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what is the meaning of ~,/ and . in ./a.out and ~/.ddd
what is the use of ~/.ddd command

. is a link to "this directory"
~ is a shortcut to "my home directory"
/ means "inside this directory"

So ./a.out means the file "a.out" located inside ".", the current directory
And ~/.ddd means the file ".ddd" located inside "~", the user's home directory. You could also use $HOME/.ddd, or /home/rammoorthy/.ddd (assuming your user name is rammoorthy and your home directory is /home/raamoorthy), they're all equivalent.

The dot in front of ddd is part of the file name, it means it's a hidden file. "ls" will not show it, but "ls -a" will.

what is .ddd file...
what is backend process while creating .ddd file

.ddd is some file you or a program on your machine created. It's not a standard file as far as I'm aware, if that's what you're asking.

I don't know what "what is backend process while creating .ddd file" means.

edit: it looks like .ddd is a directory that holds some config information for the data display debugger program. If you run "ddd", then "~/.ddd" is the directory where it looks for config information and stores logs. I have zero experience with ddd, so I can't really help beyond that.

Read the LQ Rules, and don't post your verbatim homework questions here. Also, TELLING people to 'GIMME' things is fairly rude. When you're asking for handouts, rather than doing your own homework/research, at least try to be polite.