Here Documents

              A here document is a special-purpose code block. It uses a form of I/O redirection to feed a command list to an interactive program or a command, such as ftp, cat, or the ex text editor.

COMMAND <<InputComesFromHERE
...
InputComesFromHERE

A limit string delineates (frames) the command list. The special symbol << designates the limit string. This has the effect of redirecting the output of a file into the stdin of the program or command. It is similar to interactive-program < command-file, where command-file contains

command #1
command #2
...

The here document alternative looks like this:

#!/bin/bash
interactive-program <<LimitString
command #1
command #2
...
LimitString

Choose a limit string sufficiently unusual that it will not occur anywhere in the command list and confuse matters.

Note that here documents may sometimes be used to good effect with non-interactive utilities and commands, such as, for example, wall.

#!/bin/bash

wall <<zzz23EndOfMessagezzz23
E-mail your noontime orders for pizza to the system administrator.
    (Add an extra dollar for anchovy or mushroom topping.)
# Additional message text goes here.
# Note: 'wall' prints comment lines.
zzz23EndOfMessagezzz23

# Could have been done more efficiently by
#         wall <message-file
#  However, embedding the message template in a script
#+ is a quick-and-dirty one-off solution.

exit 0

Even such unlikely candidates as the vi text editor lend themselves to here documents.

#!/bin/bash

# Non-interactive use of 'vi' to edit a file.
# Emulates 'sed'.

E_BADARGS=65

if [ -z "$1" ]
then
  echo "Usage: `basename $0` filename"
  exit $E_BADARGS
fi

TARGETFILE=$1

# Insert 2 lines in file, then save.
#--------Begin here document-----------#
vi $TARGETFILE <<x23LimitStringx23
i
This is line 1 of the example file.
This is line 2 of the example file.
^[
ZZ
x23LimitStringx23
#----------End here document-----------#

#  Note that ^[ above is a literal escape
#+ typed by Control-V <Esc>.

#  Bram Moolenaar points out that this may not work with 'vim',
#+ because of possible problems with terminal interaction.

exit 0

The above script could just as effectively have been implemented with ex, rather than vi. Here documents containing a list of ex commands are common enough to form their own category, known as ex scripts.

#!/bin/bash
#  Replace all instances of "Smith" with "Jones"
#+ in files with a ".txt" filename suffix. 

ORIGINAL=Smith
REPLACEMENT=Jones

for word in $(fgrep -l $ORIGINAL *.txt)
do
  # -------------------------------------
  ex $word <<EOF
  :%s/$ORIGINAL/$REPLACEMENT/g
  :wq
EOF
  # :%s is the "ex" substitution command.
  # :wq is write-and-quit.
  # -------------------------------------
done

Analogous to "ex scripts" are cat scripts.