You've seen scripts with `cat << EOF` followed by multiple lines of text and ending with `EOF`. What is this syntax, and how does it work? ## TL;DR `cat << EOF` is a heredoc (here document) - a way to pass multiple lines of input to a command in Bash. The `<<` redirects the following lines as input until it reaches the delimiter (EOF in this case). You can use any delimiter word, not just EOF. Variables expand inside heredocs by default, or use `<< 'EOF'` with quotes to prevent expansion. A heredoc lets you embed multi-line content directly in your script without needing separate files or complex quoting. Here's a basic example: ```bash cat << EOF This is line 1 This is line 2 This is line 3 EOF ``` This prints: ``` This is line 1 This is line 2 This is line 3 ``` The `cat` command receives all the lines between `<< EOF` and the closing `EOF` as input, then outputs them. ## Understanding the Syntax Let's break down `cat << EOF`: - `cat` - the command that receives input - `<<` - the heredoc operator (redirection) - `EOF` - the delimiter marking where input starts and ends ``` cat << EOF <- Start of heredoc text here <- Input to cat more text <- More input EOF <- End delimiter (must be on its own line) ``` The delimiter `EOF` (End Of File) is conventional but arbitrary. You can use any word: ```bash cat << STOP This works too STOP cat << MYDELIMITER So does this MYDELIMITER ``` The closing delimiter must be on a line by itself with no leading spaces (unless you use `<<-`, covered later). ## Creating Files with Heredocs A common use is creating files with multiple lines: ```bash cat << EOF > config.txt server_name=localhost port=8080 debug=true EOF ``` This creates `config.txt` with those three lines. It's cleaner than using multiple `echo` statements: ```bash # Without heredoc (verbose) echo "server_name=localhost" > config.txt echo "port=8080" >> config.txt echo "debug=true" >> config.txt # With heredoc (cleaner) cat << EOF > config.txt server_name=localhost port=8080 debug=true EOF ``` ## Variable Expansion in Heredocs Variables expand inside heredocs by default: ```bash NAME="Alice" PORT=8080 cat << EOF Hello, $NAME! Server is running on port $PORT EOF ``` Output: ``` Hello, Alice! Server is running on port 8080 ``` Command substitution works too: ```bash cat << EOF Current directory: $(pwd) Current user: $(whoami) Date: $(date +%Y-%m-%d) EOF ``` ## Preventing Variable Expansion To treat content literally (no variable expansion), quote the delimiter: ```bash NAME="Alice" cat << 'EOF' Hello, $NAME! This will print literally: $NAME EOF ``` Output: ``` Hello, $NAME! This will print literally: $NAME ``` Note the quotes around `'EOF'` - they prevent expansion. You can use single or double quotes: ```bash cat << "EOF" $NAME won't expand here either EOF ``` ## Using <<- to Ignore Leading Tabs If you're indenting heredoc content in a script for readability, use `<<-` instead of `<<`: ```bash #!/bin/bash if true; then cat <<- EOF This is indented with a tab Another line with a tab EOF fi ``` The `<<-` strips leading tab characters (not spaces) from each line, including the delimiter. This lets you indent the heredoc to match your code structure. Important: This only works with tabs, not spaces. If you use spaces, they'll appear in the output. ## Passing Heredocs to Other Commands Any command that reads from stdin can use heredocs: ```bash # Pass multi-line input to grep grep "error" << EOF This is a log entry An error occurred here Another normal entry EOF ``` Output: ``` An error occurred here ``` Passing to `mysql`: ```bash mysql -u root -p << EOF CREATE DATABASE myapp; USE myapp; CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(100)); EOF ``` Passing to `ssh`: ```bash ssh user@server << EOF cd /var/www ls -la df -h EOF ``` The remote commands execute on the server, with their output shown locally. ## Assigning Heredocs to Variables You can capture heredoc content in a variable: ```bash CONFIG=$(cat << EOF server=localhost port=8080 debug=true EOF ) echo "$CONFIG" ``` This stores the multi-line string in `$CONFIG`. Note the quotes when echoing to preserve newlines. Another syntax without `cat`: ```bash read -r -d '' CONFIG << EOF server=localhost port=8080 debug=true EOF echo "$CONFIG" ``` The `read -r -d ''` reads until it hits the null delimiter, effectively capturing everything until EOF. ## Heredocs in Functions Heredocs work inside functions: ```bash create_config() { cat << EOF > "$1" # Configuration file generated on $(date) server_name=$2 port=$3 EOF } # Usage create_config "app.conf" "localhost" 8080 ``` This function creates a config file with parameters passed as arguments. ## Appending with Heredocs Use `>>` to append instead of overwrite: ```bash # Create initial file cat << EOF > log.txt Application started EOF # Append more content cat << EOF >> log.txt User logged in Database connected EOF ``` ## Multi-line Comments with Heredocs A clever trick for multi-line comments in Bash: ```bash : << 'COMMENT' This is a multi-line comment It won't be executed Useful for temporarily disabling code blocks COMMENT echo "This will run" ``` The `:` command does nothing (it's a no-op), and the heredoc is passed to it but ignored. ## Practical Example: Generating HTML Creating an HTML file with dynamic content: ```bash #!/bin/bash TITLE="My Web Page" DATE=$(date +%Y-%m-%d) cat << EOF > index.html $TITLE

$TITLE

Generated on: $DATE

Welcome to my website!

EOF echo "HTML file created: index.html" ``` ## Practical Example: Docker Compose File Generating a Docker Compose file with environment-specific values: ```bash #!/bin/bash APP_PORT=3000 DB_PASSWORD=$(openssl rand -base64 32) cat << EOF > docker-compose.yml version: '3.8' services: app: image: myapp:latest ports: - "$APP_PORT:3000" environment: - DB_PASSWORD=$DB_PASSWORD - NODE_ENV=production database: image: postgres:14 environment: - POSTGRES_PASSWORD=$DB_PASSWORD volumes: - db-data:/var/lib/postgresql/data volumes: db-data: EOF echo "Docker Compose file created with random password" ``` ## Practical Example: Email Template Sending emails with templates: ```bash #!/bin/bash USER_NAME="Alice" RESET_LINK="https://example.com/reset?token=abc123" mail -s "Password Reset" user@example.com << EOF Hello $USER_NAME, You requested a password reset. Click the link below: $RESET_LINK This link expires in 24 hours. If you didn't request this, please ignore this email. Best regards, The Support Team EOF ``` ## Heredocs with Pipes You can pipe heredoc output to other commands: ```bash cat << EOF | grep -v "^#" | sort # This is a comment zebra apple # Another comment banana EOF ``` This filters out comments and sorts the remaining lines. ## Common Pitfalls The closing delimiter must be on its own line with no extra characters: ```bash # Wrong - has spaces before EOF cat << EOF text EOF # Wrong - has text after EOF cat << EOF text EOF # Correct cat << EOF text EOF ``` Variables in the delimiter don't expand: ```bash DELIM="END" # This looks for literal EOF, not the value of $DELIM cat << $DELIM text EOF # This won't work # You must use the actual delimiter cat << EOF text EOF ``` ## Why Use Heredocs? Heredocs are great when you need to: - Embed multi-line text in scripts without external files - Generate configuration files with variable substitution - Pass complex input to commands - Improve script readability compared to multiple echo statements - Avoid quoting nightmares with special characters For simple cases, echo might be clearer: ```bash # For single lines, echo is fine echo "server=localhost" > config.txt # For multiple lines with variables, heredoc is cleaner cat << EOF > config.txt server=localhost port=$PORT debug=true EOF ``` The `cat << EOF` syntax (heredocs) is a powerful feature for embedding multi-line text in Bash scripts. Whether you're creating configuration files, generating code, or passing complex input to commands, heredocs make your scripts cleaner and more maintainable than alternatives.

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How Does cat << EOF Work in Bash?