Operating Systems Lab

Transcription

1 Operating Systems Lab Islamic University Gaza Engineering Faculty Department of Computer Engineering Fall 2012 ECOM 4010: Operating Systems Lab Eng: Ahmed M. Ayash Lab # 4 Paths, Links & File Permissions September 29, 2012

2 Part1: Paths A path, the general form of a filename or of a directory name, specifies a unique location in a file system. A path points to a file system location by following the directory tree hierarchy expressed in a string of characters in which path components, separated by a delimiting character, represent each directory. Systems can use either absolute or relative paths: A full path or absolute path is a path that points to the same location on one file system regardless of the working directory or combined paths. It is usually written in reference to a root directory. A relative path is a path relative to the working directory of the user or application, so the full absolute path will not have to be given. In other words absolute path is from home directory to user directory, but relative path is from present working directory to user directory. Example: 1. Let us make a directory lab4 and enter in it as follows: 2. To show present working directory write pwd as follows: pwd stand for (Print Working Directory) 3. Make two directories test1 and test2 in lab4 folder and enter test1 as follows: 4. Now I m in test1 and I want to go to test2 directly, so what can I do? First, you can use relative path as follows: 1

3 Second, you can use absolute path as follows: 5. Now you are in test2 folder, think how can you back to the root folder and enter lab4 folder using relative path in one step: Converting from relative path to absolute path: There are two ways to convert relative path to absolute path and they are: 1. readlink -f [relative_path] 2. realpath [relative_path] Example: Now you are in test1 folder and you want to enter test2 folder using absolute path, but you only know the relative path, so you need to convert this relative path to its full path: 1. Using readlink as follows: 2. Using realpath as follows: Note: If realpath command not install, you can install it by writing this: $ sudo apt-get install realpath 3. Using realpath function in a CPP code as follows: 2

4 realpath.cpp #include <stdio.h> #include <limits.h> /* PATH_MAX */ #include <stdlib.h> int main(int argc, char *argv[]) { char *symlinkpath = argv[1]; char actualpath [PATH_MAX]; char *ptr; ptr = realpath(symlinkpath, actualpath); printf("%s\n", ptr); } return 0; Part2: Links Windows users are used to be able to create shortcuts to have fast access to their files and folders which is especially useful if these are buried deep in their system this feature isn t as obvious on most Linux systems as it is in Windows. Shortcuts are made using: Symbolic link: To create a symbolic link in Linux we use this syntax: ln -s /path/to/original/ /path/to/linkname hard link You can create a hard link like so: ln /path/to/original.file /path/to/link.file 3

5 The Difference between Soft and Hard Links: Hard links Only link to a file not a directory Can t reference a file on a different disk/volume Links will reference a file even if it is moved Links reference physical locations on the disk Symbolic (soft) links Can link to directories Can reference a file/folder on a different hard disk/volume Links remain if the original file is deleted Links will NOT reference the file anymore if it is moved Links reference abstract filenames/directories and NOT physical locations. Example1: Make file Shortcut Put yourself in test1 folder and do the following steps: 1. Make a new folder and name it original, then enter in it as follows: 2. let us copy a file to our original folder to test with, for example I copy a file that is shown below: Note That: o i --inode: print index number of each file. o l: use a long listing format o a, --all: do not hide entries starting with. 4

6 3. Now create a symbolic link to host.conf called soft.conf as follows: Notice how the inode for the link is different. 4. Now create a hard link to the same file as follows: Notice how the inode numbers are exactly the same for the hard link and the actual file. 5. Now open up soft.conf and edit it and save it. then show the effect on host.conf: 6. Let s move host.conf now and see if it causes any problems as follows: 5

7 Looks like that our hard link still works even though we moved the original file. This is because the hard link was linked to the inode the physical reference on the hard drive where the file resides. The soft link (symbolic link) on the other hand was linked to the abstract file name and was broken the moment we moved the file. This leads to an interesting question. What happens if I delete a hard link? Even though the hard link is referenced to the physical location of the file on the hard drive though an inode, removing a hard link will not delete the original file. And removing the original file will not delete the hard link. To create a symlink without a terminal, just hold Shift+Ctrl and drag the file or folder you want to link to the location where you want the shortcut. This method may not work with all desktop managers. Or, select the file or folder by the mouse and then press right click and choose Make Link. Example2: Make directory Shortcut Hard link does not use to make directory Shortcut, so if you used it then the result will be as follows: So, use soft link to make directory Shortcut as follows: 6

8 Part3: File Permissions Although there are already a lot of good security features built into Linux-based systems, one very important potential vulnerability can exist when local access is granted - - that is file permission based issues resulting from a user not assigning the correct permissions to files and directories. So based upon the need for proper permissions, I will go over the ways to assign permissions and show you some examples where modification may be necessary. Linux have three user classes as follows: User (u): The owner of file. Group (g): Other user who are in group (to access files). Other (o): Everyone else. Permission Types Each file or directory has three basic permission types: read (r) : The Read permission refers to a user's capability to read the contents of the file. write (w) : The Write permissions refer to a user's capability to write or modify a file or directory. execute (x) : The Execute permission affects a user's capability to execute a file or view the contents of a directory. - : Replaces "r", "w" or "x" if according access type is denied You can view the access permissions of a file by doing the long directory listing with the ls -l command. This is what a long directory listing might look like: 7

9 File type and permissions represented by a 10 character string File Type User Permissions Group Permissions Other Permissions Read Write Execute Read Write Execute Read Write Execute d r w x r w x r w x File types: 1. - : regular file. 2. d : directory. 3. L : link. 4. b : block. 5. c: character. Type and # of Files's Permission field Links Owner Example: File's Group Size in Bytes Date of last modification Filename drwxrwxr-x 2 ayash-iug ayash-iug 4096 Sep 27 10:42 original -rwxr-xr-x dr-x File, owner has read, write, execute permissions, group: only read and execute permissions, others: only read and execute permissions. Directory, owner has read and execute access, group and others have no access chmod The chmod command is used to change the permissions of a file or directory. To use it, you specify the desired permission settings and the file or files that you wish to modify. There are two ways to specify the permissions: 1. The first way to change the permission is to: o add r, w or x: +r, +w, +x, +rwx and so on. Example: chmod o+w rp o delete r, w or x -r, -w, -x, -rwx and so on. Example: chmod o-w rp 8

10 Examples OR o chmod ug=rwx rp 2. The second way to change the permission is the Numeric Method: The numbers are a binary representation of the rwx string. r = 4 w = 2 x = 1 9

11 o The umask command is used to set and determine the default file creation permissions on the system. o Default permission for directories is 777 minus umask. o Default permission for files is the directory default without execute permission. o Non-privileged users' umask is 002. o Directories will have permissions of 775 o Files will have permissions of 664 o root's umask is 022. umask Examples owner: read and write permissions, group: only read permissions, others: only read permissions. owner: read, write and execute permissions, group: read and execute permissions, others: read and execute permissions. Example: 10

12 User Id You can see the UIDs and GIDs (group identifiers) of the system's users with the command: id You can also get id using getuid() method, let us test this CPP code : stat() and lstat() lstat is a system call that is used to determine information about a file based on its filename. 11

13 lstat is exactly the same as the stat system call. The only difference between the two is when the filename refers to a link. When this is the case, lstat returns information about the link itself, whereas stat returns information about the actual file. Functions declaration int stat(const char *filename, struct stat *buf); int lstat(const char *filename, struct stat *buf); Example stat.cpp Notice that soft.conf is a link, so the stat returns information about the actual file which is not exists. 12

14 Now change stat to lstat to see the difference, the output will be as follows: Notice that lstat returns information about the link itself regardless of the existence of the actual file. Project1 Guidelines: o The code should be fully commented and the comments should be meaningful. o The variables names should be meaningful. o Your code should not make any warning. o You should write your ID in the comment section at the beginning of the program. o The student solution should be named as follows: ex01_<student_id>.c 13