A "here document" is a Bourne shell syntactic feature that allows you to feed data to a program without storing it in an external file. It works equally well in POSIX, Korn and Bash shells too.
The basic form is:
somecommand <<WORD your data go here WORD
Here, somecommand can be any program that reads from standard input (cat is by far the most common), and WORD can be any delimiter word you like. (EOF is a common choice.)
Here documents of this form have certain characteristics:
Shell substitutions (such as $variable) in the here document are performed.
- The delimiter word must appear on a line by itself, in the first column.
- Any whitespace in the here document is preserved, including leading whitespace.
If we want to avoid shell substitutions, we can quote the delimiter word:
somecommand <<'WORD' your data $go `here` WORD
If we want for the body of the here document to be indented like the rest of the script while also having bash remove all leading tab characters during execution, we can add a - (hyphen) suffix to the immediate right of the '<<'. An empty space to the left of WORD is optional, and bash will leave any (space) characters as-is.
if ... while .... somecommand <<- WORD this is an indented here document WORD done fi
In this form, all leading tab characters (not spaces!) will be removed. There is no provision for removing leading spaces, or leading tabs-and-spaces. (Recall the syntactic restrictions of Makefiles, and you'll be OK.)
Here documents are typically implemented by creating a temporary file and redirecting standard input from this file when the program is invoked.
In bash, there is a variant of the here document called the here string. It's more compact, but also more limited:
read -a octets <<< "$ipaddr"
The <<< serves a role similar to that of the << in a here document, but there is no sentinel word to tell us where the input ends. Rather, the <<< is followed by a single word (Quotes are your friend!). That word, plus a newline, become the standard input of the command.