Here Doc

When shell encounters the syntax << delimiter, it starts creating a multi-line buffer using these rules:

1. A line containing exactly delimiter ends the contents

2. Escape sequences are expanded (e.g., \t may be translated into a literal TAB)

3. Variables are expanded (e.g., $foo becomes contents of foo variable)

4. Command substitutions are performed (e.g., $(date) and `date` become the output of date command)

The buffer created is sent as stdin to the program of your choice. For example:

1 #!/bin/sh
2 person=Jane
3 cat << EOF
4 I heard from $person that "$(date +%Y) is the year of ...," but I am doubtful.
5 EOF

Produces:

I heard from Jane that "2017 is the year of ...," but I am doubtful.

Like the example for double-quoted strings, except on line 4 of the above script, double-quotes do not have to be escaped. The here document syntax can sometimes be used to avoid issues with single versus double quotes when sometimes both are required. For example:

For example:

1 #!/bin/sh
2 where="room 402"
3 what=package
4 contents="supplies for tonight's event"
5 echo "$( cat << EOF
6 $( date +"[%F %T]" ) There's a "$what" for $room containing "$contents."
7 EOF
8 )"

Produces:

[2017-11-22 10:13:39] There's a "package" for room 402 containing "supplies for tonight's event."

There's a lot to unpack here. We'll go line by line.

On line 5, our only argument to echo is:

1. Double-quoted to prevent interpolation of the contents

2. The result of expanding a sub-shell

   1. Running cat

      1. Given a here document

Line 7 is the ending delimiter for the here document and line 8 is the end of the sub-shell and double-quote started on line 5.