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--- quickreference:unix [2024/03/04 15:54] – rodolico
+++ quickreference:unix [2025/02/05 00:12] (current) – [Shell (mainly BASH)] rodolico
@@ Line 237: / Line 237: @@
 ===== Shell (mainly BASH) =====
+==== Here Documents ====
+Most unix users are familiar with echo'ing something to STDOUT so it can be used as STDIN for a following script (using the pipe). However, this is usually limited to a single line.
+A **here document** is a way of having multiple lines processed at one time. In many cases, you can have similar functionality using quotes, but here documents are more robust.
+For example, a simple test of a newly built mail system might include creating a file with all of the headers necessary, then passing that to //sendmail// for processing. However, you can skip a step and simply send the message directly using a here document.
+<code bash>
+sendmail user@example.com << EOF
+To: user@example.com
+from: root@example.org
+Subject: test
+This is a test
+EOF
+</code>
+The entire block above is one command. Here is the breakdown.
+  - //sendmail user@example.com// is a standard sendmail invocation which assumes the message itself is coming from STDIN.
+  - //<<// marks the beginning of a //here document//
+  - //EOF// is the tag which will mark the end of the text for the here document
+  - Everything up to the EOF is the actual string to be passed to sendmail
+  - //EOF// at the end marks the end of the here document. **Note**: there must be no leading or trailing whitespace. The tag must be exactly as entered after the << (case sensitive), and must be the only thing on the final line.
+This only touches the surface of here documents. See [[https://tldp.org/LDP/abs/html/here-docs.html]] for a lot more information.
 ==== Find files within date range containing text ====