Chapter 17. Here Documents
A here document uses a special form of I/O redirection to feed a command script to an interactive program, such as ftp, telnet, or ex. Typically, the script consists of a command list to the program, delineated by a limit string. The special symbol << precedes the limit string. This has the effect of redirecting the output of a file into the program, 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.
Example 17-1. dummyfile: Creates a 2-line dummy file
#!/bin/bash # Non-interactive use of 'vi' to edit a file. # (Will not work with 'vim', for some reason.) # 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 Escape. 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.
Example 17-2. 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: Comment lines printed by 'wall'.
zzz23EndOfMessagezzz23
# Could have been done more efficiently by
# wall <message-file
# However, saving a message template in a script saves work.
exit 0
|
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 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 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 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 <<- .
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
|
Quoting or escaping the "limit string" at the head of a here document disables parameter substitution within its body. This has very limited usefulness.
Example 17-6. 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 |
This is a useful script containing a here document with parameter substitution.
Example 17-7. upload: Uploads a file pair to "Sunsite" incoming directory
#!/bin/bash # upload.sh # Upload file pair (Filename.lsm, Filename.tar.gz) # to incoming directory at Sunsite (metalab.unc.edu). E_ARGERROR=65 if [ -z "$1" ] then echo "Usage: `basename $0` filename" exit $E_ARGERROR fi Filename=`basename $1` # Strips pathname out of file name. Server="metalab.unc.edu" 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 |
It is possible to use : as a dummy command accepting output from a here document. This, in effect, creates an "anonymous" here document.
Example 17-8. "Anonymous" Here Document
#!/bin/bash
: <<TESTVARIABLES
${HOSTNAME?}${USER?}${MAIL?} # Print error message if one of the variables not set.
TESTVARIABLES
exit 0
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Here documents create temporary files, but these files are deleted after opening and are not accessible to any other process.
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Some utilities will not work in a here document. |
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.