Shell by Example: Here Documents

Shell by Example: Here Documents POSIX + Bash

Here-documents (heredocs) let you include multi-line text directly in your script. They’re great for embedding configuration files, SQL queries, or help text. Basic syntax: `<<DELIMITER ... DELIMITER` The delimiter can be any word (`EOF` is common). Variables like `$HOME` are expanded.	Edit `#!/bin/sh cat <<EOF This is a here-document. It can span multiple lines. Variables like $HOME are expanded. EOF`
	Output: This is a here-document. It can span multiple lines. Variables like /root are expanded.

Use `<< 'EOF'` (quoted) to prevent variable expansion.	Edit `#!/bin/sh cat <<'EOF' This text is literal. $HOME is not expanded here. Special characters like \n are also literal. EOF`
	Output: This text is literal. $HOME is not expanded here. Special characters like \n are also literal.

Use `<<-EOF` to strip leading tabs (not spaces). This helps with indentation in scripts.	Edit `#!/bin/sh cat <<-EOF This line had a leading tab. So did this one. The tabs are stripped from output. EOF`
	Output: This line had a leading tab. So did this one. The tabs are stripped from output.

Here-documents are commonly used with commands that read from stdin. Send mail (example - won’t actually send):	Edit `#!/bin/sh send_mail() { # readin in stdin message=$(cat) echo "message: $message" } send_mail -s "Report" user@example.com <<EOF Daily report attached. Generated on $(date). EOF`
	Output: message: Daily report attached. Generated on Wed Jan 1 10:00:00 UTC 2025.

Create a configuration file file with a here-document by adding a redirection operator `>/path/to/file` to the beginning of the here-document.	Edit `#!/bin/sh cat <<EOF >/tmp/config.example # Configuration file # Generated: $(date) setting1=value1 setting2=value2 debug=false EOF echo "Created config file:" cat /tmp/config.example`
	Output: Created config file: # Configuration file # Generated: Wed Jan 1 10:00:00 UTC 2025 setting1=value1 setting2=value2 debug=false

Bash Here-strings (<<<) provide a shorthand for passing a string directly to a command’s stdin:	Edit `#!/bin/bash grep "in" <<<"search in this string"`
	Output: search in this string

Bash Here-strings are particularly useful with read:	Edit `#!/bin/bash read -r first rest <<<"hello world from bash" echo "First word: $first" # hello`
	Output: First word: hello

Bash They can include variable expansion:	Edit `#!/bin/bash message="Hello, World!" tr '[:lower:]' '[:upper:]' <<<"$message"`
	Output: HELLO, WORLD!

Here-documents work great for multi-line variables.	Edit `#!/bin/sh help_text=$( cat <<'EOF' Usage: myscript [options] <file> Options: -h, --help Show this help message -v, --verbose Enable verbose output -o FILE Output to FILE Examples: myscript input.txt myscript -v -o output.txt input.txt EOF ) echo "$help_text"`
	Output: Usage: myscript [options] <file> Options: -h, --help Show this help message -v, --verbose Enable verbose output -o FILE Output to FILE Examples: myscript input.txt myscript -v -o output.txt input.txt

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