How to copy a folder recursively in an idempotent way using cp?

Question

When I use

cp -R inputFolder outputFolder

the result is context-dependent:

1. if outputFolder does not exist, it will be created, and the cloned folder path will be outputFolder.
2. if outputFolder exists, then the clone created will be outputFolder/inputFolder

This is horrible, because I want to create some installation script, and if the user runs it twice by mistake, he will have outputFolder created the first time, then on the second run all the stuff will be created once again in outputFolder/inputFolder.

• I want always the first behavior: create a clone next to the original (as a sibling).
• I want to use cp to be portable (e.g. MINGW does not have rsync shipped)
• I checked cp -R --parents but this recreates the path all the way up the directory tree (so the clone will not be outputFolder but some/path/outputFolder)
• --remove-destination or --update in case 2 do not change anything, still things are copied into outputFolder/inputFolder

Is there a way to do this without first checking for existence of the outputFolder (if folder does not exist then...) or using rm -rf outputFolder?

What is the agreed, portable UNIX way of doing that?

Answer 1

Use this instead:

cp -R inputFolder/. outputFolder

This works in exactly the same way that, say, cp -R aaa/bbb ccc works: if ccc doesn't exist then it's created as a copy of bbb and its contents; but if ccc already exists then ccc/bbb is created as the copy of bbb and its contents.

For almost any instance of bbb this gives the undesirable behaviour that you noted in your Question. However, in this specific situation the bbb is just ., so aaa/bbb is really aaa/., which in turn is really just aaa but by another name. So we have these two scenarios:

1. ccc does not exist:

   The command cp -R aaa/. ccc means "create ccc and copy the contents of aaa/. into ccc/., i.e. copy aaa into ccc.

2. ccc does exist:

   The command cp -R aaa/. ccc means "copy the contents of aaa/. into ccc/., i.e. copy aaa into ccc.

Answer 2

Don't copy the folder, only copy the contents:

## Create the target directory. The -p suppresses error messages
## if the directory already exists
mkdir -p outputFolder

## Copy the contents recursively, this will not recreate the parent
cp -R inputfolder/* outputfolder/

This way you both ensure that the target directory is created the first time the script runs and avoid the issue when running it a second time.

Chris Down very correctly points out that in bash, this will skip files whose name starts with a .. To avoid this, you can run shopt -s dotglob before running the command above.

Both -p for mkdir and -R for cp are defined by POSIX so this should be perfectly portable.

Answer 3

Try the -T option to cp. This exists in GNU coreutils cp version 8.22; it may not be portable outside that.

Answer 4

You could also use rsync:

rsync -uav inputFolder/ outputFolder/

(note the slashes, especially after the first one)

Answer 5

You can use -t option of cp command as:

cp -R inputFolder -t outputFolder

now if target folder does not exist it will throw an error:

cp: failed to access ‘outputFolder’: No such file or directory

note above command will copy inputFolder along with its content (and not just contents inside it)

if you want to copy just the contents of inputFolder it gets bit tricky (since you have to be careful while using shell globbing while using asterisk *)

cp -R  -t outputFolder/ -- inputFolder/*

now if target folder does not exist it will throw an error:

cp: failed to access ‘outputFolder’: No such file or directory

works with cp (GNU coreutils) 8.23

Answer 6

In my opinion, there is nothing simpler and more portable than rm -rf outputFolder. So, I always stick with that. I understand that your question is different, but I think that this is the best practice.