sed: display lines selected for deleting

Question

How to use verbose flag in sed? E.g. If I'm deleting some lines using sed command then I want them to get displayed on a screen whichever lines are getting deleted. Also let me know if this can be done through a script?

Answer 1

sed doesn't have a verbose flag.

You can write a sed script that separates deleted lines from other lines, though. You can look at the deleted lines later, and decide whether deleting them was a good idea.

Here's an example. I want to delete from test.dat every line that starts with a number.

$ cat test.dat
1 First line
2 Second line
3 Third line
A Keep this one

Here's the sed script that will "do" the deleting. It looks for lines that start with a number, writes them to the file "deleted.dat", and then deletes them from the pattern space.

$ cat code/sed/delete-verbose.sed
/^[0-9]/{
w /home/myusername/deleted.dat
d
}

Here's what happens when you run it.

$ sed -f code/sed/delete-verbose.sed test.dat
A Keep this one

And here's what it wrote to "deleted.dat".

$ cat deleted.dat
1 First line
2 Second line
3 Third line

When you're confident the script is going to do the right thing, redirect output to another file, or edit the file in-place (-i option).

Answer 2

This might work for you (GNU sed);

sed -e '/pattern_to_delete/{w /dev/stderr' -e ';d}' input_file > output_file

There is no verbose flag but by sending the lines to be deleted to stderr the effect you require can be achieved.