is $$ a variable of some sort? it seems to contain a value..

GrapefruiTgirl · 06-09-2008, 07:32 AM

Code:

sasha@darkstarSSI:~$ echo $PWD
/home/Sasha
*sasha@darkstarSSI:~$ echo $$PWD
8027PWD
*sasha@darkstarSSI:~$ echo $$
8027
*sasha@darkstarSSI:~$ echo $$$
8027$
*sasha@darkstarSSI:~$ echo $
$
sasha@darkstarSSI:~$ su
Password: *****************
root@darkstarSSI:/home/Sasha# echo $$
13569
root@darkstarSSI:/home/Sasha# exit
exit
sasha@darkstarSSI:~$ echo $$PWD
8027PWD
sasha@darkstarSSI:~$

just a curiosity..
Is a double-dollarsign actually a variable? If so/not then what is the value being returned by it as seen above? A different value for Sasha than for root, too.

How I noticed this:
I was reading the kernel Makefile, and came across this little section:

Code:

help:
	@echo  '  Building external modules.'
	@echo  '  Syntax: make -C path/to/kernel/src M=$$PWD target'
	@echo  ''
	@echo  '  modules         - default target, build the module(s)'
	@echo  '  modules_install - install the module'
	@echo  '  clean           - remove generated files in module directory only'
	@echo  ''

.. and there, the second @echo line has M=$$PWD target which I thought might be a typo, so I tried the echo command as seen in the first [code] tags above.

I tried running `make help` to see what got printed, but the above help: section did not get printed to the console.

Sasha

UPDATE: The value returned by $$ is the PID of the bash console in use! However, it still seems like a typo in the Makefile (unless make interprets it differently) because "${bashPID}${PWD}" doesn't seem really useful, to me anyhow.

pixellany · 06-09-2008, 07:45 AM

ABS---Table B1

$$ = the process ID of a script

I did a few experiments entering "$$" as different users, but I still don't totally understand what is happening.

indienick · 06-09-2008, 07:47 AM

$$ is the PID of the script's current running process. I'm willing to bet if you issue another $$ and then "ps -ef | grep -i bash", you will find a Bash process with the number returned by $$.

$$ wouldn't return the same number if you wrote a script with "echo $$" in it, as a new process is spawned to execute a script. Then again, this is all theoretical, as I don't have a Linux (or UNIX) machine within reach, right now.

Ref #1. TLDP:Advanced Bash HOWTO Guide:Internal Variables

colucix · 06-09-2008, 07:56 AM

Yes, for the test with "echo $$" the meaning of the double dollar is that explained in the previous posts, but in a Makefile you have to put a double $$ to retrieve the value of a shell variable. The $$PWD is replaced by the value of the PWD variable. The single dollar sign in Makefiles is meant to call a macro.

GrapefruiTgirl · 06-09-2008, 08:11 AM

Quote:

Originally Posted by indienick

$$ is the PID of the script's current running process. I'm willing to bet if you issue another $$ and then "ps -ef | grep -i bash", you will find a Bash process with the number returned by $$.

$$ wouldn't return the same number if you wrote a script with "echo $$" in it, as a new process is spawned to execute a script. Then again, this is all theoretical, as I don't have a Linux (or UNIX) machine within reach, right now.

Ref #1. TLDP:Advanced Bash HOWTO Guide:Internal Variables

Quote:

Originally Posted by colucix

Yes, for the test with "echo $$" the meaning of the double dollar is that explained in the previous posts, but in a Makefile you have to put a double $$ to retrieve the value of a shell variable. The $$PWD is replaced by the value of the PWD variable. The single dollar sign in Makefiles is meant to call a macro.
__________________
Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

Thanks to you both for the excellent answers; I did try googling the $$ but nothing specific turned up on the first pages or so. I didn't go too mad over it as it was more a curiousity than anything, but I have learned something once again today, from each of you.

PS - Colucix, I do like that Einstein quote in your signature

PPS -

Quote:

Originally Posted by indienick

...as I don't have a Linux (or UNIX) machine within reach, right now.

-- I'm sorry to hear that, you must feel uncomfortable

I would! Heh heh, well I hope you are not in a room with a lot of windows.

Cheers!
sasha

indienick · 06-09-2008, 09:23 AM

Quote:

Originally Posted by GrapefruiTgirl

-- I'm sorry to hear that, you must feel uncomfortable I would! Heh heh, well I hope you are not in a room with a lot of windows.

Spasiba Sashinka, sadly though, I am in a Windows-only office.