03. Passing Arguments

Overview

When executing a Bash script, you can pass arguments directly from the command line. These arguments can be accessed within the script using predefined positional parameters and special variables.

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Basic Syntax

./script_name.sh arg1 arg2 arg3 ...

• Each argument separated by a space is treated as a new parameter.

• Inside the script, they can be accessed as:

  • $1 → First argument

  • $2 → Second argument

  • $3 → Third argument

  • … and so on.

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Example Script — arguments.sh

#!/bin/bash
# 03. Passing Arguments

# Access arguments directly using positional variables
echo $1 $2 $3 ' -> echo $1 $2 $3'

# Store all arguments in an array
args=("$@")
echo ${args[0]} ${args[1]} ${args[2]} ' -> args=("$@"); echo ${args[0]} ${args[1]} ${args[2]}'

# Print all arguments at once
echo $@ ' -> echo $@'

# Print number of arguments passed
echo Number of arguments passed: $# ' -> echo Number of arguments passed: $#'

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Running the Script

Execute the script and pass three arguments:

$ ./arguments.sh Bash Scripting Tutorial

Output

Bash Scripting Tutorial  -> echo $1 $2 $3
Bash Scripting Tutorial  -> args=("$@"); echo ${args[0]} ${args[1]} ${args[2]}
Bash Scripting Tutorial  -> echo $@
Number of arguments passed: 3  -> echo Number of arguments passed: $#

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Explanation

Command / Variable	Description	Example Output
$1, $2, $3	Accesses positional arguments (first, second, third)	Bash Scripting Tutorial
args=("$@")	Stores all arguments into an array	args[0] = Bash
${args[index]}	Access argument by array index	${args[1]} → Scripting
$@	Expands all arguments as a single string	Bash Scripting Tutorial
$#	Prints the number of arguments passed	3

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Important Notes

• $0 refers to the script name itself.

• Arguments beyond $9 should be accessed using curly braces, e.g. ${10}, ${11}, etc.

• "$@" and "$*" behave differently:

  • "$@" treats each argument as a separate quoted string (preferred).

  • "$*" treats all arguments as a single string.

Example difference:

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

→ Prints each argument separately.

for arg in "$*"; do
  echo "Arg: $arg"
done

→ Prints all arguments as one combined string.

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Bonus Tip: Argument Validation

You can ensure required arguments are provided:

#!/bin/bash
if [ $# -lt 2 ]; then
  echo "Usage: $0 <arg1> <arg2>"
  exit 1
fi
echo "Arguments received: $1 and $2"

Output when missing arguments:

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

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Summary

Feature	Variable / Command	Purpose
Access single argument	$1, $2, $3	Gets specific argument
Access all arguments	$@	Lists all arguments
Store arguments	args=("$@")	Stores arguments in an array
Count arguments	$#	Shows number of arguments
Script name	$0	Name of the running script

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Example in One Go

$ ./arguments.sh Hello Cyber Security
Hello Cyber Security  -> echo $1 $2 $3
Hello Cyber Security  -> args=("$@"); echo ${args[0]} ${args[1]} ${args[2]}
Hello Cyber Security  -> echo $@
Number of arguments passed: 3  -> echo Number of arguments passed: $#

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In Short:
Bash scripts can take arguments just like programs. You can use positional variables, arrays, and special variables like $@ and $# to handle them efficiently.

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Real-World Use Case: Automated File Backup Script

Let’s apply what we’ve learned in a real-world automation scenario.
Here’s a Bash script that creates a timestamped backup of a file or directory — using arguments to specify the source and destination paths.

Script: backup.sh

#!/bin/bash
# ===============================
# Automated Backup Script
# Usage: ./backup.sh <source> <destination>
# ===============================

# Validate input arguments
if [ $# -ne 2 ]; then
  echo "Usage: $0 <source_path> <destination_directory>"
  exit 1
fi

# Store arguments in variables for readability
SOURCE=$1
DESTINATION=$2
TIMESTAMP=$(date +"%Y%m%d_%H%M%S")
BASENAME=$(basename "$SOURCE")
BACKUP_NAME="${BASENAME}_backup_${TIMESTAMP}.tar.gz"

# Create backup
tar -czf "$DESTINATION/$BACKUP_NAME" "$SOURCE"

# Display result
if [ $? -eq 0 ]; then
  echo "Backup created successfully!"
  echo "Source: $SOURCE"
  echo "Destination: $DESTINATION/$BACKUP_NAME"
else
  echo "Backup failed!"
fi

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Example Execution

$ ./backup.sh /home/user/documents /home/user/backups

Output

Backup created successfully! Source: /home/user/documents Destination: /home/user/backups/documents_backup_20251112_163000.tar.gz

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What’s Happening?

Component	Explanation
$1, $2	Used to get source and destination paths
$(date +"%Y%m%d_%H%M%S")	Generates a timestamp for uniqueness
tar -czf	Compresses the source folder into a .tar.gz archive
$?	Checks the exit status of the last command (0 = success)
${BASENAME}	Extracts the file/folder name from a full path

This script demonstrates how Bash argument handling can be used to create dynamic, flexible automation tools.

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