redirection
 

I want do this
cat abc.txt | tr -d '\n' > abc.txt
I want output in same file, How can I do?

you would want a temporary file, e.g. abc.temp which you then rename to abc.txt in a second command.

you want to remove all newlines you mean?

perl -i -pe 'tr/\n//d' file

no, his code is fine, but wants to put it back in the file he read it from. you could run it through ex nto acheive the same result i guess.

Right, so --
A bit crude, but it works.

I don't know anything about sed/awk but they could manage it without a temp file.