Is it possible to use "/" in a filename?

It depends on what filesystem you are using. Of some of the more popular ones:

• ext3: No
• ext4: No
• jfs: Yes
• reiserfs: No
• xfs: No

The answer is that you can't, unless your filesystem has a bug. Here's why:

There is a system call for renaming your file defined in fs/namei.c called renameat:

SYSCALL_DEFINE4(renameat, int, olddfd, const char __user *, oldname,
                int, newdfd, const char __user *, newname)

When the system call gets invoked, it does a path lookup (do_path_lookup) on the name. Keep tracing this, and we get to link_path_walk which has this:

static int link_path_walk(const char *name, struct nameidata *nd)
{
       struct path next;
       int err;
       unsigned int lookup_flags = nd->flags;

       while (*name=='/')
              name++;
       if (!*name)
              return 0;
...

This code applies to any file system. What's this mean? It means that if you try to pass a parameter with an actual '/' character as the name of the file using traditional means, it will not do what you want. There is no way to escape the character. If a filesystem "supports" this, it's because they either:

• Use a unicode character or something that looks like a slash but isn't.
• They have a bug.

Furthermore, if you did go in and edit the bytes to add a slash character into a file name, bad things would happen. That's because you could never refer to this file by name :( since anytime you did, Linux would assume you were referring to a nonexistent directory. Using the 'rm *' technique would not work either, since bash simply expands that to the filename. Even rm -rf wouldn't work, since a simple strace reveals how things go on under the hood (shortened):

$ ls testdir
myfile2 out
$ strace -vf rm -rf testdir
...
unlinkat(3, "myfile2", 0)               = 0
unlinkat(3, "out", 0)                   = 0
fcntl(3, F_GETFD)                       = 0x1 (flags FD_CLOEXEC)
close(3)                                = 0
unlinkat(AT_FDCWD, "testdir", AT_REMOVEDIR) = 0
...

Notice that these calls to unlinkat would fail because they need to refer to the files by name.

Only with an agreed-upon encoding. For example, you could agree that % will be encoded as %% and that %2F will mean a /. All the software that accessed this file would have to understand the encoding.

You could use a Unicode character that displays as / (for example the fraction slash), assuming your filesystem supports it.