shell script $$ value

Question

When I use these shell commands:

[root@linux /tmp]# a=$$ [root@linux /tmp]# echo $a 3985 

where does the value 3985 come from? And why?

Answer 1

man bash 

explains it.

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

Answer 2

$$ is the pid of the current process

try

 host@controller:~$ echo $$ 12481 host@controller:~$ ps -p 12481 PID TTY TIME CMD 12481 pts/2 00:00:01 bash host@controller:~$ 

Since we execute echo $$ in bash, we get it's current pid

know also

echo $? is the return code of the last executed command.

$# is the number of arguments

$* is the list of arguments passed to the current process

$[1 or 2 or ... n] value of each corresponding argument

Answer 3

That's why some people use it to construct a filename that's only used temporarily and then destroyed, as in this script fragment.

SCRATCHFILE=/tmp/emphemeral.$$ ; # Now have the script write to and read from the temp file rm $SCRATCHFILE ; 

As mentioned above, the $$ in the filename will be the PID of the main script.

Answer 4

Try "echo $$" you will get the answer.