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 (Section 35.9, Section 35.3) are evaluated during this operation. Here is a way to transfer a file using anonymous ftp (Section 1.21)[3] from a shell script:
Go to http://examples.oreilly.com/upt3 for more information on:
ftpfile
#!/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, variable and command substitutions (Section 28.14) 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 Section
36.19.
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.
Other shells, notably zsh and later versions of ksh, but in the future possibly also bash, support a method for taking input from a string:
$ tr ... <<< "$xyzzy" | ...
— BB
[3] You might be better off using wget or curl for downloads, but this method can be useful for automated uploads.