Why does echo $$ return a number?

Question

Why do I get a number when doing that :

echo $$

which returns

If I open a new terminal it returns another number. It seems it's related to the pid of the terminal session, but why ?

Also see: http://stackoverflow.com/questions/2493642/how-does-a-script-know-its-own-pid — anubhava, Mar 17 '14 at 14:39
And bash special parameters: http://www.gnu.org/software/bash/manual/bashref.html#Special-Parameters — glenn jackman, Mar 17 '14 at 14:42

fedorqui · Accepted Answer · 2014-03-17T14:45:38.917

$$ means your current PID.

As seen in Bash Reference Manual - 3.4.2 Special Parameters:

$

Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the invoking shell, not the subshell.

You can test it by doing ps -ef | grep 489, and it will show the process in which you are logged in.

For example in my case:

$ echo $$
3470

$ ps -ef | grep 3470
1000      3470  3469  0 10:59 pts/3    00:00:00 -bash    <---- this process
1000      8151  3470  0 15:37 pts/3    00:00:00 ps -ef
1000      8152  3470  0 15:37 pts/3    00:00:00 grep --color=auto 3470

score 2 · Answer 2 · answered Mar 17 '14 at 14:38

2

Because that's how it is defined. $$ is a special shell variable (like e.g. $!, $_, $@, $1, ...) referring to the PID of the invoked shell.

answered Mar 17 '14 at 14:38

Adrian Frühwirth

42,970
10
60
71

score 1 · Answer 3 · edited May 23 '17 at 10:31

1

You will find an excellent explanation in this post.

$$ pid of the current shell (not subshell)

edited May 23 '17 at 10:31

Community

1
1

answered Mar 17 '14 at 14:39

Saucier

4,200
1
25
46

Why does echo $$ return a number?

3 Answers3

Linked