User Input & Arguments

7.4. User Input & Arguments#

Make scripts interactive by accepting arguments and reading user input.

7.4.1. Real-World Example: Interactive Configuration#

#!/bin/bash

# Validate we got a config file
if [ $# -ne 1 ]; then
  echo "Usage: $0 <config_file>"
  exit 1
fi

config_file="$1"

echo "=== System Configuration ==="
echo

# Check if config exists
if [ -f "$config_file" ]; then
  read -p "Config exists. Overwrite? (y/n): " overwrite
  if [ "$overwrite" != "y" ]; then
    echo "Aborted"
    exit 0
  fi
fi

# Get configuration from user
read -p "Hostname: " hostname
read -p "IP Address: " ip_addr
read -p "Gateway: " gateway

# Validate IP
if ! [[ "$ip_addr" =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
  echo "Invalid IP address"
  exit 1
fi

# Save configuration
cat > "$config_file" << EOF
hostname=$hostname
ip_addr=$ip_addr
gateway=$gateway
EOF

echo "Configuration saved to $config_file"

7.4.1.1. read Multiple Variables#

#!/bin/bash

read -p "Enter first and last name: " first last

echo "First: $first"
echo "Last: $last"

Usage:

$ bash myscript.sh
Enter first and last name: John Doe
First: John
Last: Doe

7.4.1.2. read from File or Pipe#

#!/bin/bash

# Read from file
while read -r line; do
  echo "Read: $line"
done < /etc/hostname

# Read from pipe
echo "hello world" | read first second
echo "$first $second"  # "hello world"

7.4.1.3. Basic read#

#!/bin/bash

echo "What is your name?"
read name

echo "Hello, $name!"

Usage:

$ bash myscript.sh
What is your name?
Alice
Hello, Alice!

7.4.1.4. read with Prompt#

#!/bin/bash

read -p "Enter username: " username
read -p "Enter password: " password

echo "Logged in as: $username"

7.4.1.5. read -s: Silent (Password)#

#!/bin/bash

read -sp "Enter password: " password
echo
echo "Password accepted (length: ${#password})"

The -s flag hides input (no echo)

7.4.2. read: Interactive Input#

Get input from user during script execution:

7.4.2.1. Validating Arguments#

#!/bin/bash

# Check if argument provided
if [ $# -eq 0 ]; then
  echo "Usage: $0 <filename>"
  exit 1
fi

# Check if too many arguments
if [ $# -gt 2 ]; then
  echo "Too many arguments"
  exit 1
fi

# Check specific argument count
if [ $# -ne 2 ]; then
  echo "Exactly 2 arguments required"
  exit 1
fi

echo "Arguments: $1, $2"

Usage:

$ bash myscript.sh
Usage: ./myscript.sh <filename>

$ bash myscript.sh a b c
Too many arguments

$ bash myscript.sh a b
Arguments: a, b

7.4.2.2. Accessing Arguments#

#!/bin/bash

# $0 = script name
echo "Script: $0"

# $1, $2, $3... = first, second, third argument
echo "First arg: $1"
echo "Second arg: $2"

# $# = total number of arguments
echo "Total args: $#"

# $@ = all arguments (as array)
echo "All args: $@"

# $* = all arguments (as string)
echo "As string: $*"

Usage:

$ bash myscript.sh foo bar baz
Script: ./myscript.sh
First arg: foo
Second arg: bar
Total args: 3
All args: foo bar baz
As string: foo bar baz

7.4.3. Script Arguments: \(@, \)#, \(0-\)9#

Access command-line arguments passed to script:

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