Why does echo $$ return a number? [duplicate]

问题

This question already has answers here:

What are the special dollar sign shell variables? (4 answers)

Closed 5 years ago.

Why do I get a number when doing that :

echo $$

which returns

489

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

回答1:

$$ 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

回答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.

回答3:

You will find an excellent explanation in this post.

$$ pid of the current shell (not subshell)

来源：https://stackoverflow.com/questions/22457166/why-does-echo-return-a-number