Special Variables

Bash provides several built-in special variables that give you access to script arguments, process information, and command status. These are essential for writing flexible and robust scripts.

Positional Parameters

These variables hold command-line arguments passed to your script:

Variable	Description
$0	Script name (or shell name if interactive)
$1 to $9	First 9 arguments
${10}, ${11}	Arguments 10 and beyond (braces required)
$#	Number of arguments
$@	All arguments as separate words
$*	All arguments as a single word

#!/bin/bash
# save as show_args.sh
echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "Number of arguments: $#"
echo "All arguments: $@"

$@ vs $*

The difference becomes important with quoting:

# Given: ./script.sh "hello world" foo bar

# $@ preserves each argument separately
for arg in "$@"; do
    echo "Arg: $arg"
done
# Output:
# Arg: hello world
# Arg: foo
# Arg: bar

# $* treats all as one string (separated by first char of IFS)
for arg in "$*"; do
    echo "Arg: $arg"
done
# Output:
# Arg: hello world foo bar

Rule: Almost always use "$@" when iterating over arguments.

Exercise: Count Arguments

Work with positional parameters:

Process-Related Variables

Variable	Description
$$	Current shell's process ID (PID)
$!	PID of last background process
$?	Exit status of last command
$-	Current shell options

echo "This script's PID: $$"

# Run something in background
sleep 10 &
echo "Background job PID: $!"

# Check last command status
ls /nonexistent
echo "Exit status: $?"    # Non-zero (error)

ls /tmp
echo "Exit status: $?"    # 0 (success)

The Exit Status $?

The exit status is crucial for error handling:

# 0 = success, non-zero = failure
grep "pattern" file.txt
if [ $? -eq 0 ]; then
    echo "Pattern found"
else
    echo "Pattern not found"
fi

# Common exit codes
# 0   - Success
# 1   - General error
# 2   - Misuse of command
# 126 - Permission denied
# 127 - Command not found
# 128+N - Killed by signal N

Exercise: Check Exit Status

Understand how exit status works:

Environment Variables

These are inherited from the parent process:

Variable	Description
$HOME	User's home directory
$USER	Current username
$PATH	Command search path
$PWD	Current working directory
$OLDPWD	Previous working directory
$SHELL	Current shell path
$HOSTNAME	System hostname
$RANDOM	Random number (0-32767)
$SECONDS	Seconds since shell started
$LINENO	Current line number in script

echo "Home: $HOME"
echo "User: $USER"
echo "Shell: $SHELL"
echo "Random: $RANDOM"

The $RANDOM Variable

Generate random numbers:

# Random number between 0 and 32767
echo $RANDOM

# Random number between 0 and 99
echo $((RANDOM % 100))

# Random number between 1 and 10
echo $((RANDOM % 10 + 1))

# Random number between MIN and MAX
MIN=5
MAX=15
echo $((RANDOM % (MAX - MIN + 1) + MIN))

The $SECONDS Variable

Track script execution time:

#!/bin/bash
SECONDS=0   # Reset timer

# ... do some work ...
sleep 2

echo "Script ran for $SECONDS seconds"

The $LINENO Variable

Useful for debugging:

#!/bin/bash
echo "This is line $LINENO"
echo "This is line $LINENO"
echo "This is line $LINENO"
# Output:
# This is line 2
# This is line 3
# This is line 4

IFS - Input Field Separator

Controls word splitting:

# Default IFS is space, tab, newline
echo "$IFS" | cat -A

# Custom IFS for parsing
DATA="apple,banana,cherry"
IFS=',' read -ra FRUITS <<< "$DATA"
for fruit in "${FRUITS[@]}"; do
    echo "Fruit: $fruit"
done

Shifting Arguments

Use shift to move through arguments:

#!/bin/bash
# Process all arguments one at a time
while [ $# -gt 0 ]; do
    echo "Processing: $1"
    shift   # Remove $1, shift others down
done

# After shift:
# $2 becomes $1
# $3 becomes $2
# $# decreases by 1

The set Command

Modify positional parameters:

# Set new positional parameters
set -- "arg1" "arg2" "arg3"
echo "$1"   # arg1
echo "$2"   # arg2
echo "$#"   # 3

# Clear all parameters
set --
echo "$#"   # 0

Exercise: Display Environment Info

Show multiple special variables:

Quick Reference

# Positional
$0        # Script name
$1-$9     # Arguments 1-9
${10}     # Argument 10+
$#        # Argument count
$@        # All args (separate)
$*        # All args (single)

# Status
$?        # Last exit status
$$        # Current PID
$!        # Last background PID

# Environment
$HOME     # Home directory
$USER     # Username
$PWD      # Current directory
$PATH     # Command path
$RANDOM   # Random number
$SECONDS  # Seconds elapsed
$LINENO   # Line number

Key Takeaways

• $1, $2, etc. hold script arguments; $# is the count
• Use "$@" to iterate over arguments (preserves quoting)
• $? contains the exit status of the last command (0 = success)
• $$ is the current process ID, $! is the last background job
• Environment variables like $HOME, $USER, $PWD are always available
• $RANDOM generates random numbers; $SECONDS tracks time
• shift moves through arguments, discarding the first one