Command-line arguments ($1, $2, ...) in Bash Scripting

Introduction

Command-line arguments let you give information to a script when you start it. This helps the script do different things without changing its code.

You want to tell a script which file to process without editing the script.

You need to pass a username or password to a script securely.

You want to run the same script with different options or settings.

You want to automate tasks that require different inputs each time.

You want to make your script flexible and reusable.

Syntax

Bash Scripting

script_name.sh arg1 arg2 arg3 ...

Inside the script:
$1  # first argument
$2  # second argument
$3  # third argument
...
$#  # number of arguments
$@  # all arguments as separate words
$*  # all arguments as a single string

The first argument is accessed with $1, the second with $2, and so on.

$# tells how many arguments were given.

Examples

This script prints the first and second arguments you give it.

Bash Scripting

#!/bin/bash

echo "First argument: $1"
echo "Second argument: $2"

This script checks if you gave at least two arguments and tells you how many you gave.

Bash Scripting

#!/bin/bash

if [ "$#" -lt 2 ]; then
  echo "Please provide at least two arguments."
  exit 1
fi

echo "You gave $# arguments."

This script loops through all arguments and prints each one.

Bash Scripting

#!/bin/bash

for arg in "$@"; do
  echo "Argument: $arg"
done

Sample Program

This script expects exactly two arguments: a name and an age. It then greets the user using those values.

Bash Scripting

#!/bin/bash

# This script greets the user with their name and age

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

name=$1
age=$2

echo "Hello, $name! You are $age years old."

Important Notes

Always check if the right number of arguments is given to avoid errors.

Use quotes around variables like "$1" to handle spaces correctly.

$0 holds the script's name, useful for usage messages.

Summary

Command-line arguments let scripts get input when they start.

Use $1, $2, etc., to access each argument.

Check $# to know how many arguments were given.

Practice

1. What does $1 represent in a bash script?

easy

A. The last command-line argument passed to the script

B. The total number of arguments passed to the script

C. The name of the script file

D. The first command-line argument passed to the script

Solution

1. Step 1: Understand command-line arguments in bash

   In bash scripts, $1 refers to the first argument given after the script name.

2. Step 2: Differentiate from other special variables

   $# is the count of arguments, $0 is the script name, and $@ is all arguments.

3. Final Answer:

   The first command-line argument passed to the script -> Option D

4. Quick Check:

   $1 = first argument [OK]

Hint: Remember $1 is always the first argument after script name [OK]

Common Mistakes:

• Confusing $1 with $# (argument count)
• Thinking $1 is the script name
• Mixing $1 with $0 or $@

2. Which of the following is the correct way to access the third argument in a bash script?

easy

A. $0

B. $2

C. $3

D. $#

Solution

1. Step 1: Recall argument numbering in bash

   Arguments start from $1 for the first, so the third argument is $3.

2. Step 2: Identify incorrect options

   $2 is second argument, $# is count, $0 is script name.

3. Final Answer:

   $3 -> Option C

4. Quick Check:

   Third argument = $3 [OK]

Hint: Count arguments starting at $1, so third is $3 [OK]

Common Mistakes:

• Using $2 instead of $3 for third argument
• Confusing $# with argument value
• Using $0 which is script name

3. What will be the output of this script if run as ./script.sh apple banana cherry?

echo "First: $1"
echo "Second: $2"
echo "Count: $#"

medium

A. First: apple
Second: banana
Count: 3

B. First: apple
Second: cherry
Count: 3

C. First: ./script.sh
Second: apple
Count: 3

D. First: banana
Second: cherry
Count: 2

Solution

1. Step 1: Identify argument values

   Arguments are apple ($1), banana ($2), cherry ($3). Script name is $0.

2. Step 2: Understand output commands

   echo prints $1 as apple, $2 as banana, and $# as 3 (number of arguments).

3. Final Answer:

   First: apple Second: banana Count: 3 -> Option A

4. Quick Check:

   Arguments match output [OK]

Hint: Remember $1, $2 are first and second args; $# is count [OK]

Common Mistakes:

• Mixing script name ($0) with arguments
• Confusing argument order
• Miscounting number of arguments

4. Given this script snippet:

echo "Argument 1: $1"
echo "Argument 2: $2"

What is the error if you run the script without any arguments?

medium

A. Script prints empty lines for $1 and $2

B. No output is printed

C. Script throws a syntax error

D. Script prints the script name instead of arguments

Solution

1. Step 1: Understand missing arguments behavior

   If no arguments are passed, $1 and $2 are empty strings, not errors.

2. Step 2: Check script output

   echo prints "Argument 1: " and "Argument 2: " with nothing after colon.

3. Final Answer:

   Script prints empty lines for $1 and $2 -> Option A

4. Quick Check:

   Missing args = empty $1, $2 [OK]

Hint: No args means $1, $2 are empty, no errors [OK]

Common Mistakes:

• Expecting syntax or runtime error
• Thinking script name prints as argument
• Assuming no output at all

5. You want a bash script to print all arguments passed to it, each on a new line, using only $1, $2, etc. Which approach works best?

hard

A. Use a loop with a counter and access arguments by $1, $2, ... until counter > $#

B. Use shift command in a loop to access each argument and print it

C. Use $0 to print all arguments one by one

D. Use echo $@ to print all arguments on one line

Solution

1. Step 1: Understand how to access all arguments one by one

   Using shift moves arguments left, so $1 becomes next argument each time.

2. Step 2: Compare options

   Use a loop with a counter and access arguments by $1, $2, ... until counter > $# is complex and error-prone; Use echo $@ to print all arguments on one line prints all on one line; Use $0 to print all arguments one by one prints script name; Use shift command in a loop to access each argument and print it correctly loops and prints each argument on new line.

3. Final Answer:

   Use shift command in a loop to access each argument and print it -> Option B

4. Quick Check:

   Shift loops through args one by one [OK]

Hint: Use shift in loop to print each argument separately [OK]

Common Mistakes:

• Using $0 instead of arguments
• Trying to manually count without shift
• Printing all args on one line instead of separate lines