Special Variables

Bash provides many special variables for script information, arguments, and process management.

Positional Parameters

Variable	Description
$0	Script name or shell name
$1 - $9	Positional arguments 1-9
${10}	10th argument (braces required for 10+)
$#	Number of positional arguments
$@	All arguments as separate words
$*	All arguments as a single word

$@ vs $*

#!/usr/bin/env bash
# Called with: ./script.sh "hello world" foo bar

echo "Using \$@:"
for arg in "$@"; do
    echo "  $arg"
done
# Output:
#   hello world
#   foo
#   bar

echo "Using \$*:"
for arg in "$*"; do
    echo "  $arg"
done
# Output:
#   hello world foo bar

Rule: Always use "$@" to preserve argument boundaries.

shift Command

Move positional parameters:

echo "$1"    # first
shift
echo "$1"    # second (was $2)
shift 2
echo "$1"    # fourth (was $4)

Exit Status

Variable	Description
$?	Exit status of last command
${PIPESTATUS[@]}	Exit statuses of pipeline commands

ls /nonexistent
echo $?     # 1 (error)

ls /
echo $?     # 0 (success)

# Pipeline statuses
false | true | false
echo "${PIPESTATUS[@]}"    # 1 0 1

Process IDs

Variable	Description
$$	Current shell's PID
$!	PID of last background command
$BASHPID	Actual current process PID (differs in subshells)
$PPID	Parent process PID

echo "Shell PID: $$"
echo "Parent PID: $PPID"

sleep 10 &
echo "Background PID: $!"

(echo "Subshell BASHPID: $BASHPID")
echo "Main BASHPID: $BASHPID"

$$ vs $BASHPID

echo "$$: $$, BASHPID: $BASHPID"
(echo "Subshell - $$: $$, BASHPID: $BASHPID")
# $$ stays the same, $BASHPID changes

Miscellaneous

Variable	Description
$_	Last argument of previous command
$-	Current shell options
$IFS	Internal Field Separator

echo "hello" "world"
echo $_              # world

echo $-              # himBH (varies)
# h - hash commands
# i - interactive
# m - monitor mode
# B - brace expansion
# H - history expansion

Environment Variables

Variable	Description
$HOME	User's home directory
$USER	Current username
$LOGNAME	Login name
$SHELL	User's default shell
$PATH	Executable search path
$PWD	Current working directory
$OLDPWD	Previous working directory
$HOSTNAME	System hostname
$HOSTTYPE	Host architecture
$OSTYPE	Operating system type
$MACHTYPE	Machine type (arch-vendor-os)
$TERM	Terminal type
$LANG	Language/locale
$TZ	Timezone
$EDITOR	Default text editor
$VISUAL	Visual editor
$PAGER	Default pager
$BROWSER	Default web browser

Bash-Specific Variables

Shell Information

Variable	Description
$BASH	Path to bash
$BASH_VERSION	Bash version string
$BASH_VERSINFO	Version array
$BASH_SOURCE	Source filename array
$BASH_LINENO	Line number array
$FUNCNAME	Function name array
$SHELLOPTS	Enabled shell options
$BASHOPTS	Enabled shopt options

echo "$BASH_VERSION"    # 5.2.15(1)-release

echo "${BASH_VERSINFO[0]}"   # 5 (major)
echo "${BASH_VERSINFO[1]}"   # 2 (minor)

History

Variable	Description
$HISTSIZE	History entries in memory
$HISTFILESIZE	History entries in file
$HISTFILE	History file path
$HISTCONTROL	History control (ignoredups, etc.)
$HISTIGNORE	Patterns to ignore
$HISTTIMEFORMAT	Timestamp format

Prompt

Variable	Description
$PS1	Primary prompt
$PS2	Continuation prompt
$PS3	Select prompt
$PS4	Debug trace prompt
$PROMPT_COMMAND	Command run before prompt

Random

Variable	Description
$RANDOM	Random number 0-32767
$SRANDOM	Random 32-bit number (Bash 5.1+)

echo $RANDOM              # 12345
echo $((RANDOM % 100))    # 0-99

Time

Variable	Description
$SECONDS	Seconds since shell start
$EPOCHSECONDS	Unix timestamp (Bash 5.0+)
$EPOCHREALTIME	Unix timestamp with microseconds (Bash 5.0+)

echo "Shell running for $SECONDS seconds"
echo "Current timestamp: $EPOCHSECONDS"

Debugging

Variable	Description
$LINENO	Current line number
$FUNCNAME	Array of function names
$BASH_SOURCE	Array of source files
$BASH_LINENO	Array of line numbers
$BASH_COMMAND	Command being executed

debug() {
    echo "Function: ${FUNCNAME[1]}"
    echo "Line: ${BASH_LINENO[0]}"
    echo "File: ${BASH_SOURCE[1]}"
}

Completion

Variable	Description
$COMP_WORDS	Array of words in current line
$COMP_CWORD	Index of current word
$COMP_LINE	Current command line
$COMP_POINT	Cursor position
$COMPREPLY	Array of completions

Read-Only Variables

Some variables cannot be changed:

UID         # Real user ID
EUID        # Effective user ID
GROUPS      # Group memberships
HOSTNAME    # Hostname
HOSTTYPE    # Host type
OSTYPE      # OS type
MACHTYPE    # Machine type

Default Values

Set defaults for potentially unset variables:

Syntax	Meaning
${var:-default}	Use default if unset/empty
${var:=default}	Set and use default if unset/empty
${var:+alternate}	Use alternate if set
${var:?message}	Error with message if unset/empty

# Use default
name="${1:-Anonymous}"

# Set default
: "${config:=/etc/default.conf}"

# Error if missing
: "${required_var:?Must be set}"

Indirect References

Access variable by name:

name="value"
var="name"
echo "${!var}"           # value

# List variables matching prefix
echo "${!BASH@}"         # BASH BASHOPTS BASHPID BASH_...

Arrays Special Variables

Syntax	Description
${arr[@]}	All elements
${arr[*]}	All elements as single string
${!arr[@]}	All indices
${#arr[@]}	Number of elements
${#arr[0]}	Length of first element

Try It

1. Positional parameters:

   show_args() {
       echo "Count: $#"
       echo "All: $@"
       echo "First: $1"
   }
   show_args one two three
   

2. Exit status:

   ls /etc/passwd
   echo "Exit: $?"
   ls /nonexistent
   echo "Exit: $?"
   

3. Process IDs:

   echo "Current: $$"
   sleep 1 &
   echo "Background: $!"
   

4. Shell info:

   echo "Version: $BASH_VERSION"
   echo "Running: $SECONDS seconds"
   echo "Random: $RANDOM"
   

Summary

Most Used

Variable	Purpose
$1-$9	Script arguments
$@	All arguments (use quoted)
$#	Argument count
$?	Last exit code
$$	Current PID
$!	Last background PID
$0	Script name
$_	Last argument

Environment

Variable	Purpose
$HOME	Home directory
$PATH	Executable path
$PWD	Current directory
$USER	Username

Debugging

Variable	Purpose
$LINENO	Current line
$FUNCNAME	Function names
$BASH_SOURCE	Source files
$BASH_COMMAND	Current command