Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I try using tar to backup my files. With --exclude=/proc/, the archive contains files under /proc, and --exclude=/proc does not. What causes the difference?

P.S. The shell I use is Bash.

share|improve this question
up vote 2 down vote accepted

There is really no major difference between using a trailing slash or not in general for a directory, except in symlink handling. See Open Group Base Specifications, 4.11 Pathname Resolution (also see this answer at Unix SE).

As GNU tar is concerned: a trailing slash is simply not handled in --exclude patterns, as you've noticed. The difference is simply caused by its specific source code, if one should point to something :-) . See this question at Server Fault SE (though it does not give much detail - it merely states the fact). See this mail thread for some related information. A clue might be found in this passage (though I'm not entirely sure what to make of it):

Otto Moerbeek wrote:

This is the piece of code that is relevant:

             * Some programs that create ustar archives append a '/'
             * to the pathname for directories. This clearly violates
             * ustar specs, but we will silently strip it off anyway. 
            if (arcn-> name[arcn-> nlen - 1] == '/')
                    arcn-> name[--arcn-> nlen] = '\0';

As you can see with hexdump -C of archives created by gtar, gtar is such a program. I do not feel like changing tar just to accomodate non-conforming programs.

share|improve this answer

Usually, it's down to the convention of the program. With (GNU) tar it's a little inconsistent because a trailing / does not change included directories. The behaviour is partly explained here "patterns and names are used as-is". Excludes can match any part of a name (unless you use --anchor).

Documentation isn't clear that there is a difference, or why.

Internally, the reason a trailing / doesn't match in an exclude is that tar uses opendir(3), readdir(3) and fnmatch(3) -- opendir() accepts trailing / on a directory, readdir() doesn't put a slash on directory names, and fnmatch() simply matches patterns without regard to existing files or canonicalization. Specifically, fnmatch("proc/","proc",0) returns 1 (no match)..

Compare rsync which has very distinct (and well documented) distinctions for trailing / on directory names.

A related issue is that sometimes you want to backup a directory because it's a mount point, though you don't want its contents. The intent being your backup can contain all the required mount points (/dev proc /sys ) so usually you would want to do:

tar --exclude=/proc/* --exclude=/sys/* --exclude=/dev/* [...]
share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.