-1

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 i.e., 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?

user1766169
  • 1,932
  • 3
  • 22
  • 44
Jhansi Rani
  • 449
  • 1
  • 5
  • 11

1 Answers1

2

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` islrwxrwxrwx 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' F S 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.

Youmu
  • 78
  • 4