/5 00:00:00 ps

What execve does is replacing the current process image with a new process image. fork creates a new process.