Here Documents
Here documents (heredocs) are a convenient way to handle multi-line text blocks in Bash scripts. They're perfect for generating configuration files, creating SQL queries, writing multi-line messages, and passing large text blocks to commands. Here strings are a related feature for single-line text input. Both features make your scripts cleaner and more readable!
Basic Here Document Syntax
A here document starts with << followed by a delimiter (commonly EOF for "End Of File"). Everything until the delimiter is treated as input to the command.
Click Run to execute your code
# Basic heredoc syntax
command <
Key Points:
- The delimiter can be any word (EOF, END, STOP, etc.)
- The ending delimiter must be on a line by itself, with no leading/trailing spaces
- Variables and command substitution are expanded by default
- Heredocs work with any command that reads from stdin
Variable Expansion in Heredocs
By default, variables and command substitutions are expanded in here documents, making them perfect for templates!
name="Alice"
age=25
cat <
Pro Tip: Heredocs are excellent for generating configuration files, SQL queries, HTML templates, and email messages where you need variable substitution!
Quoted Delimiters (No Expansion)
If you quote the delimiter (with single or double quotes, or a backslash), variable expansion is disabled. This is useful when you want literal text.
name="Alice"
# With expansion (default)
cat <
Important: Use quoted delimiters (<<'EOF' or <<\EOF) when generating scripts, config files with literal dollar signs, or when you want to preserve special characters exactly as written.
Here Strings: <<<
Here strings are a shorthand way to pass a single string to a command's stdin. They're useful for quick one-liners.
# Here string syntax: <<< "text"
# Convert to uppercase
tr 'a-z' 'A-Z' <<< "hello world"
# Output: HELLO WORLD
# Pass to grep
grep "error" <<< "This line has an error message"
# Pass variable to command
text="Hello World"
wc -w <<< "$text"
# Output: 2
Use Cases:
- Quick text processing with commands like tr, grep, sed
- Testing commands with sample input
- Passing variables to commands that expect stdin
- One-liners without creating files
Practical Examples
Here documents shine in real-world scenarios. Here are some common use cases:
# 1. Generate configuration file
cat > /tmp/config.txt < deploy.sh <<'EOF'
#!/bin/bash
# Auto-generated deployment script
echo "Deploying application..."
# More commands here
EOF
chmod +x deploy.sh
Common Mistakes
1. Spaces around delimiter
# Wrong - spaces break the delimiter
cat <
2. Using wrong delimiter
# Wrong - delimiter mismatch
cat <
3. Forgetting to quote when needed
# Wrong - unwanted expansion when generating script
cat > script.sh < script.sh <<'EOF'
#!/bin/bash
echo "Price: $10" # This stays as $10 (literal)
EOF
Exercise: Generate a Configuration File
Task: Use a here document to generate a configuration file!
Requirements:
- Create a config file with at least 3 settings
- Use variables for at least one setting
- Include a comment in the config file
- Use a here string to count words in a message
Hint: Remember to use quoted delimiter if you want literal dollar signs in your config!
Show Solution
#!/bin/bash
# Generate configuration file
# Variables for config
APP_NAME="MyApp"
PORT=8080
# Create config file with heredoc
cat > app.conf <
Summary
- Here Document:
<<DELIMITER ... DELIMITER for multi-line text blocks
- Variable Expansion: Variables expand by default in heredocs (perfect for templates)
- Quoted Delimiter: Use
<<'EOF' or <<\EOF to disable expansion
- Here String:
<<< "text" for single-line input
- Delimiter Rules: Must match exactly, no spaces, on its own line
- Use Cases: Config files, SQL queries, script generation, email templates
- Common Delimiters: EOF, END, STOP (use descriptive names for clarity)
What's Next?
Excellent! You've completed the Strings & Text Processing module. Next, we'll move on to Operators & Arithmetic - learning how to perform mathematical operations, comparisons, logical operations, and test file properties. These are essential for conditionals and calculations in your scripts!
Enjoying these tutorials?