Bash Here Documents

A here document feeds several lines of text into a command. The text sits right inside the script, so no separate file is needed. Scripts use it for long messages, configuration files, and menus.

The Idea in a Picture

  command << MARKER
  line one
  line two
  line three
  MARKER
     |
     v
  The command receives the three lines as input

The marker word starts the block and ends it. Any word works. Writers pick EOF (end of file) by habit.

Basic Example

#!/bin/bash
cat << EOF
Welcome to the Backup Tool
Version 1.0
Press Ctrl+C to stop
EOF

Output:

Welcome to the Backup Tool
Version 1.0
Press Ctrl+C to stop

The closing marker must stand alone at the start of its line, with no spaces before or after it.

Variables Inside the Block

Bash replaces variables and commands inside a here document:

#!/bin/bash
name="Asha"
cat << EOF
Hello, $name
Today is $(date +%A)
Your folder is $(pwd)
EOF

Output:

Hello, Asha
Today is Friday
Your folder is /home/asha

Stop Expansion With Quotes

Quotation marks around the marker switch replacement off. The text prints exactly as typed.

cat << 'EOF'
The variable $name stays as text
The command $(date) stays as text
EOF

Indent With a Dash

The form <<- removes leading tab characters. Code inside functions and loops stays neatly aligned.

show_help() {
	cat <<- EOF
	Usage: tool [option]
	  -h   show help
	  -v   show version
	EOF
}

This form strips tabs only. Spaces remain in the output.

Write a File

cat > settings.conf << EOF
host=localhost
port=8080
mode=testing
EOF

The redirect > saves the block into a new file. Use >> to append to an existing file instead.

Write a File With Administrator Rights

sudo tee /etc/myapp.conf << EOF > /dev/null
mode=production
EOF

A plain redirect fails here because the redirect runs without administrator rights. The tee command receives the rights from sudo.

Send Input to Another Command

sort << EOF
mango
apple
banana
EOF

Output:

apple
banana
mango

Run Multi-Line Commands

bash << 'EOF'
for i in 1 2 3
do
  echo "Run $i"
done
EOF

Bash reads the block as a small script and runs it.

Here Strings

A here string passes one short piece of text with three angle brackets:

grep "apple" <<< "I like apple pie"
wc -w <<< "count these four words"

The second line prints 4.

Practical Example: Help Message

#!/bin/bash
usage() {
  cat << EOF
Usage: $0 [options] file

Options:
  -h    Show this help message
  -v    Show detailed output
  -o    Set the output folder
EOF
}

if [ $# -eq 0 ]; then
  usage
  exit 1
fi

The function prints a tidy help screen when a user runs the script without arguments.

Common Mistakes

• Spaces before the closing marker keep the block open and trigger a "here-document delimited by end-of-file" warning.
• Different spelling in the opening and closing markers has the same effect.
• Dollar signs in text need a quoted marker or a backslash to stay as typed.

Key Takeaways

• << MARKER sends a block of text to a command.
• Variables and commands expand unless the marker sits in quotes.
• <<- removes leading tabs for tidy code.
• Combine with > to create files from a script.
• <<< sends a single string as input.