What are the special dollar sign shell variables?

Question

In Bash, there appear to be several variables which hold special, consistently-meaning values. For instance,

./myprogram &; echo $! 

will return the PID of the process which backgrounded myprogram. I know of others, such as $? which I think is the current TTY. Are there others?

Answer 1

• $1, $2, $3, ... are the positional parameters.
• "$@" is an array-like construct of all positional parameters, {$1, $2, $3 ...}.
• "$*" is the IFS expansion of all positional parameters, $1 $2 $3 ....
• $# is the number of positional parameters.
• $- current options set for the shell.
• $$ pid of the current shell (not subshell).
• $_ most recent parameter (or the abs path of the command to start the current shell immediately after startup).
• $IFS is the (input) field separator.
• $? is the most recent foreground pipeline exit status.
• $! is the PID of the most recent background command.
• $0 is the name of the shell or shell script.

Most of the above can be found under Special Parameters in the Bash Reference Manual. There are all the environment variables set by the shell.

For a comprehensive index, please see the Reference Manual Variable Index.

Answer 2

• $_ last argument of last command
• $# number of arguments passed to current script
• $* / $@ list of arguments passed to script as string / delimited list

off the top of my head. Google for bash special variables.