Here-doc and here-string: data inside the script

Here-doc: multi-line stdin

When you need to feed a command multi-line text without creating a temp file:

cat <<EOF > /etc/myapp/config.ini

[server]

host = localhost

port = 8080

user = ${USER}

EOF

Everything between <<EOF and the closing EOF goes to the command's stdin (here that is cat, which writes to a file through >). EOF is just a marker; you can use any word (END, DONE, __SCRIPT__).

Variable expansion: with quotes or without

By default ${VAR} and $(...) expand inside a here-doc:

cat <<EOF

Hello, ${USER}!

Today is $(date +%F)

EOF

If you want literal text with no expansion, quote the marker:

cat <<'EOF' > script.template

Hello, ${USER}!     # ← does NOT expand, stays as ${USER}

$HOME too           # ← does NOT expand

EOF

This matters when you generate scripts, templates, or Dockerfile fragments where $VAR must stay as literal text.

Stripping leading tabs: <<-

Inside indented blocks (if, function) a here-doc looks ugly, because the indentation ends up in the text. The fix is <<- (minus): bash strips leading tabs (not spaces):

if [[ -d /etc ]]; then

    cat <<-EOF

		this message

		has leading tabs

		that bash will strip

	EOF

fi

Tabs only. If your editor inserts spaces, it will not work.

Here-string: <<< for a single line

The single-line counterpart:

grep apple <<< "i like apple pie"

Handy for bc, jq, any command that wants stdin data:

bc <<< "2 + 2"           # 4

jq '.name' <<< '{"name":"alice"}'   # "alice"

Common mistakes

• A marker with trailing spaces: bash will not close the here-doc:
  bashcopy

  cat <<EOF

  text

  EOF       <-- space after EOF! will not close

• <<- with spaces instead of tabs: the stripping will not happen.
• Cross-platform: <<< exists in bash and zsh, not in /bin/sh. For POSIX compatibility, use printf '...' | cmd.