Special $ (dollar) shell variables

If you use Linux or another UNIX-like operating system and find yourself forgetting exactly which $ prefixed command does what, this is for you:

• $$ - the PID of the current process
• $! - the PID of the most recent background process
• $? - the most recent exit code
• $# - the number of passed parameters
• $* / $@ - a string / array of the passed parameters
• $_ - the most recent parameter
• $0 - script/program name (0th parameter)
• $1...$9 - positional parameters 1 through 9
• $NAME - references environment variable "NAME"