The Unexpected Importance of the Trailing Slash
tookmund.com
tookmund.com
Sometimes people would complain that they didn't need a "beginner" class but I bet there wasn't a single person who didn't learn some important subtlety like how the trailing / works in different situations.
I've never had another company require or even offer a training like that. Do other companies still teach this stuff?
edit: typos
I will also note that I think skills like this end up being way more valuable to me as a mentor than hard CS knowledge.
It was a tech support job for mostly dial up customers. The manager had developed a binder of training materials that included many things to help us troubleshoot.
I once heard a story about some SunOS consultant who was used to "rm" being an alias for "rm -i", and the first thing this consultant did, as root, was to "cd /etc" and then "rm *".
This way there can be no confusion whether you're copying the contents of a directory or the whole directory, and you can't create a duplicated directory within your destination by mistake.
rsync has "weird" syntax for a reason. Unlike other unix-like commands, it treats trailing slashes as significant AND consistently. If a directory has a trailing slash, it means "contents" of the directory". No slash means "the directory itself". These are two different concepts, and a program that copies directories should take the difference into account. scp (and cp, for that matter) don't take this difference into account. That leads to gotchas with recursive (-r) copying. Most importantly, scp isn't idemopotent:
scp -r fromdir todir
If todir doesn't exist, scp will copy the contents of directory fromdir to a new directory named todir.
Execute the same command again (now that todir exists), and scp will copy fromdir to todir/fromdir .
On the other hand:
rsync -a fromdir/ todir
will always copy the contents of fromdir into a directory named todir (effectively, a directory rename operation), whether todir exists or not.
rsync -a fromdir todir
will always copy the directory fromdir into the directory todir, whether todir exists or not.
These rsync operations are idempotent, which it important because rsync is designed to incrementally re-sync directories. It is expected that it will commonly be run more than once, which is why it needed to address this IMHO fundamental bug/limitation in cp and scp.
I think it's `cp` that uses that syntax
I don't think having to create a target directory first is obvious, although neither is the fact that bare "cp" doesn't copy directories.
Finally, for remote copying it would be a nightmare for scp to require the destination directory to exist. To copy a directory tree to a remote server with a new name you'd need to either ssh a remote mkdir command, or add a flag to scp to specify the new name.
Because such cases are so rare, it does not make any difference for me if I have to add "/*" instead of the slightly shorter "/" (even when I also have to add "/.*" for hidden files/directories).
So I would like if rsync would have a command-line option to ignore trailing slashes, like other programs have (and which I always use with them).
As it is, rsync is the only program where I have to be extra cautious before executing a command-line, because the directory names are in most cases provided by auto-complete, and most shells terminate them with a trailing slash, which I have to be very careful to always delete, which slows me a lot.
An exception is when using zsh, which sometimes, but not always, deletes itself the trailing slash added by auto-complete (zsh might be confused by the aliases used by me, so it usually deletes the trailing slashes when they do not matter, but retains them when I would want them to be deleted).
I think this is the wrong way to think of it. It's more like "rsync foo/ bar/" means "make these two directories the same" - it does change the last-modified timestamp on bar/ to match foo/. If you use "foo/*" it excludes the directory and last-modified becomes now.
For all the combinations this is what's in my head, and I find it for the most part very consistent and predictable:
* rsync -a foo bar/ -> put this file in this directory (a directory is a special type of file, so you end up with bar/foo/)
* rsync -a foo/ bar/ -> put the contents of foo/ into bar/ (but also we're syncing the directories, which is why bar/'s last-modified is changed)
* rsync -a foo bar -> put this file in here, whatever "bar" is (makes a file named bar, unless foo is a directory - you can't put a directory inside a non-directory file, so it makes directory bar and puts foo inside it)
* rsync -a foo/ bar -> This is the only one I have issues with because it doesn't particularly make sense so I never have reason to even try using it - put a directory into a file? But it's the same as "foo/ bar/" case.
True, but instead of adding "/" to the source you obtain the same effect by deleting the last component of the destination (which is normally a non-action, you just do not auto-complete far enough).
If you meant that you might want to have "foo" and "bar" directories with the same content and timestamps, but with different names, you can rename the target directory before and after the rsync.
This is negligible overhead, because in many decades of using computers for all kinds of applications, I have not encountered a single case when I would have needed to have 2 directories with identical content and even with identical directory timestamps, but with different directory names.
Making a backup directory in the same directory with the original directory would not be a good example, that would be just stupid. Any backup directory should be placed as far as possible from the original directory, when not on a different filesystem, in which case they can have the same name.
The way how rsync handles names without trailing slashes is completely right.
The way how rsync handles trailing slashes to encode alternative behaviors might have been OK if this feature would not have been in conflict with the auto-complete feature of the shells.
For all the rsync behaviors that can be obtained by adding some "/", there are alternatives almost as simple as that.
A slight simplification in the encoding of the seldom used alternative behaviors is paid by a large complication in the typing of the frequently used behaviors, where you must always insert a backspace + space sequence after each directory name, to delete the dangerous trailing slash.
The problem is that the ergonomics of renaming a directory on a remote machine is vastly inferior to doing so on a local machine. Locally, you can use mv. There is no "rmv".
Even worse, rsync is from a different generation of unix utilities than cp/mv or rcp/rsh or scp/ssh. It is more "Swiss army knife" style, able to stand on its own. It shouldn't need to depend on a mythical rmv command.
Finally, I very frequently use the "copy a directory to a new name" feature, both locally and remotely. Locally, I use it to make a quick backup of a directory for emergency recovery. Remotely, syncing a local log directory to a remote "log.machine-name" is pretty common I think. Without renaming, you would do "log -> remote:machine-name", but machine-name would have to exist on the remote machine, necessitating a separate "rmkdir" command.
When you are worrying about idempotent re-sync of trees, I think it is so much more important to think about where the roots of the synchronization should match up between local and remote, and to make that explicit in the calling convention.
rsync -Ra /src-only/./{a,b,c} remote:/remote-only/./
I would have to think about how it fits in with the philosophy of the trailing slash sometime, but I don't want to hurt my head right now.
To make backups, you clone a directory into the rsync repository, and when you later want to update the backup, you pull the newest changes from the directory into the repository.
And it would be impossible to destroy the backup by forgetting a slash or syncing another directory on the wrong backup because it would remember which directory belongs to which backup repository.
Unfortunately, rsync lacks a helpful option like the "--strip-trailing-slashes" of GNU cp and mv.
I never use bare cp or mv, but only aliases which replace their undesirable default options with other more useful options, including "--strip-trailing-slashes", which makes the presence or absence of a trailing slash irrelevant.
This annoys people who know rsync well, but newbies are very happy with it!
Even though I know rsync extremely well I am super careful with this. Test first with `--dry-run`!
tl;dr: Basically, it's a way to say "do this operation inside this folder".
For example, if you do
cp -r A B
This is pretty terrible behavior. The question is how do you avoid this. On Linux, if you always want A to be overlaid on top of B, then you can do
cp -T A B
cp A B/.
Note: I have not checked whether this behavior is uniform across other POSIX systems, but I hope it is.
Reflink copies are basically invisible, except when you need to meter disk usage. Symlinks and hardlinks add a billion corner cases to file handling.