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If while an application is running one of the shared libraries it uses is written to or truncated, then the application will crash. Moving the file or removing it wholesale with 'rm' will not cause a crash, because the OS (Solaris in this case but I assume this is true on Linux and other *nix as well) is smart enough to not delete the inode associated with the file while any process has it open.
I have a shell script that performs installation of shared libraries. Sometimes, it may be used to reinstall versions of shared libraries that were already installed, without an uninstall first. Because applications may be using the already installed shared libraries, it's important the the script is smart enough to rm the files or move them out of the way (e.g. to a 'deleted' folder that cron could empty at a time when we know no applications will be running) before installing the new ones so that they're not overwritten or truncated.
Unfortunately, recently an application crashed just after an install. Coincidence? It's difficult to tell. The real solution here is to switch over to a more robust installation method than an old gigantic shell script, but it'd be nice to have some extra protection until the switch is made. Is there any way to wrap a shell script to protect it from overwriting or truncating files (and ideally failing loudly), but still allowing them to be moved or rm'd?
Standard UNIX file permissions won't do the trick because you can't distinguish moving/removing from overwriting/truncating. Aliases could work but I'm not sure what entirety of commands need to be aliased. I imagine something like truss/strace except before each action it checks against a filter whether to actually do it. I don't need a perfect solution that would work even against an intentionally malicious script.
You can prevent a script from overwriting through I/O redirection by
set noclobber
Preventing overwriting by cp and the like is harder. My inclination would be to reset the PATH for the script to run with PATH containing just a single entry, a "blessed" directory where you place commands that you know are safe. This might mean, for example, that your version of cp is arranged always to use the --remove-destination option (probably a GNU-ism). In any case, you arrange for the script to execute only commands from the blessed directory. You can then vet each such command individually.
It would be good if you could prevent a script from executing a command by absolute pathname, but I don't know how to do that. If you are doing installations in your regular directories, a chroot jail probably does not help unless you do a lot of loopback mounting to make those directories visible. And if the directories into which you're installing contain dangerous commands, I don't see how you can protect yourself against them completely.
Incidentally, I tried and failed to learn if install(1) removes the desitination before installing. It would be interseting to learn.
Bash script I presume? Is the script very long? If not, you can do this manually:
if [ ! -f /tmp/foo.txt ] #If file does not exist
then
...code
fi
But I think you're asking for a way to wrap this around the script. You can certainly monitor the file writes with strace but AFAIK it doesn't have the functionality to interrupt them, unless you set up some sort of Intrusion Detection System with rules etc.
But to be honest, unless it's a huge script, that's probably more trouble than it's worth
Write your own safe_install() functions and make sure they're the only methods used. If you really need to be sure, run two processes. One would have permissions to make changes and the other would drop all privileges early and tell the other script to do the actual disk work.
My university has quite a lot files (like past papers etc) which I need to be on the uni network to access them. I ssh, using port forwarding, into the network and set my browser (Firefox) to go through that port. I know it would be quite easy to write something in most languages that would enter the command into a terminal, but which language/method could I use to make this happen? I expect some sort of scripting language could but I really know nothing about them, but I'd be willing to learn one just to have a little fun with this.
I'm using a cygwin terminal on Windows 7, if that will make a difference.
To neaten up my question: Which language would be best to acheive the above effect? Not necessarily the best, just possible and relatively simple to learn. I used the word best so I suppose I'll put a subjective tag.
EDIT:
Just in case anyway wants to do this precise thing, I'll show my solution. I ended up using a batch file because a few things made the bash more difficult. Anyway, in order to SSH into the network and open a new firefox window with the proxy changed, I just have to open this batch file and enter my password.
cd C:/cygwin/bin
ssh -fND 9001 username#your.website.here
cd "C:/Program Files/Mozilla Firefox"
firefox -no-remote -P SSH_profile
Save as .bat
In order for this to work you need to set up a profile with name SSH_profile (or whatever) and set the connection settings to go through port 9001. The script will open use the ssh command in /cygwin/bin and enter in the information. You enter your password and then it goes to your firefox directory, uses those arguments which opens a separate profile with the connection settings changed (means you can view unaccessible files in one window and anything else you don't want to be on your uni network in another. Just close the windows when you are done.
Quite a specific case but I thought I'd share my answer in the hope of helping someone, some day.
Since you are using cygwin, write a bash shell script.. If you don't want to rely on bash, powershell would be what you want, although you could probably get it working using a batch file.
This is exactly the kind of thing shell scripting is for.
Edit
You can change the firefox settings on the command line by passing the -override flag. This will let you have special settings for the invoked process.
I noticed when a file is executed on Windows (.exe or .dll), it is locked and cannot be deleted, moved or modified.
Linux, on the other hand, does not lock executing files and you can delete, move, or modify them.
Why does Windows lock when Linux does not? Is there an advantage to locking?
Linux has a reference-count mechanism, so you can delete the file while it is executing, and it will continue to exist as long as some process (Which previously opened it) has an open handle for it. The directory entry for the file is removed when you delete it, so it cannot be opened any more, but processes already using this file can still use it. Once all processes using this file terminate, the file is deleted automatically.
Windows does not have this capability, so it is forced to lock the file until all processes executing from it have finished.
I believe that the Linux behavior is preferable. There are probably some deep architectural reasons, but the prime (and simple) reason I find most compelling is that in Windows, you sometimes cannot delete a file, you have no idea why, and all you know is that some process is keeping it in use. In Linux it never happens.
As far as I know, linux does lock executables when they're running -- however, it locks the inode. This means that you can delete the "file" but the inode is still on the filesystem, untouched and all you really deleted is a link.
Unix programs use this way of thinking about the filesystem all the time, create a temporary file, open it, delete the name. Your file still exists but the name is freed up for others to use and no one else can see it.
Linux does lock the files. If you try to overwrite a file that's executing you will get "ETXTBUSY" (Text file busy). You can however remove the file, and the kernel will delete the file when the last reference to it is removed. (If the machine wasn't cleanly shutdown, these files are the cause of the "Deleted inode had zero d-time" messages when the filesystem is checked, they weren't fully deleted, because a running process had a reference to them, and now they are.)
This has some major advantages, you can upgrade a process thats running, by deleting the executable, replacing it, then restarting the process. Even init can be upgraded like this, replace the executable, and send it a signal, and it'll re-exec() itself, without requiring a reboot. (THis is normally done automatically by your package management system as part of it's upgrade)
Under windows, replacing a file that's in use appears to be a major hassle, generally requiring a reboot to make sure no processes are running.
There can be some problems, such as if you have an extremely large logfile, and you remove it, but forget to tell the process that was logging to that file to reopen the file, it'll hold the reference, and you'll wonder why your disk didn't suddenly get a lot more free space.
You can also use this trick under linux for temporary files. open the file, delete it, then continue to use the file. When your process exits (for no matter what reason -- even power failure), the file will be deleted.
Programs like lsof and fuser (or just poking around in /proc//fd) can show you what processes have files open that no longer have a name.
I think linux / unix doesn't use the same locking mechanics because they are built from the ground up as a multi-user system - which would expect the possibility of multiple users using the same file, maybe even for different purposes.
Is there an advantage to locking? Well, it could possibly reduce the amount of pointers that the OS would have to manage, but now a days the amount of savings is pretty negligible. The biggest advantage I can think of to locking is this: you save some user-viewable ambiguity. If user a is running a binary file, and user b deletes it, then the actual file has to stick around until user A's process completes. Yet, if User B or any other users look on the file system for it, they won't be able to find it - but it will continue to take up space. Not really a huge concern to me.
I think largely it's more of a question on backwards compatibility with window's file systems.
I think you're too absolute about Windows. Normally, it doesn't allocate swap space for the code part of an executable. Instead, it keeps a lock on the excutable & DLLs. If discarded code pages are needed again, they're simply reloaded. But with /SWAPRUN, these pages are kept in swap. This is used for executables on CD or network drives. Hence, windows doesn't need to lock these files.
For .NET, look at Shadow Copy.
If executed code in a file should be locked or not is a design decision and MS simply decided to lock, because it has clear advantages in practice: That way you don't need to know which code in which version is used by which application. This is a major problem with Linux default behaviour, which is simply ignored by most people. If system wide libs are replaced, you can't easily know which apps use code of such libs, most of the times the best you can get is that the package manager knows some users of those libs and restarts them. But that only works for general and well know things like maybe Postgres and its libs or such. The more interesting scenarios are if you develop your own application against some 3rd party libs and those get replaced, because most of the times the package manager simply doesn't know your app. And that's not only a problem of native C code or such, it can happen with almost everything: Just use httpd with mod_perl and some Perl libs installed using a package manager and let the package manager update those Perl libs because of any reason. It won't restart your httpd, simply because it doesn't know the dependencies. There are plenty of examples like this one, simply because any file can potentially contain code in use in memory by any runtime, think of Java, Python and all such things.
So there's a good reason to have the opinion that locking files by default may be a good choice. You don't need to agree with that reasons, though.
So what did MS do? They simply created an API which gives the calling application the chance to decide if files should be locked or not, but they decided that the default value of this API is to provide an exclusive lock to the first calling application. Have a look at the API around CreateFile and its dwShareMode argument. That is the reason why you might not be able to delete files in use by some application, it simply doesn't care about your use case, used the default values and therefore got an exclusive lock by Windows for a file.
Please don't believe in people telling you something about Windows doesn't use ref counting on HANDLEs or doesn't support Hardlinks or such, that is completely wrong. Almost every API using HANDLEs documents its behaviour regarding ref counting and you can easily read in almost any article about NTFS that it in deed does support Hardlinks and always did. Since Windows Vista it has support for Symlinks as well and the Support for Hardlinks has been improved by providing APIs to read all of those for a given file etc.
Additionally, you may simply want to have a look at the structures used to describe a file in e.g. Ext4 compared to those of NTFS, which have a lot in common. Both work with the concept of extents, which separates data from attributes like file name, and inodes are pretty much just another name for an older, but similar concept of that. Even Wikipedia lists both file systems in its article.
There's really a lot of FUD around file locking in Windows compared to other OSs on the net, just like about defragmentation. Some of this FUD can be ruled out by simply reading a bit on the Wikipedia.
NT variants have the
openfiles
command, which will show which processes have handles on which files. It does, however, require enabling the system global flag 'maintain objects list'
openfiles /local /?
tells you how to do this, and also that a performance penalty is incurred by doing so.
Executables are progressively mapped to memory when run. What that means is that portions of the executable are loaded as needed. If the file is swapped out prior to all sections being mapped, it could cause major instability.
UNIX file-locking is dead-easy: The operating system assumes that you know what you are doing and lets you do what you want:
For example, if you try to delete a file which another process has opened the operating system will usually let you do it. The original process still keeps it's file-handles until it terminates - at which point the the file-system will quietly re-cycle the disk-resources. No fuss, that's the way I like it.
How different things are on Windows: If I try to delete a file which another process is using I get an Operating-System error. The file is untouchable until the original process releases it's lock on the file. That was great back in the single-user days of MS-DOS when any locking process was likely to be on the same computer that contained the files, however on a network it's a nightmare:
Consider what happens when a process hangs while writing to a shared file on a Windows file-server. Before the file can be deleted we have to locate the computer and ID the process on that computer which originally opened the file. Only then can we kill the process and delete our unwanted file.
What a nuisance!
Is there a way to make this better? What I want is for file-locking on Windows to behave a like file-locking in UNIX. I want the operating system to just let me do what I want because I'm in charge and I know what I'm doing...
...so can it be done?
No. Windows is designed for the "average user", that is people who don't understand anything about a computer. Therefore, the OS tries to be smart to avoid PEBKACs. To quote Bill Gates: "There are no issues with Windows that any number of people want to be fixed." Of course, he knows that 99.9999% of all Windows users can't tell whether the program just did something odd because of them or the guy who wrote it.
Unix was designed when the world was more simple and anyone close enough to a computer to touch it, probably knew how to assemble it from dirty sand. Therefore, the OS usually lets you do what you want because it assumes that you know better (and if you didn't, you will next time).
Technical answer: Unix allocates an "i-nodes" if you create a file. I-nodes can be shared between processes. If two processes create the same file (that is, two processes call create() with the same path), then you end up with two i-nodes. This is by design. It allows for a fancy security feature: You can create files which no one can open but yourself:
Open a file
Delete it (but keep the file handle)
Use the file any way you like
Close the file
After step #2, the only process in the universe who can access the file is the one who created it (unless you want to read the hard disk block by block). The OS will keep the data alive until you either close the file or your process dies (at which time Unix will clean up after you).
This design is the foundation of all Unix filesystems. The Windows file system NTFS works much the same way but the high level API is different. Many applications open files in exclusive mode (which prevents anyone, even backup programs) to read the file. This is even true for applications which just display information like PDF viewers.
That means you'll have to fix all the Windows applications to achieve the desired effect. If you have access to the source, you can create a file in a shared mode. That would allow other processes to access it at the same time but then, you will have to check before every read/write if the file still exists, whether someone has made changes, etc.
According to MSDN you can specify to CreateFile() 3rd parameter (dwSharedMode) shared mode flag FILE_SHARE_DELETE which:
Enables subsequent open operations on a file or device to request delete access.
Otherwise, other processes cannot open the file or device if they request delete access.
If this flag is not specified, but the file or device has been opened for delete access, the function fails.
Note Delete access allows both delete and rename operations.
http://msdn.microsoft.com/en-us/library/aa363858(VS.85).aspx
So if you're can control your applications you can use this flag.
Note that Process Explorer allow for force closing of file handles (for processes local to the box on which you are running it) via Handle -> Close Handle.
Unlocker purports to do a lot more, and provides a helpful list of other tools.
Also deleting on reboot is an option (though this sounds like not what you want)
That doesn't really help if the hung process still has the handle open. It won't release the resources until that hung process releases the handle. But anyway, in Windows it is possible to force close a file out from under a process that's using it. Process Explorer from sysinternals.com will let you look at and close handles that a process has open.
When I defragment my XP machine I notice that there is a block of "Unmovable Files". Is there a file attribute I can use to make my own files unmovable?
Just to clarify, I want a way to programmatically tell Windows that a file that I create should be unmovable. Is this possible, and if so, how can I do it?
Thanks,
Terry
A lot of system files cannot be moved after the system boots, such as the page file and registry database files.
This utility runs before Windows boots to defragment those files. I have it set to run at every boot, and it works well for me on several machines.
Note that the very first time you boot up with this utility set to run, it may take several minutes to defrag. After that first run though, it finishes in just 3 or 4 seconds.
Edit0: To respond to your clarification- that link says windows has marked the page file and registry files as open for exclusive access. So you should be able to do the same thing with the LockFile API Call. However, that's not an attribute of the file itself. You'd have to actually run some background program that locks the file for exclusive access.
There are no file attributes that you can place on your files to mark them as immovable. The only way that a file cannot be moved (I think) during defragmentation is to have some other process have the file open (for read or write, I'm not even sure that you need to have the file open in exclusive mode or not).
Quite frankly, I cannot think of a reason that you'd want your files not to move, unless you have specific requirements about where on the disk platter your files reside. Defragmentation should generally lead to faster disk access and that seems to be desireable in all cases :-)
This usually means that the file is in use by some process. If you're defragmenting, you'll likely see this with a lot of system files. If the file should legitimately be movable and is stuck (it's being held by a process that runs at startup but shouldn't be, for example), the most useful way of resolving the problem is to remove all permissions on the file, reboot, restore the permissions, and then get rid of the file/run the program that's trying to use it.
I suppose the ugly way is to have an application boot on startup, check every few seconds if defrag is running and if so open the file in exclusive mode.
This is really ugly and I don't recommend it unless there is no cleaner solution.
Terry, the answers all mention ways to prevent files from becoming unmovable during defragmentation. From your question it appears that you are in fact wanting to make your personal files unmovable. Can you please clarify what is appealing about making your files unmovable.
I assume you're using the defragger that comes with Windows. Some commercial ones like DiskKeeper can move some of these files (usually system files). You can try their trial versions.
Contig might serve your purpose http://technet.microsoft.com/en-us/sysinternals/bb897428.aspx
I'm relatively certain I ran across some methods/attributes you could access programatically to do exactly what you want. This was back in NT4 days though and my memory isn't that good.
For a little more complete solution try Raxco's PerfectDisk. While it is a commercial product it does a very good job and supports boot time defrag of system files. The first defrag takes longer than say DiskKeeper but its a single pass defragger and supports defragging with very little free space left on the drive. Overall its a much smarter defrag program then any other I've seen and supports systems of any size.
http://www.raxco.com/
first try to move(or delete) the files within safe mode. If can not, try to move(or delete) the files with linux.
But be careful if those are the windows system files, then you are failed to boot up your windows.
Some reason why the files are unmovable are : the file size is too big, the files are being in open/in use condition, insufficient security privileges, being access by other computer/s, and many other things.