what happens when the file in which grep is searching for pattern is blank?

ameylimaye · 09-30-2011, 12:44 AM

what happens when the file in which grep is searching for pattern is blank?

how do i tackle this problem?...i want to print "--" instead of skipping a line.

Nylex · 09-30-2011, 12:53 AM

I don't think grep can distinguish between an empty file and the pattern not being found. If you need to check for an empty file, in Bash, you can do so with -s, which returns true if the file is not empty:

Code:

myfile=/home/me/foo
if [ ! -s $myfile ] 
then 
  echo "$myfile is empty"
fi

See this for more on file tests in Bash.

David the H. · 09-30-2011, 12:55 AM

I don't understand your question. Could you explain in more detail?

Show us exactly what you're doing, with examples of the file(s), the commands you're using, the output you're getting, and what kind of output you want to get.

ameylimaye · 09-30-2011, 01:53 AM

Hi,
In my script I have to print a line in file using grep and then again grab data from the line.
But if the first grep does not find the matching pattern..it will not print anything in that file...so I want to read an empty file...and then instead of grabbing nothing from the empty file..i want to put "--" indicating the file was empty.

If this description helps!

David the H. · 10-01-2011, 02:10 AM

I think I understand the basic point, but it would help a lot more if we could see the actual code you're using, rather than just a vague description of what it does. The best solutions to use in cases like this often depend on the exact context of the problem.

As Nylex pointed out, at the very least you can store the grep results in a variable, and test that in order to determine what to print. You can also often use shell built-in features extract substrings if necessary.

Code:

line="$( grep "string" inputfile )"

if [[ "$line" ]]; then
	<extract and print substring>
else
	echo "--"
fi

But there may be even better options, depending on just what you need to do.