Special Parameters: Positional, IFS, and PATH

Special Parameters: Positional, IFS, and PATH

Bash reserves single-character parameters for tracking runtime state, command-line arguments, and environment boundaries:

Positional Arguments and $@ vs $*

Parameter	Meaning	Quoted Expansion
$0	Name of the script or shell	"$0"
$1 .. $9	Individual positional arguments	"$1"
${10}	Arguments beyond 9 require braces	"${10}"
$#	Number of positional arguments	"$#"
$@	All arguments as individual words	"$@" expands to "$1" "$2" "$3" (Safe)
$*	All arguments concatenated	"$*" expands to "$1 $2 $3" joined by $IFS

# ALWAYS use "$@" when iterating over command-line arguments
for arg in "$@"; do
    echo "Processing: $arg"
done

Process & Runtime Parameters

• $?: Exit status of the most recently executed foreground pipeline (0 = success).
• $$: Process ID (PID) of the current shell.
• $!: Process ID of the most recently spawned background process.
• $_: The last argument of the previous command executed.