19. Here Documents (Revision #2)

Here and now, boys.
--Aldous Huxley, Island

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 << precedes the limit string. This has the effect of redirecting the output of a command block 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 equivalent looks like this:

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.

Example 19-1. broadcast: Sends message to everyone logged in

#!/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

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

Example 19-2. dummyfile: Creates a 2-line dummy file

#!/bin/bash

# Noninteractive use of 'vi' to edit a file.
# Emulates 'sed'.

E_BADARGS=85

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

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 f orm 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. 

Example 19-3. Multi-line message using cat

#!/bin/bash

#  'echo' is fine for printing single line messages,
#+  but somewhat problematic for for message blocks.
#   A 'cat' here document overcomes this limitation.

cat <<End-of-message
-------------------------------------
This is line 1 of the message.
This is line 2 of the message.
This is line 3 of the message.
This is line 4 of the message.
This is the last line of the message.
-------------------------------------
End-of-message

#  Replacing line 7, above, with
#+   cat > $Newfile <<End-of-message
#+       ^^^^^^^^^^
#+ writes the output to the file $Newfile, rather than to stdout.

exit 0

#--------------------------------------------
# Code below disabled, due to "exit 0" above.

# S.C. points out that the following also works.
echo "-------------------------------------
This is line 1 of the message.
This is line 2 of the message.
This is line 3 of the message.
This is line 4 of the message.
This is the last line of the message.
-------------------------------------"
# However, text may not include double quotes unless they are escaped.

The - option to mark a here document limit string (<<-LimitString) suppresses leading tabs (but not spaces) in the output. This may be useful in making a script more readable.

Example 19-4. Multi-line message, with tabs suppressed

#!/bin/bash
# Same as previous example, but...

#  The - option to a here document <<-
#+ suppresses leading tabs in the body of the document,
#+ but *not* spaces.

cat <<-ENDOFMESSAGE
    This is line 1 of the message.
    This is line 2 of the message.
    This is line 3 of the message.
    This is line 4 of the message.
    This is the last line of the message.
ENDOFMESSAGE
# The output of the script will be flush left.
# Leading tab in each line will not show.

# Above 5 lines of "message" prefaced by a tab, not spaces.
# Spaces not affected by   <<-  .

# Note that this option has no effect on *embedded* tabs.

exit 0

A here document supports parameter and command substitution. It is therefore possible to pass different parameters to the body of the here document, changing its output accordingly.

Example 19-5. Here document with replaceable parameters

#!/bin/bash
# Another 'cat' here document, using parameter substitution.

# Try it with no command-line parameters,   ./scriptname
# Try it with one command-line parameter,   ./scriptname Mortimer
# Try it with one two-word quoted command-line parameter,
#                           ./scriptname "Mortimer Jones"

CMDLINEPARAM=1     #  Expect at least command-line parameter.

if [ $# -ge $CMDLINEPARAM ]
then
  NAME=$1          #  If more than one command-line param,
                   #+ then just take the first.
else
  NAME="John Doe"  #  Default, if no command-line parameter.
fi  

RESPONDENT="the author of this fine script"  
 

cat <<Endofmessage

Hello, there, $NAME.
Greetings to you, $NAME, from $RESPONDENT.

# This comment shows up in the output (why?).

Endofmessage

# Note that the blank lines show up in the output.
# So does the comment.

exit

This is a useful script containing a here document with parameter substitution.

Example 19-6. Upload a file pair to Sunsite incoming directory

#!/bin/bash
# upload.sh

#  Upload file pair (Filename.lsm, Filename.tar.gz)
#+ to incoming directory at Sunsite/UNC (ibiblio.org).
#  Filename.tar.gz is the tarball itself.
#  Filename.lsm is the descriptor file.
#  Sunsite requires "lsm" file, otherwise will bounce contributions.


E_ARGERROR=85

if [ -z "$1" ]
then
  echo "Usage: `basename $0` Filename-to-upload"
  exit $E_ARGERROR
fi  


Filename=`basename $1`           # Strips pathname out of file name.

Server="ibiblio.org"
Directory="/incoming/Linux"
#  These need not be hard-coded into script,
#+ but may instead be changed to command-line argument.

Password="your.e-mail.address"   # Change above to suit.

ftp -n $Server <<End-Of-Session
# -n option disables auto-logon

user anonymous "$Password"       #  If this doesn't work, then try:
                                 #  quote user anonymous "$Password"
binary
bell                             # Ring 'bell' after each file transfer.
cd $Directory
put "$Filename.lsm"
put "$Filename.tar.gz"
bye
End-Of-Session

exit 0

Quoting or escaping the "limit string" at the head of a here document disables parameter substitution within its body. The reason for this is that quoting/escaping the limit string effectively escapes the $, `, and \ special characters, and causes them to be interpreted literally. (Thank you, Allen Halsey, for pointing this out.)

Example 19-7. Parameter substitution turned off

#!/bin/bash
#  A 'cat' here-document, but with parameter substitution disabled.

NAME="John Doe"
RESPONDENT="the author of this fine script"  

cat <<'Endofmessage'

Hello, there, $NAME.
Greetings to you, $NAME, from $RESPONDENT.

Endofmessage

#   No parameter substitution when the "limit string" is quoted or escaped.
#   Either of the following at the head of the here document would have
#+  the same effect.
#   cat <<"Endofmessage"
#   cat <<\Endofmessage



#   And, likewise:

cat <<"SpecialCharTest"

Directory listing would follow
if limit string were not quoted.
`ls -l`

Arithmetic expansion would take place
if limit string were not quoted.
$((5 + 3))

A a single backslash would echo
if limit string were not quoted.
\\

SpecialCharTest


exit

Disabling parameter substitution permits outputting literal text. Generating scripts or even program code is one use for this.

Example 19-8. A script that generates another script

#!/bin/bash
# generate-script.sh
# Based on an idea by Albert Reiner.

OUTFILE=generated.sh         # Name of the file to generate.


# -----------------------------------------------------------
# 'Here document containing the body of the generated script.
(
cat <<'EOF'
#!/bin/bash

echo "This is a generated shell script."
#  Note that since we are inside a subshell,
#+ we can't access variables in the "outside" script.

echo "Generated file will be named: $OUTFILE"
#  Above line will not work as normally expected
#+ because parameter expansion has been disabled.
#  Instead, the result is literal output.

a=7
b=3

let "c = $a * $b"
echo "c = $c"

exit 0
EOF
) > $OUTFILE
# -----------------------------------------------------------

#  Quoting the 'limit string' prevents variable expansion
#+ within the body of the above 'here document.'
#  This permits outputting literal strings in the output file.

if [ -f "$OUTFILE" ]
then
  chmod 755 $OUTFILE
  # Make the generated file executable.
else
  echo "Problem in creating file: \"$OUTFILE\""
fi

#  This method can also be used for generating
#+ C programs, Perl programs, Python programs, Makefiles,
#+ and the like.

exit 0

It is possible to set a variable from the output of a here document. This is actually a devious form of command substitution.

variable=$(cat <<SETVAR
This variable
runs over multiple lines.
SETVAR)

echo "$variable"

A here document can supply input to a function in the same script.

Example 19-9. Here documents and functions

#!/bin/bash
# here-function.sh

GetPersonalData ()
{
  read firstname
  read lastname
  read address
  read city
  read state
  read zipcode
} # This certainly looks like an interactive function, but...


# Supply input to the above function.
GetPersonalData <<RECORD001
Bozo
Bozeman
2726 Nondescript Dr.
Baltimore
MD
21226
RECORD001


echo
echo "$firstname $lastname"
echo "$address"
echo "$city, $state $zipcode"
echo

exit 0

It is possible to use : as a dummy command accepting output from a here document. This, in effect, creates an "anonymous" here document.

Example 19-10. "Anonymous" Here Document

#!/bin/bash

: <<TESTVARIABLES
${HOSTNAME?}${USER?}${MAIL?}  # Print error message if one of the variables not set.
TESTVARIABLES

exit $?