Finding Truth with ps -p $$

Parent - children process confusion in SHELL

Confusing Behaviour in Shell

SHELL does not reflect the currently running shell. If you launch bash inside a zsh session:

zsh

echo $SHELL     # → /bin/zsh   ( says zsh! )
echo $$         # → 3429
bash
echo $SHELL     # → /bin/zsh   ( STILL says zsh! )
echo $$         # → 5102       ( new PID — you're in bash now )

$SHELL stayed the same because it’s just an inherited environment variable — it was set at login and never updated. $$ changed because a new process actually started.

Process inheritance (specifically environment inheritance)

When you ran:

bash

• The current shell (zsh) performed a fork()

• The child process executed bash

• The child inherited the environment variables from the parent

So $SHELL stayed /bin/zsh because :

Environment variables are inherited from the parent process to the child process unless explicitly changed.

ps -p $$

zsh

ps -p $$
# shell expands this first $$ -> PID 
ps -p 5102

then it pick the process with PID - 5102

ps -p $$

# Ground Truth
 PID TTY        TIME CMD           
 5102 ttys003    0:00 bash 

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“Like father, like son.”

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