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 $$?

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:

<ul><li><a href="http://unixhelp.ed.ac.uk/scrpt/scrpt2.2.2.html" rel="nofollow">http://unixhelp.ed.ac.uk/scrpt/scrpt2.2.2.html</a></li> </ul>

<a href="http://tldp.org/LDP/abs/html/internalvariables.html#PIDVARREF" rel="nofollow">$!</a> is the process ID of the last job run in the background.

<a href="http://tldp.org/LDP/abs/html/special-chars.html#PROCESSIDREF" rel="nofollow">$$</a> is the process ID of the script itself.

(Both of the above are links to the <a href="http://tldp.org/LDP/abs/html/" rel="nofollow">Advanced Bash Scripting Guide</a> on TDLP.)