$$  –  Shell Process ID

$$ expands to the process ID of the invoked shell. In a subshell it expands to the same ID as that of the parent process.

Links

• POSIX specification
• Z Shell manual

Examples

Print PID of the main shell twice

1 2 3 4

# main echo "$$" # subshell (echo "$$")

Output:

1 2	140352 140352