Bash Special Variables

Some of the most useful bash special variables.

• $0 is the name of the shell or shell script.
• $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.

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