/ Linux Reviews / Beginners: Learn Linux / Advanced Bash-Scripting Guide - en


Chapter 17. Here Documents

 

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 << 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.

Example 17-1. 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 0

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

Example 17-2. dummyfile: Creates a 2-line dummy file

#!/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.

Example 17-3. 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 17-4. 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 17-5. Here document with parameter substitution

#!/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 0

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

Example 17-6. 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=65

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"
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.

Example 17-7. 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

exit 0

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

Example 17-8. 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.

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 17-9. 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 17-10. "Anonymous" Here Document

#!/bin/bash

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

exit 0

Tip

A variation of the above technique permits "commenting out" blocks of code.

Example 17-11. Commenting out a block of code

#!/bin/bash
# commentblock.sh

: <<COMMENTBLOCK
echo "This line will not echo."
This is a comment line missing the "#" prefix.
This is another comment line missing the "#" prefix.

&*@!!++=
The above line will cause no error message,
because the Bash interpreter will ignore it.
COMMENTBLOCK

echo "Exit value of above \"COMMENTBLOCK\" is $?."   # 0
# No error shown.


#  The above technique also comes in useful for commenting out
#+ a block of working code for debugging purposes.
#  This saves having to put a "#" at the beginning of each line,
#+ then having to go back and delete each "#" later.

: <<DEBUGXXX
for file in *
do
 cat "$file"
done
DEBUGXXX

exit 0

Tip

Yet another twist of this nifty trick makes "self-documenting" scripts possible.

Example 17-12. A self-documenting script

#!/bin/bash
# self-document.sh: self-documenting script
# Modification of "colm.sh".

DOC_REQUEST=70

if [ "$1" = "-h"  -o "$1" = "--help" ]     # Request help.
then
  echo; echo "Usage: $0 [directory-name]"; echo
  sed --silent -e '/DOCUMENTATIONXX$/,/^DOCUMENTATIONXX$/p' "$0" |
  sed -e '/DOCUMENTATIONXX$/d'; exit $DOC_REQUEST; fi


: <<DOCUMENTATIONXX
List the statistics of a specified directory in tabular format.
---------------------------------------------------------------
The command line parameter gives the directory to be listed.
If no directory specified or directory specified cannot be read,
then list the current working directory.

DOCUMENTATIONXX

if [ -z "$1" -o ! -r "$1" ]
then
  directory=.
else
  directory="$1"
fi  

echo "Listing of "$directory":"; echo
(printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG-NAME\n" \
; ls -l "$directory" | sed 1d) | column -t

exit 0

See also Example A-27 for an excellent example of a self-documenting script.

Note

Here documents create temporary files, but these files are deleted after opening and are not accessible to any other process.

bash$ bash -c 'lsof -a -p $$ -d0' << EOF
> EOF
lsof    1213 bozo    0r   REG    3,5    0 30386 /tmp/t1213-0-sh (deleted)
	      

Caution

Some utilities will not work inside a here document.

Warning

The closing limit string, on the final line of a here document, must start in the first character position. There can be no leading whitespace. Trailing whitespace after the limit string likewise causes unexpected behavior. The whitespace prevents the limit string from being recognized.

#!/bin/bash

echo "----------------------------------------------------------------------"

cat <<LimitString
echo "This is line 1 of the message inside the here document."
echo "This is line 2 of the message inside the here document."
echo "This is the final line of the message inside the here document."
     LimitString
#^^^^Indented limit string. Error! This script will not behave as expected.

echo "----------------------------------------------------------------------"

#  These comments are outside the 'here document',
#+ and should not echo.

echo "Outside the here document."

exit 0

echo "This line had better not echo."  # Follows an 'exit' command.

For those tasks too complex for a "here document", consider using the expect scripting language, which is specifically tailored for feeding input into interactive programs.

17.1. Here Strings

A here string can be considered as a stripped-down form of here document. It consists of nothing more than COMMAND <<<$WORD, where $WORD is expanded and fed to the stdin of COMMAND.

Example 17-13. Prepending a line to a file

#!/bin/bash
# prepend.sh: Add text at beginning of file.
#
#  Example contributed by Kenny Stauffer,
#+ and slightly modified by document author.


E_NOSUCHFILE=65

read -p "File: " file   # -p arg to 'read' displays prompt.
if [ ! -e "$file" ]
then   # Bail out if no such file.
  echo "File $file not found."
  exit $E_NOSUCHFILE
fi

read -p "Title: " title
cat - $file <<<$title > $file.new

echo "Modified file is $file.new"

exit 0

# from 'man bash':
# Here Strings
# 	A variant of here documents, the format is:
# 
# 		<<<word
# 
# 	The word is expanded and supplied to the command on its standard input.

Exercise: Find other uses for here strings.


/ Linux Reviews / Beginners: Learn Linux / Advanced Bash-Scripting Guide


Meet new people

Adult Dating