[Java] How to append something to each line in a text file ?

Kunsheng · 04-15-2009, 11:53 AM

Hello everyone,

I am thinking of appending something to each line in a text file with Java. I prefer not write a new file with content appended from the old one.

That 'something' would probably be Time Stamp when the file is created (which is same for each line).

I am not sure Java provide some easy way for it or not.

Appreciate any idea from you guys,

Thanks in advance,

-Kun

Robhogg · 04-15-2009, 12:19 PM

I don't think the standard Java library provides anything ready-built to do this (though I did a quick search and it looks like a few people have had a go at implementing sed in Java). In fact, sed -i does create a new temporary file, outputs the modifications to it, then overwrites the original. You can see this if you attempt to use it in a folder in which you do not have write permissions:

Code:

rob:secdir$ sed -i "s/$/ $date/" file
sed: couldn't open temporary file ./sedXlilKP: Permission denied

... In other words,

Code:

sed -i "s/$/ $date/" file

is equivalent to:

Code:

sed "s/$/ $date/" file > tmp_file && mv tmp_file file

You could, of course, read the contents of the file into an ArrayList, append the timestamp to each element of this, and then truncate the file and write the data back again (so long as it was not too large to hold efficiently in memory).

Kunsheng · 04-15-2009, 02:38 PM

Thanks, Rob! I think I will take this instead of Javaing.

Quote:

Originally Posted by Robhogg

I don't think the standard Java library provides anything ready-built to do this (though I did a quick search and it looks like a few people have had a go at implementing sed in Java). In fact, sed -i does create a new temporary file, outputs the modifications to it, then overwrites the original. You can see this if you attempt to use it in a folder in which you do not have write permissions:

Code:

rob:secdir$ sed -i "s/$/ $date/" file
sed: couldn't open temporary file ./sedXlilKP: Permission denied

... In other words,

Code:

sed -i "s/$/ $date/" file

is equivalent to:

Code:

sed "s/$/ $date/" file > tmp_file && mv tmp_file file

You could, of course, read the contents of the file into an ArrayList, append the timestamp to each element of this, and then truncate the file and write the data back again (so long as it was not too large to hold efficiently in memory).

jay73 · 04-15-2009, 09:08 PM

Well, most if not all of that can be done if you learn java regular expressions.