Why doesn’t find . -delete delete current directory

directoryfindrm

I would expect

find . -delete

to delete the current directory, but it doesn't. Why not?

Best Answer

The members of findutils aware of it, it's for compatible with *BSD:

One of the reasons that we skip deletion of "." is for compatibility with *BSD, where this action originated.

The NEWS in findutils source code shows that they decided to keep the behavior:

#20802: If -delete fails, find's exit status will now be non-zero. However, find still skips trying to delete ".".

[UPDATE]

Since this question become one of the hot topic, so i dive into FreeBSD source code and come out a more convincing reason.

Let's see the find utility source code of FreeBSD:

int
f_delete(PLAN *plan __unused, FTSENT *entry)
{
    /* ignore these from fts */
    if (strcmp(entry->fts_accpath, ".") == 0 ||
        strcmp(entry->fts_accpath, "..") == 0)
        return 1;
...
    /* rmdir directories, unlink everything else */
    if (S_ISDIR(entry->fts_statp->st_mode)) {
        if (rmdir(entry->fts_accpath) < 0 && errno != ENOTEMPTY)
            warn("-delete: rmdir(%s)", entry->fts_path);
    } else {
        if (unlink(entry->fts_accpath) < 0)
            warn("-delete: unlink(%s)", entry->fts_path);
    }
...

As you can see, if it doesn't filter out dot and dot-dot, then it will reach rmdir() C function defined by POSIX's unistd.h.

Do a simple test, rmdir with dot/dot-dot argument will return -1:

printf("%d\n", rmdir(".."));

Let's take a look how POSIX describe rmdir:

If the path argument refers to a path whose final component is either dot or dot-dot, rmdir() shall fail.

No reason was given why shall fail.

I found rename explain some reason:

Renaming dot or dot-dot is prohibited in order to prevent cyclical file system paths.

Cyclical file system paths ?

I look over The C Programming Language (2nd Edition) and search for directory topic, surprisingly i found the code is similar:

if(strcmp(dp->name,".") == 0 || strcmp(dp->name,"..") == 0)
    continue;

And the comment !

Each directory always contains entries for itself, called ".", and its parent, ".."; these must be skipped, or the program will loop forever.

"loop forever", this is same like how rename describe it as "cyclical file system paths" above.

I slightly modify the code and to make it run in Kali Linux based on this answer:

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h> 
#include <dirent.h>
#include <unistd.h>

void fsize(char *);
void dirwalk(char *, void (*fcn)(char *));

int
main(int argc, char **argv) {
    if (argc == 1)
        fsize(".");
    else
        while (--argc > 0) {
            printf("start\n");
            fsize(*++argv);
        }
    return 0;
}

void fsize(char *name) {
    struct stat stbuf;
    if (stat(name, &stbuf) == -1 )  {
        fprintf(stderr, "fsize: can't access %s\n", name);
        return;
    }
    if ((stbuf.st_mode & S_IFMT) == S_IFDIR)
        dirwalk(name, fsize);
    printf("%81d %s\n", stbuf.st_size, name);
}

#define MAX_PATH 1024
void dirwalk(char *dir, void (*fcn)(char *))
{
    char name[MAX_PATH];
    struct dirent *dp;

    DIR *dfd;

    if ((dfd = opendir(dir)) == NULL) {
            fprintf(stderr, "dirwalk: can't open %s\n", dir);
            return;
    }

    while ((dp = readdir(dfd)) != NULL) {
            sleep(1);
            printf("d_name: S%sG\n", dp->d_name);
            if (strcmp(dp->d_name, ".") == 0
                            || strcmp(dp->d_name, "..") == 0) {
                    printf("hole dot\n");
                    continue;
                    }
            if (strlen(dir)+strlen(dp->d_name)+2 > sizeof(name)) {
                    printf("mocha\n");
                    fprintf(stderr, "dirwalk: name %s/%s too long\n",
                                    dir, dp->d_name);
                    }
            else {
                    printf("ice\n");
                    (*fcn)(dp->d_name);
            }
    }
    closedir(dfd);
}

Let's see:

xb@dnxb:/test/dot$ ls -la
total 8
drwxr-xr-x 2 xiaobai xiaobai 4096 Nov 20 04:14 .
drwxr-xr-x 3 xiaobai xiaobai 4096 Nov 20 04:14 ..
xb@dnxb:/test/dot$ 
xb@dnxb:/test/dot$ cc /tmp/kr/fsize.c -o /tmp/kr/a.out 
xb@dnxb:/test/dot$ /tmp/kr/a.out .                     
start
d_name: S..G
hole dot
d_name: S.G
hole dot
                                                                             4096 .
xb@dnxb:/test/dot$

It work correctly, now what if I comment out the continue instruction:

xb@dnxb:/test/dot$ cc /tmp/kr/fsize.c -o /tmp/kr/a.out 
xb@dnxb:/test/dot$ /tmp/kr/a.out .
start
d_name: S..G
hole dot
ice
d_name: S..G
hole dot
ice
d_name: S..G
hole dot
ice
^C
xb@dnxb:/test/dot$

As you can see, I have to use Ctrl+C to kill this infinitely loop program.

The '..' directory read its first entry '..' and loop forever.

Conclusion:

  1. GNU findutils try to compatible with find utility in *BSD.

  2. find utility in *BSD internally use rmdir POSIX-compliant C function which dot/dot-dot is not allow.

  3. The reason of rmdir do not allow dot/dot-dot is prevent cyclical file system paths.

  4. The C Programming Language written by K&R shows the example of how dot/dot-dot will lead to forever loop program.

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