What does mean $$ or $! in bash? [closed]

Question

It's kind of easy question but I didn't find any information. What does mean $! or $$ in bash?

For example: ps -p $! or pstree $$?

Answer 1

Actually, these variables were inherited by bash from the Bourne shell.

"$$" means current PID.

"$!" is the PID of the last program your shell ran in the background (e.g. "myprog &")

Here is a list of shell variables:

• http://unixhelp.ed.ac.uk/scrpt/scrpt2.2.2.html

Answer 2

$! is the process ID of the last job run in the background.

$$ is the process ID of the script itself.

(Both of the above are links to the Advanced Bash Scripting Guide on TDLP.)