Is there a way to append files efficiently using the DOS copy command?

Using the DOS copy command syntax to concatenate files:

copy file1.txt+file2.txt all.txt

I know I can do this...

copy file1.txt+file2.txt file1.txt

Is this efficient? Is it doing what I'm expecting? It works, but I want to know is it actually appending to file1.txt or is it copying file1.txt (bad), concatenating file2 and then renaming to file1.txt (which is not efficient)?

Solution 1:

copy is copying file1.txt and file2.txt into memory, concatenating them then writing out to file1.txt. It's not copying to a new file then renaming that file so really there's not much extra disk I/O.

You can also use type.

type file2.txt >> file1.txt

The >> operator appends text. But that will, of course, not work for binary files.

Solution 2:

Is this efficient?

Sure. However, using the the /b switch can/may increase performance by simply concatenating the bytes instead of processing the files as text. This is particularly noticeable when concatenating very large text files.

Is it doing what I'm expecting?

Usually yes, but if the file was made in Linux, Mac, or other system with differing file-/line-terminators, then it may give unexpected results. It is a good idea to use the /b switch in general, even for text files.

I want to know is it actually appending to file1.txt or is it copying file1.txt (bad), concatenating file2 and then renaming to file1.txt (which is not efficient)?

Yes, it is creating a new, temporary file, deleting the original, and renaming the temp file to the original name, but deleting and renaming take no time and unless the original file is massive, you won’t normally notice even the (redundant) copying of the original file.