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If I run this command in a script it doesn't produce output except header from the ps:

 # Taken from Advanced Bash Usage on youtube:

 echo "$(echo "$(echo "$(echo "$(ps wwf -s $$)")")")"

This is the output:

$./testing.bash
 PID TTY STAT TIME COMMAND

but here it is run in a terminal which produces the expected output:

$echo "$(echo "$(echo "$(echo "$(ps wwf -s $$)")")")"
 PID TTY STAT TIME COMMAND
18289 pts/4 Ss+ 0:00 /bin/bash
17917 pts/4 S+ 0:00 _ /bin/bash
17918 pts/4 S+ 0:00 _ /bin/bash
17919 pts/4 S+ 0:00 _ /bin/bash
17920 pts/4 S+ 0:00 _ /bin/bash
17921 pts/4 R+ 0:00 _ ps wwf -s 18289

QUESTION:

Can you explain the difference and show me the right way to do this?

I've tried a lot of things and searched google for 4 hours, if you need I can list what I tried but I don't think that's relevant here.

$echo $SHELL
/bin/bash

and:

$head -1 testing.bash 
#!/bin/bash

ps' -s sessionid option is to select processes based on their session id.

You can use ps -j to see the session id of processes. Sessions along with process groups are generally used for shell job control (hence the -j).

Your terminal emulator creates a new session with the process that it then reuses to execute your preferred shell. So, in a terminal, the session id will generally be the same as the pid of that shell.

So, if you run ps -j -s "$$" in that shell, you'll get the processes in the session because "$$" happens to be the same as the session id.

If you run that command in any other shell (like the shell that is executed in a child process to interpret your testing script), any shell that is not a session leader, you'll get nothing because there's no session with an id that corresponds to the pid of that shell.

$ ps -j -s "$$"
 PID PGID SID TTY TIME CMD
 7239 7239 7239 pts/7 00:00:00 zsh
21002 21002 7239 pts/7 00:00:00 ps

$$ is 7239, the session leader. So that ps -j -s 7239 gives me all the processes in that session.

$ sh -xc 'ps -j -s "$$"; ps -j -p "$$"'
+ ps -j -s 21044
 PID PGID SID TTY TIME CMD
+ ps -j -p 21044
 PID PGID SID TTY TIME CMD
21044 21044 7239 pts/7 00:00:00 sh

The first ps command returns nothing because, as the second ps shows, there's no session with id 21044 as the process of id 21044 is not a session leader. The session leader is still 7239, the shell started by the terminal emulator.

$ sh -xc 'ps -j -s "$(($(ps -o sid= -p "$$")))"'
+ ps -o sid= -p 21215
+ ps -j -s 7239
 PID PGID SID TTY TIME CMD
 7239 7239 7239 pts/7 00:00:00 zsh
21215 21215 7239 pts/7 00:00:00 sh
21217 21215 7239 pts/7 00:00:00 ps

Now, we see all the processes in the session. We used ps -o sid= -p "$$" to get the id of the session that $$ belongs to.