Linux Basic — Here-Documents

Here-Documents (<<EOF)

A here-document feeds a block of literal text as standard input to a command. It replaces fragile echo -e "line1\nline2" chains and is the canonical way to embed SQL, config templates, or multi-line emails inside a script.

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Basic Syntax

cat <<EOF
Hello, $USER
Today is $(date +%F).
EOF

The delimiter (EOF) is your choice — it just has to appear unindented on a line by itself to end the heredoc. Variables ($USER) and command substitution ($(date)) are expanded by default.

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Disable Expansion: Quote the Delimiter

cat <<'EOF'
Literal $USER, no expansion.
Backticks `like this` stay literal too.
EOF

Quoting the opening delimiter ('EOF' or "EOF") treats the body as raw text. Use this for scripts within scripts, AWK programs, or anything containing $ you don’t want expanded.

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Strip Leading Tabs: <<-

<<-EOF strips leading tab characters (not spaces) from each line — letting you indent the body to match surrounding code:

if true; then
    cat <<-EOF
	this line has leading tabs
	they will be stripped
	EOF
fi

Only tabs are stripped. Mixing tabs and spaces silently breaks this.

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Heredocs to Commands Other Than cat

Anything that reads stdin works:

# Send mail
mail -s "Report" ops@example.com <<EOF
Run completed at $(date).
EOF

# Pipe SQL to psql
psql -d mydb <<SQL
SELECT count(*) FROM users WHERE active = true;
SQL

# Build a config file
sudo tee /etc/myapp.conf >/dev/null <<EOF
listen_port = 8080
log_level   = info
EOF

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Capture a Heredoc into a Variable

template=$(cat <<EOF
host: $HOSTNAME
user: $USER
EOF
)
echo "$template"

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Common Pitfalls

Pitfall	Fix
Closing EOF is indented with spaces	Must be flush-left (or use <<- with tabs only)
Trailing whitespace after opening <<EOF	Bash treats the literal delimiter strictly — clean line endings
Variable expansion happens when you don’t want it	Quote the delimiter: <<'EOF'
Heredoc inside a pipeline runs in a subshell	Variables set inside aren’t visible after

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Quick Reference

Form	Expansion	Indent stripping
<<EOF	yes	none
<<'EOF'	no	none
<<-EOF	yes	leading tabs
<<-'EOF'	no	leading tabs

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