> ## Documentation Index > Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt > Use this file to discover all available pages before exploring further. # Hashtag > This guide explains how to use `$#` in Bash to count command-line arguments for better error handling and input validation in scripts. In shell scripting, robust error handling and input validation are crucial. Earlier, we saw how `$?` 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 `$#` Matters](#why-) 2. [Basic Usage of `$#`](#basic-usage-of-) 3. [Positional Parameters and Empty Defaults](#positional-parameters-and-empty-defaults) 4. [Guard Clauses with `$#`](#guard-clauses-with-) * [Exact Number of Arguments](#exact-number-of-arguments) * [Minimum Number of Arguments](#minimum-number-of-arguments) * [Range of Arguments](#range-of-arguments) 5. [Real-World Example: Walking Calorie Expenditure Script](#real-world-example-walking-calorie-expenditure-script) * [Flawed Version](#flawed-version) * [Improved Version with `set -e` and `terminate()`](#improved-version-with-set-e-and-terminate) 6. [Summary of Key Variables](#summary-of-key-variables) 7. [Links and References](#links-and-references) *** ## Why `$#` 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 `$#` allows you to: * Validate inputs before executing critical logic * Provide clear usage messages * Exit early on misuse Always validate positional parameters to avoid silent errors and improve script maintainability. *** ## Basic Usage of `$#` Create a script called `count-args.sh`: ```bash theme={null} #!/usr/bin/env bash echo "Number of arguments: $#" ``` Run it with different argument counts: ```bash theme={null} $ ./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: ```bash theme={null} #!/usr/bin/env bash echo "First: '${1}'" echo "Second: '${2}'" ``` ```bash theme={null} $ ./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: | Pattern | Description | Example | | | | | | ------------------ | ------------------------- | ---------------------------------------------------------------------- | ------------------------ | ------------------- | - | ----------------------------------------------------------- | | `[[ $# -ne N ]]` | Exact number of arguments | `if [[ $# -ne 1 ]]; then echo "Usage: $0 " >&2; exit 1; fi` | | | | | | `[[ $# -lt N ]]` | At least N arguments | `if [[ $# -lt 1 ]]; then echo "Need at least one arg" >&2; exit 1; fi` | | | | | | \`\[\[ \$# -lt MIN | | \$# -gt MAX ]]\` | Between MIN and MAX args | \`if \[\[ \$# -lt 1 | | \$# -gt 2 ]]; then echo "Expect 1–2 args" >&2; exit 1; fi\` | ### Exact Number of Arguments ```bash theme={null} #!/usr/bin/env bash if [[ $# -ne 1 ]]; then echo "Usage: $0 " >&2 exit 1 fi echo "Argument received: $1" exit 0 ``` ### Minimum Number of Arguments ```bash theme={null} #!/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 ```bash theme={null} #!/usr/bin/env bash readonly MIN=1 readonly MAX=2 if [[ $# -lt $MIN || $# -gt $MAX ]]; then echo "Usage: $0 [arg2]" >&2 echo "Provide between $MIN and $MAX arguments." >&2 exit 1 fi echo "Arguments are within the expected range." ``` *** ## Real-World Example: Walking Calorie Expenditure Script Calculating calories burned from step counts is a practical script. Let’s see a flawed version and then improve it with input validation. ### Flawed Version ```bash theme={null} #!/usr/bin/env bash steps=${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`: ```bash theme={null} $ ./calorie.sh (standard_in) 1: syntax error $ echo $? 0 ``` Scripts that continue after errors can hide critical failures. Always combine guard clauses with strict error handling. ### Improved Version with `set -e` and `terminate()` ```bash theme={null} #!/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 ```bash theme={null} $ ./calorie.sh Error: Please provide exactly one argument (number of steps) $ echo $? 155 $ ./calorie.sh 10000 Calories burned for 10000 steps: 400.00 $ echo $? 0 ``` *** ## Summary of Key Variables | Variable | Description | Example | | -------- | -------------------------------------------------- | -------------------------------- | | `$?` | Exit status of the last command | `echo $?` | | `$#` | Number of positional parameters passed to a script | `if [[ $# -ne 1 ]]; then ... fi` | | `$1, $2` | Positional parameters (first, second, etc.) | `echo "First: $1"` | *** ## Links and References * [Bash Reference Manual](https://www.gnu.org/software/bash/manual/bash.html) * [Advanced Bash-Scripting Guide](https://tldp.org/LDP/abs/html/) * [Shell Parameter Expansion](https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html)