Linux – cp -L vs. cp -H

command linecoreutilscplinuxoptions

Problem

When copying files with cp -H or cp -L, I get the same results:

$ ls -l fileA
  fileA -> fileB
$ cp fileA somewhere/ -H
$ ls -l somewhere/
  fileA     # fileA is a copy of fileB, only renamed, with same properties!

This answer here describes both options as similar UNLESS used in combination with -R. Not for me. Soft- as hardlinked files become renamed copies of the files they point to at the source.

Question:

What is the proper use of cp -H and cp -L? Is this the expected behavior?

My attempt to solve:
man cp tells me quite the same for both options, but info cp's wording makes it even more confusing for me. Maybe one can help me break this down a bit:

-H
If a command line argument specifies a symbolic link, then copy the file it points to rather than the symbolic link itself. However, copy (preserving its nature) any symbolic link that is encountered via recursive traversal.

This sounds like a contradiction to me: I guess that »a symbolic link's nature« is that it points somewhere…

-L, --dereference
Follow symbolic links when copying from them. With this option, cp cannot create a symbolic link. For example, a symlink (to regular file) in the source tree will be copied to a regular file in the destination tree.

I do know that a symlink isn't a regular file, but… I admit I'm overchallenged with this explanation here.

Best Answer

With symlinks, tools have two things they can do:

Treat the symlink as a symlink ("preserving its nature"), or
Treat the symlink as the type of file that it points to.

Saying that -H "preserves its nature" is not a contradiction. Consider the alternative. If you use -L, any symlinks cp finds will be opened, and their contents copied to the target file name. So the source was a symlink, but its copy is not a symlink. So it "lost its nature as a symlink".

Consider

$ mkdir subdir
$ echo "some contents" > subdir/file
$ ln -s file subdir/link

# definition of "list", the abbreviated ls -l output used below
$ list() { ls -l "$@" | \
    awk '$0 !~ /^total/ { printf "%s %s\t%s %s %s\n", $1, $5, $9, $10, $11 }' ; }

$ list subdir
-rw-rw-r-- 14   file  
lrwxrwxrwx 4    link -> file

$ cp -rH subdir subdir-with-H
$ list subdir-with-H
-rw-rw-r-- 14   file  
lrwxrwxrwx 4    link -> file

$ cp -rL subdir subdir-with-L
$ list subdir-with-L
-rw-rw-r-- 14   file  
-rw-rw-r-- 14   link

Related Solutions

Prevent Perl -i from Clobbering Symlinks – How to Guide

I wonder whether the small sponge general-purpose utility ("soak up standard input and write to a file") from moreutils will be helpful in this case and whether it will follow the symlink.

The author describes sponge like this:

It addresses the problem of editing files in-place with Unix tools, namely that if you just redirect output to the file you're trying to edit then the redirection takes effect (clobbering the contents of the file) before the first command in the pipeline gets round to reading from the file. Switches like sed -i and perl -i work around this, but not every command you might want to use in a pipeline has such an option, and you can't use that approach with multiple-command pipelines anyway.

I normally use sponge a bit like this:
sed '...' file | grep '...' | sponge file

Centos – How to copy a symbolic link

Use cp -P (capital P) to never traverse any symbolic link and copy the symbolic link instead.

This can be combined with other options such as -R to copy a directory hierarchy — cp -RL traverses all symbolic links to directories, cp -RP copies all symbolic links as such. cp -R might do one or the other depending on the unix variants; GNU cp (as found on CentOS) defaults to -P.

Even with -P, you can copy the target of a symbolic link to a directory on the command line by adding a / at the end: cp -RP foo/ bar copies the directory tree that foo points to.

GNU cp has a convenient -a option that combines -R, -P, -p and a little more. It makes an exact copy of the source (as far as possible), preserving the directory hierarchy, symbolic links, permissions, modification times and other metadata.

Question:

Best Answer

Related Solutions

Prevent Perl -i from Clobbering Symlinks – How to Guide

Centos – How to copy a symbolic link

Related Question