Here Document And Here String in Bash on Linux

Here documents and here strings are powerful Bash features that allow users to pass multi-line input to commands without using external files. These redirection mechanisms are essential for shell scripting, enabling clean and readable code for tasks involving multi-line text processing, configuration generation, and command automation.

What is a Here Document?

A here document (heredoc) is a type of input redirection that allows you to specify multiple lines of input for a command using the << operator followed by a delimiter. The input continues until a line containing only the delimiter is encountered.

Basic Here Document Syntax

command <<DELIMITER
line 1
line 2
line 3
DELIMITER

Example Using cat with Here Document

cat <<EOF
This is line 1 of the input.
This is line 2 of the input.
This is line 3 of the input.
EOF

This is line 1 of the input.
This is line 2 of the input.
This is line 3 of the input.

Creating Files with Here Documents

#!/bin/bash
# Create a configuration file
cat > config.txt <<EOF
database_host=localhost
database_port=5432
database_name=myapp
EOF

What is a Here String?

A here string uses the <<< operator to pass a single string (which can contain newlines) as input to a command. Unlike here documents, here strings are simpler and don't require delimiters.

Here String Syntax

command <<<"string content"

Example Using grep with Here String

grep 'line 2' <<<"line 1
line 2
line 3"

line 2

Practical Examples

Processing Data with awk

# Using here document with awk
awk '{print "Field 1:", $1}' <<EOF
apple red fruit
car blue vehicle
book thick object
EOF

Field 1: apple
Field 1: car
Field 1: book

Variable Substitution in Here Documents

#!/bin/bash
name="John"
age=30

# Variables are expanded by default
cat <<EOF
Hello, my name is $name
I am $age years old
EOF

Preventing Variable Expansion

# Use quoted delimiter to prevent expansion
cat <<'EOF'
Hello, my name is $name
I am $age years old
EOF

Advanced Features

Indented Here Documents

if true; then
    cat <<-EOF
	This text is indented
	But leading tabs are removed
	EOF
fi

Common Use Cases

• Configuration file generation Creating config files with dynamic content

• SQL queries Passing multi-line SQL statements to database clients

• Email templates Generating email content with variable substitution

• Script documentation Embedding help text or usage information

• Data processing Feeding sample data to text processing commands

Conclusion

Here documents and here strings are essential Bash features for handling multi-line input elegantly. Here documents excel at creating large blocks of text with variable substitution, while here strings provide a concise way to pass simple multi-line strings to commands. Both features improve script readability and eliminate the need for temporary files in many scenarios.