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Awk: Can anyone tell me why this doesn't work?

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Top Forums Shell Programming and Scripting Awk: Can anyone tell me why this doesn't work?
# 1  
Old 11-10-2008
Awk: Can anyone tell me why this doesn't work?
If there exists a field in stdin, print it, otherwise, print hello.....

These print nothing:

Code:
cat /dev/null | awk '{if ( length > 0 )  print $1; else print "hello"}'

cat /dev/null | awk '{if ( $1 ) print $1; else print "hello"}'

But the scripts work if I run them directly in a terminal:

Code:
root@home# awk '{if ( $1 ) print $1; else print "hello"}'

hello

hello
dsfs
dsfs
sdfz
sdfz

Please help I am going insane trying to work this out!!
# 2  
Old 11-10-2008
cat /dev/null gives an empty string without a newline just like echo -n "" so you won't get any output.
awk gets a line in the buffer, only if it ends with a newline.

Regards
# 3  
Old 11-10-2008
You beat me to it. The file MUST have a newline

and you dont need to cat

Code:
awk '{if ( $1 ) print $1;else print "hello"}'  /path/to/file

# 4  
Old 11-10-2008
That makes sense. So awk chokes when it receives an empty string. That is a huge drawback. Thanks for letting me know.

BTW, I was trying to replace the following code with a one-liner:

Code:
temp=$(grep string /tmp/file | awk '{print $1}')
if [[ -z $temp ]]
	then
		printf "\n"
	else
		printf "$temp\n"
fi

Cheers guys Smilie
# 5  
Old 11-10-2008
You should get the same output like this command:

Code:
awk '/string/{print $1}' /tmp/file

# 6  
Old 11-10-2008
This will work with text files, not /dev/null, as it is not a text file.

Code:
awk '{if ( length > 0 )  print $1; else print "hello"}' /path/to/testfile.txt

# 7  
Old 11-10-2008
Hi.
Code:
[[ -z $temp ]] && printf "\n" || printf "$temp\n"

but not as readable in my opinion. I generally don't like one-liners in production code (but they are sometimes interesting from a puzzle viewpoint) ... cheers, drl
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