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In shell scripting, robust error handling and input validation are crucial. Earlier, we saw how $? captures the exit status of the last command. In this guide, we’ll dive into $#, which returns the number of positional parameters passed to your script or function. By checking $# early, you can prevent unexpected behavior and make your scripts more reliable.

Table of Contents

1. Why $# Matters
2. Basic Usage of $#
3. Positional Parameters and Empty Defaults
4. Guard Clauses with $#
   • Exact Number of Arguments
   • Minimum Number of Arguments
   • Range of Arguments
5. Real-World Example: Walking Calorie Expenditure Script
   • Flawed Version
   • Improved Version with set -e and terminate()
6. Summary of Key Variables
7. Links and References

Why $# Matters

Shell scripts often rely on user-supplied arguments. If your script assumes a certain number of inputs but none (or too many) are provided, it can silently fail or produce erroneous results. Checking $# allows you to:

• Validate inputs before executing critical logic
• Provide clear usage messages
• Exit early on misuse

Basic Usage of $#

Create a script called count-args.sh:

#!/usr/bin/env bash
echo "Number of arguments: $#"

Run it with different argument counts:

$ ./count-args.sh
Number of arguments: 0

$ ./count-args.sh alpha
Number of arguments: 1

$ ./count-args.sh alpha beta
Number of arguments: 2

$ ./count-args.sh ""
Number of arguments: 1

Positional Parameters and Empty Defaults

By default, referencing an unset positional parameter expands to an empty string without an error:

#!/usr/bin/env bash
echo "First: '${1}'"
echo "Second: '${2}'"

$ ./show-params.sh hello
First: 'hello'
Second: ''
$ echo $?
0

Without validation, scripts can continue with missing data, leading to downstream failures.

Guard Clauses with $#

Validate $# early in your script to enforce expected usage. Below are common patterns:

Exact Number of Arguments

#!/usr/bin/env bash

if [[ $# -ne 1 ]]; then
  echo "Usage: $0 <arg>" >&2
  exit 1
fi

echo "Argument received: $1"
exit 0

Minimum Number of Arguments

#!/usr/bin/env bash

if [[ $# -lt 1 ]]; then
  echo "Error: At least one argument is required." >&2
  exit 1
fi

echo "Received $# argument(s)."

Range of Arguments

#!/usr/bin/env bash
readonly MIN=1
readonly MAX=2

if [[ $# -lt $MIN || $# -gt $MAX ]]; then
  echo "Usage: $0 <arg1> [arg2]" >&2
  echo "Provide between $MIN and $MAX arguments." >&2
  exit 1
fi

echo "Arguments are within the expected range."

Real-World Example: Walking Calorie Expenditure Script

Calculating calories burned from step counts is a practical script. Let’s see a flawed version and then improve it with input validation.

Flawed Version

#!/usr/bin/env bash
steps=${1}
cal_per_step=0.04
calories=$(echo "${steps} * ${cal_per_step}" | bc)
echo "Calories burned for ${steps} steps: ${calories}"

Running without arguments shows a syntax error but exits with status 0:

$ ./calorie.sh
(standard_in) 1: syntax error
$ echo $?
0

Improved Version with set -e and terminate()

#!/usr/bin/env bash
set -e

CL_ARGS_ERROR=155
CAL_PER_STEP=0.04

terminate() {
  local msg="$1"
  local code="${2:-160}"
  echo "Error: $msg" >&2
  exit "$code"
}

# Guard clause: expect exactly one argument
if [[ $# -ne 1 ]]; then
  terminate "Please provide exactly one argument (number of steps)" "$CL_ARGS_ERROR"
fi

steps=$1
calories=$(echo "$steps * $CAL_PER_STEP" | bc)
echo "Calories burned for $steps steps: $calories"
exit 0

Usage

$ ./calorie.sh
Error: Please provide exactly one argument (number of steps)
$ echo $?
155

$ ./calorie.sh 10000
Calories burned for 10000 steps: 400.00
$ echo $?
0

Summary of Key Variables

Links and References

• Bash Reference Manual
• Advanced Bash-Scripting Guide
• Shell Parameter Expansion