Information on heredoc

Shell Programming and Scripting
1

Hi ,

I am using ksh on Aix 7.1 and found the below code in a script which is a bit confusing to me, any help will be appreciated

<<comments
put ${pathforfiles}/${ftpfilename}
put ${logs}/${filename}.sch
put ${logs}/${filename}.tgr
comments

I have searched for information on heredoc but did not find any article that says what would happen if there is no command to which heredoc is passed

Thanks

2

Shell Command Language
This indicates that it writes to stdin by default.
Nested here documents example:

#!/bin/bash
$cat <<eofa; cat <<eofb
$>>test1
$>>test2
test1 
test2

$<<eofa; cat <<eofb
$>>test1
$>>test2

test2

In the second block the first instance does not have stdin defined as an output device, and so nothing is there to output or process the data "test1".

I think Don Cragun probably knows more.

3

This is cute...

There is a comment in the POSIX Standard's rationale for the simple commands section of the shell command language description that says:

But, I don't see anything in the standard that clearly specifies for what a here-document without a command is providing input. With the command exec a here-document does replace standard input for the current shell execution environment.

Playing around a little bit with the above example code in the POSIX rationale and the code supplied in post #1 in this thread, it appears to me that the here-document:

<<comments
put ${pathforfiles}/${ftpfilename}
put ${logs}/${filename}.sch
put ${logs}/${filename}.tgr
comments

is a shell language comment similar to the C language comment:

/*
put ${pathforfiles}/${ftpfilename}
put ${logs}/${filename}.sch
put ${logs}/${filename}.tgr
*/

except that any expansions occurring in that here-document may have side effects. For example:

count=0
IFS=:
while read a b
do	echo $a
	<<comment
	This comment counts lines read from the input file
	$((++count))
	comment
done </etc/passwd
echo "$count lines were read from /etc/passwd"

might produce abbreviated output similar to:

root
nobody
daemon
...
108 lines were read from /etc/passwd

If you wanted a multi-line shell comment without side effects (like the C example I showed earlier) you could quote one or more characters in the EOF string of the here-document:

count=0
IFS=:
while read a b
do	echo $a
	<<"comment"
	This comment counts lines read from the input file
	$((++count))
	comment
done </etc/passwd
echo "$count lines were read from /etc/passwd"

which would then produce abbreviated output similar to the following (with the last line being exactly like that shown below):

root
nobody
daemon
...
0 lines were read from /etc/passwd

Are we having fun yet? :cool:

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