A here document uses a special form
of I/O redirection (see Section 3.13)
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 3-93. dummyfile: Creates a 2-line dummy file
#!/bin/bash
# Non-interactive use of 'vi' to edit a file.
# Emulates 'sed'.
if [ -z $1 ]
then
echo "Usage: `basename $0` filename"
exit 1
fi
TARGETFILE=$1
vi $TARGETFILE <<x23LimitStringx23
i
This is line 1 of the example file.
This is line 2 of the example file.
^[
ZZ
x23LimitStringx23
# 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 3-94. broadcast: Sends message to everyone logged in
#!/bin/bash
wall <<zzz23EndOfMessagezzz23
Dees ees a message frrom Central Headquarters:
Do not keel moose!
# Other message text goes here.
# Note: Comment lines printed by 'wall'.
zzz23EndOfMessagezzz23
# Could have been done more efficiently by
# wall <message-file
exit 0 |
Example 3-95. 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 |
Example 3-96. upload: Uploads a file pair to "Sunsite"
incoming directory
#!/bin/bash
# upload
# upload file pair (filename.lsm, filename.tar.gz)
# to incoming directory at Sunsite
if [ -z $1 ]
then
echo "Usage: `basename $0` filename"
exit 1
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,
# 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 |
Note: Some utilities will not work in a
"here document". The pagers, more and
less are among these.
For those tasks too complex for a "here document",
consider using the expect scripting language, which
is specifically tailored for feeding input into non-interactive programs.
