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8.18 Here Documents

So far, we've talked about three different kinds of quoting: backslashes (\), single quotes ('), and double quotes (<">). The shells support yet one more kind of quoting, called here documents. A here document is useful when you need to read something from standard input, but you don't want to create a file to provide that input; you want to put that input right into your shell script (or type it directly on the command line). To do so, use the << operator, followed by a special word:

sort >file <<EndOfSort
zygote
abacus
EndOfSort

This is very useful because variables (6.8, 6.1) are evaluated during this operation. Here is a way to transfer a file using anonymous ftp (52.7) from a shell script:


#!/bin/sh
# Usage:
#     ftpfile machine file
# set -x
SOURCE=$1
FILE=$2
GETHOST="uname -n"
BFILE=`basename $FILE`
ftp -n $SOURCE <<EndFTP
ascii
user anonymous $USER@`$GETHOST`
get $FILE /tmp/$BFILE
EndFTP

As you can see, variables and command substitutions (9.16) are done. If you don't want those to be done, put a backslash in front of the name of the word:

cat >file <<\FunkyStriNG

Notice the funky string. This is done because it is very unlikely that I will want to put that particular combination of characters in any file. You should be warned that the C shell expects the matching word (at the end of the list) to be escaped the same way, i.e., \FunkyStriNG, while the Bourne shell does not. See article 45.26.

[Most Bourne shells also have the <<- operator. The dash (-) at the end tells the shell to strip any TAB characters from the beginning of each line. Use this in shell scripts to indent a section of text without passing those TABs to the command's standard input. -JP]

- BB


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