If you are wondering what $$ (double-dollar) is in a bash shell script, well it is a special variable that can be used to know the Process ID or PID of the current shell.
Let's try running this on a bash shell.bash_script.sh
#!/bin/bash echo "The process ID of the current shell is: $$"
./bash_script.sh The process ID of the current shell is: 4181
List of Special Variable that starts with $ and their meaning
|Process ID of the most recent background command|
|Process ID of the current shell|
|Exit status of the most recently executed command|
|Number of command-line arguments passed to the script|
|All command-line arguments as a single string (positional parameters)|
|All command-line arguments as separate strings (positional parameters)|
|Name of the script itself (or the shell)|
|Individual command-line arguments (positional parameters)|
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