Positional Parameters

Positional parameters allow scripts to accept command-line arguments:

• $0 - Name of the currently executing script
• $n - nth argument passed to the script (use braces for n > 10, e.g., ${10})
• $# - Number of arguments passed to the script
• $* - All arguments as a single string
• $@ - All arguments as separate strings

When unquoted, $* and $@ behave identically. When quoted, "$*" combines all arguments into a single string, while "$@" preserves each argument as a separate string.

Example: Parameter Validation

#!/bin/bash

if [ $# -ne 2 ]; then
    echo "Usage: $0 arg1 arg2"
    exit 1
fi

echo "Arguments received: $1 $2"

When executed without arguments:

$ ./script.sh
Usage: ./script.sh arg1 arg2

When executed with two arguments:

$ ./script.sh hello world
Arguments received: hello world

Status Variables

These variables provide information about the shell and executed commands:

Exit Status Handling

The $? variable is crucial for error handling in scripts. Different commands may return different exit codes, but 0 typically indicates success while any other value indicates failure.

Common uses of $? include:

• Verifying if commands executed successfully
• Checking backup operations completed without errors
• Controlling script flow based on command results

Example: Exit Status in Function

stop_service() {
    echo -n "Stopping $service: "
    killproc $service
    local result=$?
    echo
    
    if [ $result -eq 0 ]; then
        rm -f "/var/lock/subsys/$service"
        rm -f "/var/run/$service*"
    fi
    
    return $result
}

Parameter Expansion Operators

These operators allow for more sophisticated variable handling:

When the colon is omitted (e.g., ${var-default}), the check applies only to unset variables. When included (e.g., ${var:-default}), it also checks for empty variables.

Example: Parameter Expansion

#!/bin/bash

# Test default value substitution
echo "Unset test: ${unset_var:-default_value}"
echo "After substitution: $unset_var"

# Test variable assignment
echo "Assignment test: ${unset_var:=assigned_value}"
echo "After assignment: $unset_var"

# Test alternate value usage
echo "Alternate test: ${unset_var:+alternate_value}"
echo "Variable remains: $unset_var"

# Test error message
unset unset_var
echo "Error test: ${unset_var:?Variable is not set}"

Practical Application

These operators are particularly useful for handling optional configuration parameters:

# Search for old files in specified directory or /tmp
find "${search_path:-/tmp}" -name "*.tar.gz" -type f -mtime +7 | xargs rm -f

This ensures that even if search_path is unset, the command will default to /tmp, preventing errors and maintaining functionality.