ps命令中的`$$`代表什麼？

Question

可能重複：
What does $$ mean in the shell?

$ ./cruncher & ./cruncher & ./cruncher & 
$ ps -C cruncher -p $$ -o pid,state,cmd 
PID S CMD 
2588 S bash 
2657 R /bin/sh ./cruncher 
2658 R /bin/sh ./cruncher 
2659 R /bin/sh ./cruncher 

-C cmdlist  Select by command name. 
         This selects the processes whose executable name is 
         given in cmdlist. 

-p pidlist  Select by PID. 
         This selects the processes whose process ID numbers 
         appear in pidlist. 

問題>什麼是$$意思？

Answer 1

它與ps命令無關，$$被shell替換爲其進程ID爲。

Answer 2

這是一個內置的shell變量，它保存了PID進程ID。