I have a need for this hack every now and then, and I *always* forget it for some reason, so I’m putting it here for safe keeping. I’ll start a “hacks” category, too, so I can locate these quickly, and so you can too So, here’s the hack:

[jonesy@cranford testing]$ ls
bar  foo
[jonesy@cranford testing]$ ls foo
1  2  3
[jonesy@cranford testing]$ ls bar
1  2  3  4  5  6
[jonesy@cranford testing]$ yes n | cp -i bar/* foo 2>/dev/null
[jonesy@cranford testing]$ ls foo
1  2  3  4  5  6
[jonesy@cranford testing]$ ls bar
1  2  3  4  5  6

I’m piping a constant “no” to the “cp -i” command, which asks you if you want to overwrite (or not) any files that already exist in the target directory. You don’t have to send STDERR to /dev/null — there’s just some messy output if you don’t. The end result is that the copy command will only copy files into the destination directory if they don’t already exist. It’ll skip files that exist in both directories (Well, technically the *copy* command won’t, but the operation will when you do it this way).

Of course, I could just forcibly overwrite the directory contents, but I don’t know if the files that exist in both directories are identical. Or I could move one sideways and the other into place, but the same issue exists.

11 Responses to “Linux/Unix File Copy Trick”

1. Billings says:

   June 17, 2009 at 12:00 pm

   Why not use the -n switch to cp?
2. m0j0 says:

   June 17, 2009 at 12:06 pm

   I think you mean the ‘-u’ switch. There is no ‘-n’ switch. The reason I don’t use ‘-u’ is because I don’t want to worry about checking dates and ages or running touch or any of that nonsense — I *do not* want files that already exist in the destination directory to be overwritten regardless of their age. I just want files that aren’t already there added.
3. Billings says:

   June 17, 2009 at 8:28 pm

   I mean the -n switch, which both my BSD cp and GNU cp systems have. I suppose it’s not POSIX so it’s not portable.
4. jinks says:

   June 18, 2009 at 6:17 am

   from cp –help:
   -n, –no-clobber do not overwrite an existing file (overrides a previous -i option)

   this is the gnu version. If you are on Solaris, the gnu-tools should be available iirc. Try gcp, gnu-cp, …
5. m0j0 says:

   June 18, 2009 at 7:08 am

   Ok, well, redhat apparently doesn’t ship gnu cp, although the man page seems to indicate that it is:
   /> cp –version
   cp (GNU coreutils) 5.97

   If you log into a RH box with the coreutils package installed, you’ll find that the RH-supplied /bin/cp does not have a -n flag.
6. goozbach says:

   June 18, 2009 at 8:23 am

   try this:

   rsync -r –ignore-existing bar/. foo/.

   Here’s what happens when I do it on my box:

   dcarter@ord-mandell:/tmp/test$ ls foo/
   1 2 3
   dcarter@ord-mandell:/tmp/test$ cat foo/*
   foo
   foo
   foo
   dcarter@ord-mandell:/tmp/test$ ls bar/
   1 2 3 4 5 6
   dcarter@ord-mandell:/tmp/test$ cat bar/*
   bar
   bar
   bar
   bar
   bar
   bar
   dcarter@ord-mandell:/tmp/test$ rsync -r –ignore-existing bar/. foo/.
   dcarter@ord-mandell:/tmp/test$ ls bar/
   1 2 3 4 5 6
   dcarter@ord-mandell:/tmp/test$ cat bar/*
   bar
   bar
   bar
   bar
   bar
   bar
   dcarter@ord-mandell:/tmp/test$ ls foo/
   1 2 3 4 5 6
   dcarter@ord-mandell:/tmp/test$ cat foo/*
   foo
   foo
   foo
   bar
   bar
   bar
7. Saint Aardvark says:

   June 18, 2009 at 11:02 am

   I’m with goozbach; this is the sort of thing I usually invoke rsync for.
8. m0j0 says:

   June 18, 2009 at 11:04 am

   rsync is great, but if it’s not installed, that’s a problem, and it’s not a given that it’ll be installed wherever I find myself today. It’s also not usually the case that I’m given free reign to install anything. In addition, rsync’s flags have never cemented themselves in my brain very well. I have to use the man page to do even really trivial things with rsync. I know. I’m a dunce with rsync.
9. Oubliette says:

   July 16, 2009 at 1:51 pm

   I’m here because I want to do exactly this and I have already tried rsync. The source files are on a samba share and rsync can’t see them. Well, actually, it can see them, it builds the copy list fine, but when it goes to get them it hits a “file has vanished” error for every single file. I think this might be a unicode issue, the files are written to the samba share by iTunes from a Mac, but I can’t fiddle with the code page because the share is on a dumb NAS device. I’m left with cp and this trick to not overwrite files…

   … or -n now I know it exists. It wasn’t on any of the man pages I looked at on the net and I don’t have my Ubuntu box with me
10. Redbrick says:

    November 12, 2010 at 5:53 am

    Thanks for this tip, it’s an easy and quick way to sync to directories that don’t support rsync properly (e.g. certain SMB shares) and on a platform without the -n switch to cp (e.g. Debian). I sometimes like to add the -v switch to cp to make it list the files it’s copying.
11. dimangi says:

    November 23, 2010 at 1:48 pm

    I had also problems with rsync, but the cp hack worked just fine…