Function arguments ($1, $2 inside function) in Bash Scripting

Introduction

Functions can take inputs to work with different data each time. Using $1, $2 inside a function lets you access these inputs easily.

You want to reuse a set of commands but with different values each time.

You need to pass a filename or user input to a function to process it.

You want to write a script that can handle multiple tasks by changing arguments.

You want to keep your script organized by breaking tasks into functions with inputs.

Syntax

Bash Scripting

function_name() {
  echo "First argument is $1"
  echo "Second argument is $2"
}

Inside a function, $1, $2, etc. refer to the arguments passed to that function, not the script.

You can use as many arguments as you want, accessed by $1, $2, $3, and so on.

Examples

This function greets the person whose name is passed as the first argument.

Bash Scripting

greet() {
  echo "Hello, $1!"
}
greet Alice

This function adds two numbers passed as arguments and prints the result.

Bash Scripting

add() {
  sum=$(( $1 + $2 ))
  echo "Sum is $sum"
}
add 5 7

This function prints three arguments passed to it.

Bash Scripting

show_args() {
  echo "Arg1: $1"
  echo "Arg2: $2"
  echo "Arg3: $3"
}
show_args one two three

Sample Program

This script defines a function that prints a name and age passed as arguments.

Bash Scripting

#!/bin/bash

print_info() {
  echo "Name: $1"
  echo "Age: $2"
}

print_info "John" 30

Important Notes

If you call a function without enough arguments, the missing ones will be empty strings.

You can use "$@" inside a function to access all arguments as a list.

Always quote your variables like "$1" to avoid issues with spaces or special characters.

Summary

Use $1, $2, etc. inside functions to access the inputs given when calling the function.

Functions help reuse code with different data by passing arguments.

Remember that $1 inside a function is different from $1 in the main script.

Practice

1. In a bash function, what does $1 represent?

easy

A. The first argument passed to the function

B. The name of the function

C. The total number of arguments passed to the script

D. The exit status of the last command

Solution

1. Step 1: Understand function arguments in bash

   Inside a bash function, $1 refers to the first argument passed specifically to that function, not the script.

2. Step 2: Differentiate from script arguments

   The script's arguments are accessed outside functions, but inside a function, $1 resets to the function's first argument.

3. Final Answer:

   The first argument passed to the function -> Option A

4. Quick Check:

   $1 in function = first function argument [OK]

Hint: Inside functions, $1 is the first function argument, not script's [OK]

Common Mistakes:

• Confusing $1 inside function with script's $1
• Thinking $1 is function name
• Mixing $1 with number of arguments

2. Which of the following is the correct way to define a bash function that uses arguments $1 and $2?

easy

A. function myfunc { echo $1 $2

B. function myfunc($1, $2) { echo $1 $2 }

C. def myfunc($1, $2) { echo $1 $2 }

D. myfunc() { echo $1 $2; }

Solution

1. Step 1: Recall bash function syntax

   Bash functions are defined using name() { commands; } or function name { commands; } without parameters in parentheses.

2. Step 2: Identify correct syntax for arguments

   Arguments are accessed inside the function via $1, $2, not declared in parentheses.

3. Final Answer:

   myfunc() { echo $1 $2; } -> Option D

4. Quick Check:

   Function args accessed inside, no params in definition [OK]

Hint: Define functions with () and access args inside, no params in () [OK]

Common Mistakes:

• Trying to declare parameters in function definition
• Using other language syntax like def or parentheses with args
• Missing semicolon or braces

3. What is the output of this script?

greet() {
  echo "Hello, $1 and $2!"
}
greet Alice Bob

medium

A. Hello, $1 and $2!

B. Hello, Alice and Bob!

C. Hello, Alice and !

D. Hello, and Bob!

Solution

1. Step 1: Understand function call and arguments

   The function greet is called with two arguments: Alice and Bob. Inside the function, $1 is Alice and $2 is Bob.

2. Step 2: Substitute arguments in echo

   The echo command prints: "Hello, Alice and Bob!" exactly as expected.

3. Final Answer:

   Hello, Alice and Bob! -> Option B

4. Quick Check:

   Function args $1=Alice, $2=Bob replaced correctly [OK]

Hint: Arguments replace $1, $2 inside function call [OK]

Common Mistakes:

• Expecting literal $1 and $2 printed
• Mixing argument order
• Forgetting to pass both arguments

4. Consider this function:

sum() {
  echo $(($1 + $2))
}
sum 5

What error or output will this produce?

medium

A. bash: + : syntax error: operand expected (error token is "+ ")

B. 5

C. bash: syntax error: unexpected end of file

D. 10

Solution

1. Step 1: Analyze function call with missing argument

   The function sum expects two arguments, but only one (5) is passed. So, $2 is empty.

2. Step 2: Evaluate arithmetic expression

   The expression $(($1 + $2)) becomes (5 + ), which is invalid syntax causing an error about operand expected.

3. Final Answer:

   bash: + : syntax error: operand expected (error token is "+ ") -> Option A

4. Quick Check:

   Missing $2 causes arithmetic syntax error [OK]

Hint: Always pass all args used in arithmetic to avoid errors [OK]

Common Mistakes:

• Assuming missing args default to zero
• Ignoring error messages
• Passing arguments incorrectly

5. You want a bash function concat that joins two strings passed as arguments with a dash between them. Which function definition correctly does this?

hard

A. concat() { echo "$1+$2"; }

B. concat() { echo "$1 $2"; }

C. concat() { echo "$1-$2"; }

D. concat() { echo "$1$2"; }

Solution

1. Step 1: Understand the requirement

   The function should join two strings with a dash - between them.

2. Step 2: Check each option's output

   concat() { echo "$1-$2"; } prints $1-$2 which is exactly the two strings joined by a dash. Others use space, plus, or no separator.

3. Final Answer:

   concat() { echo "$1-$2"; } -> Option C

4. Quick Check:

   Dash between args = concat() { echo "$1-$2"; } [OK]

Hint: Use echo "$1-$2" to join with dash inside function [OK]

Common Mistakes:

• Using space or plus instead of dash
• Forgetting quotes causing word splitting
• Concatenating without separator