# 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.


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