Input_output

Chapter 14: Input/Output

Overview

This chapter covers input/output operations in bash, including reading from stdin, writing to stdout/stderr, file redirection, and advanced I/O techniques. These skills are essential for creating interactive scripts, processing data, and handling errors in DevOps workflows.

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Basic Output

echo Command

echo_basic.sh

#!/usr/bin/env bash
# Basic output
echo "Hello, World!"

# Without newline
echo -n "No newline: "

# Enable escape sequences
echo -e "Line1\nLine2"

# Print literal
echo -E "No expansion: \n"

printf Command

printf_basic.sh

#!/usr/bin/env bash
# Basic printf
printf "Hello %s\n" "World"

# Format specifiers
printf "%d %s %.2f\n" 42 "answer" 3.14159

# Width and padding
printf "|%10s|\n" "hello"
printf "|%-10s|\n" "hello"
printf "|%05d|\n" 42

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Basic Input

read Command

read_basic.sh

#!/usr/bin/env bash
# Basic read
echo "Enter your name:"
read name
echo "Hello, $name"

# Read with prompt
read -p "Enter your name: " name
echo "Hello, $name"

# Read with timeout
read -t 3 -p "Enter value (3s timeout): " value
echo "Value: $value"

Reading into Array

read_array.sh

#!/usr/bin/env bash
# Read line into array
echo "Enter fruits (space-separated):"
read -a fruits

echo "You entered: ${fruits[@]}"
echo "First fruit: ${fruits[0]}"

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File I/O

Reading from File

read_file.sh

#!/usr/bin/env bash
# Method 1: While loop
while IFS= read -r line; do
    echo "Line: $line"
done < /etc/passwd

# Method 2: Using cat
cat /etc/passwd | while read line; do
    echo "Line: $line"
done

# Method 3: Using mapfile
mapfile -t lines < /etc/passwd
for line in "${lines[@]}"; do
    echo "Line: $line"
done

Writing to File

write_file.sh

#!/usr/bin/env bash
# Overwrite file
echo "Content" > file.txt

# Append to file
echo "More content" >> file.txt

# Using printf
printf "%s\n" "Line 1" "Line 2" > output.txt

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Redirection

Input/Output Redirection

redirection.sh

#!/usr/bin/env bash
# Redirect stdout to file
echo "Output" > output.txt

# Redirect stderr to file
command 2> error.log

# Redirect both to same file
command &> all.log

# Redirect and append
echo "More" >> output.txt

# From file to command
command < input.txt

Here Strings and Here Documents

here_string.sh

#!/usr/bin/env bash
# Here string
read -r word <<< "hello world"
echo "$word"  # hello

# Here document
cat << EOF
This is a multi-line
string that preserves
formatting
EOF

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Practical Examples

Example 1: Interactive Menu

menu.sh

#!/usr/bin/env bash
show_menu() {
    echo "1. View status"
    echo "2. Start service"
    echo "3. Stop service"
    echo "4. Exit"
}

while true; do
    show_menu
    read -p "Choice: " choice

    case "$choice" in
        1) echo "Status: Running" ;;
        2) echo "Starting service..." ;;
        3) echo "Stopping service..." ;;
        4) echo "Goodbye!"; break ;;
        *) echo "Invalid choice" ;;
    esac
    echo
done

Example 2: Read Configuration

read_config.sh

#!/usr/bin/env bash
# Read key=value pairs
while IFS='=' read -r key value; do
    # Skip comments and empty lines
    [[ "$key" =~ ^# ]] && continue
    [[ -z "$key" ]] && continue

    echo "Setting: $key = $value"
    declare "$key=$value"
done < config.txt

Example 3: Logging

logging.sh

#!/usr/bin/env bash
LOG_FILE="/var/log/script.log"

log() {
    local level="$1"
    shift
    local message="$*"
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] [$level] $message" | tee -a "$LOG_FILE"
}

log "INFO" "Script started"
log "WARN" "This is a warning"
log "ERROR" "This is an error"

Example 4: Interactive Confirmation

confirm.sh

#!/usr/bin/env bash
confirm() {
    local prompt="$1"
    local response

    while true; do
        read -p "$prompt [y/n]: " response
        case "$response" in
            [yY]|[yY][eE][sS]) return 0 ;;
            [nN]|[nN][oO]) return 1 ;;
            *) echo "Please answer y or n" ;;
        esac
    done
}

if confirm "Continue with deployment?"; then
    echo "Continuing..."
else
    echo "Cancelled"
    exit 1
fi

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Advanced I/O

File Descriptors

descriptors.sh

#!/usr/bin/env bash
# Open file descriptor
exec 3> output.txt

# Write to descriptor
echo "Line to fd 3" >&3

# Close descriptor
exec 3>&-

# Read from descriptor
exec 3< input.txt
read -u 3 line
echo "Read: $line"
exec 3<&-

tee Command

tee_example.sh

#!/usr/bin/env bash
# Write to file and stdout
echo "Output" | tee file.txt

# Append
echo "More" | tee -a file.txt

# Multiple outputs
echo "Output" | tee file1.txt file2.txt

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Summary

In this chapter, you learned about:

• ✅ Basic output with echo and printf
• ✅ Basic input with read
• ✅ Reading and writing files
• ✅ Redirection operators
• ✅ Here strings and documents
• ✅ Interactive menus
• ✅ Configuration reading
• ✅ Logging
• ✅ File descriptors
• ✅ tee command

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Exercises

Level 1: Basics

1. Create a script that asks for user input
2. Write to a file using echo
3. Read from a file line by line

Level 2: Intermediate

4. Implement an interactive menu
5. Create a logging function
6. Build a configuration file parser

Level 3: Advanced

7. Implement file descriptor operations
8. Create a confirmation prompt function
9. Build a pipeline with tee

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Next Steps

Continue to the next chapter to learn about functions.

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Previous Chapter: String Manipulation Next Chapter: Functions