Linux - Unix Concept
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1. In filename linkname creates another directory entry for filename called linkname i e linkname is a hard link Both directory entries appear identical and both now have a link count of 2 If either filename or linkname is modified the change will be reflected in the other file since they are in fact just two different directory entries pointing to the same file In s filename linkname creates a shortcut called linkname i e linkname is a soft link The shortcut appears as an entry with a special type T In s hello txt bye txt 4 Is l1 bye txt lrwxrwxrwx 1 alex finance 13 bye txt gt hello txt The link count of the source file remains unaffected Notice that the permission bits on a symbolic link are not used always appearing as rwxrwxrwx Instead the permissions on the link are determined by the permissions on the target hello txt in this case Note that you can create a symbolic link to a file that doesn t exist but not a hard link Another difference between the two is that you can create symbolic links across different physical disk devices or partitions but hard links are restricted to the same disk partition Finally most current UNIX implementations do not allow hard links to point to directories matches all three character filenames ell matches any five character filenames with ell in the middle he matches any filename beginning with he m z a l matches any filename that beg2. VFS defines a set of functions that every filesystem has to implement This interface is made up of a set of operations associated to three kinds of objects filesystems inodes and open files The VFS knows about filesystem types supported in the kernel It uses a table defined during the kernel configuration Each entry in this table describes a filesystem type it contains the name of the filesystem type and a pointer on a function called during the mount operation When a filesystem is to be mounted the appropriate mount function is called This function is responsible for reading the superblock from the disk initializing its internal variables and returning a mounted filesystem descriptor to the VFS After the filesystem is mounted the VFS functions can use this descriptor to access the physical filesystem routines A mounted filesystem descriptor contains several kinds of data informations that are common to every filesystem types pointers to functions provided by the physical filesystem kernel code and private data maintained by the physical filesystem code The function pointers contained in the filesystem descriptors allow the VFS to access the filesystem internal routines Two other types of descriptors are used by the VFS an inode descriptor and an open file descriptor Each descriptor contains informations related to files in use and a set of operations provided by the physical filesystem code While the inode descriptor contains pointer
3. an existing system a fixed number of inodes per partition is created This number will be the maximum amount of files of all types including directories special files links etc that can exist at the same time on the partition We typically count on having 1 inode per 2 to 8 kilobytes of storage At the time a new file is created it gets a free inode In that inode is the following information e Owner and group owner of the file e File type regular directory e Permissions on the file e Date and time of creation last read and change Date and time this information has been changed in the inode Number of links to this file File size An address defining the actual location of the file data The only information not included in an inode is the file name and directory These are stored in the special directory files By comparing file names and inode numbers the system can make up a tree structure that the user understands Users can display inode numbers using the i option to ls The inodes have their own separate space on the disk Basic File System Concepts Every Linux filesystem implements a basic set of common concepts derivated from the Unix operating system files are represented by inodes directories are simply files containing a list of entries and devices can be accessed by requesting I O on special files Inodes Each file is represented by a structure called an inode Each inode contains the description of the
4. are not even required they are only a convention The tree of the file system starts at the trunk or slash indicated by a forward slash This directory containing all underlying directories and files is also called the root directory or the root of the file system Directories that are only one level below the root directory are often preceded by a slash to indicate their position and prevent confusion with other directories that could have the same name When starting with a new system it is always a good idea to take a look in the root directory Let s see what you could run into emmy gt cd emmy gt Is bin dev home lib misc opt root tmp var boot etc initrd lost found mnt proc sbin ust Subdirectories of the root directory Directory Content bin Common programs shared by the system the system administrator and the users The startup files and the kernel vmlinuz In some recent distributions also grub data Grub is the boot GRand Unified Boot loader and is an attempt to get rid of the many different boot loaders we know today Contains references to all the CPU peripheral hardware which are represented as files with special dev i properties Directory Content Most important system configuration files are in etc this directory contains data similar to those in the ai Control Panel in Windows home Home directories of the common users initrd on some distributions Information for boot
5. essentially ext2 with added support for journaling Ext3 has a significant advantage over the other options described below It is backwards compatible Ext2 partitions can be converted to ext3 and vice versa without reformatting the partition An Ext3 partition can be mounted by an older kernel with no ext3 support it is just seen as a normal ext2 partition Ext3 partitions like ext2 allow files to be undeleted Init The kernel once it is loaded finds init in sbin and executes it When init starts it becomes the parent or grandparent of all of the processes that start up automatically on your Linux system The first thing init does is reading its initialization file etc inittab This instructs init to read an initial configuration script for the environment which sets the path starts swapping checks the file systems and so on Basically this step takes care of everything that your system needs to have done at system initialization setting the clock initializing serial ports and so forth Then init continues to read the etc inittab file which describes how the system should be set up in each run level and sets the default run level A run level is a configuration of processes All UNIX like systems can be run in different process configurations such as the single user mode which is referred to as run level 1 or run level S or s In this mode only the system administrator can connect to the system It is used to perform maintenance tas
6. for use by mainly one and the same person The selected software for installation reflects this and the stress is on common user packages such as nice desktop themes development tools client programs for E mail multimedia software web and other services Everything is put together on one large partition swap space twice the amount of RAM is added and your generic workstation is complete providing the largest amount of disk space possible for personal use but with the disadvantage of possible data integrity loss during problem situations On a server system data tends to be separate from user data Programs that offer services are kept in a different place than the data handled by this service Different partitions will be created on such systems e a partition with all data necessary to boot the machine a partition with configuration data and server programs one or more partitions containing the server data such as database tables user mails an ftp archive etc a partition with user programs and applications one or more partitions for the user specific files home directories e one or more swap partitions virtual memory Servers usually have more memory and thus more swap space Certain server processes such as databases may require more swap space than usual see the specific documentation for detailed information For better performance swap is often divided into different swap partitions Mount points All partitions are attached to the
7. having networking available There are times in which you may want to operate the system in a lower mode Examples are fixing disk corruption problems in run level 1 so no other users can possibly be on the system or leaving a server in run level 3 without an X session running In these cases running services that depend upon a higher system mode to function does not make sense because they will not work correctly anyway By already having each service assigned to start when its particular run level is reached you ensure an orderly start up process and you can quickly change the mode of the machine without worrying about which services to manually start or stop Available run levels are generally described in etc inittab which is partially shown below inittab This file describes how the INIT process should set up the system in a certain run level Default runlevel The runlevels ar 0 imate De NOW See tinsieclewawllic co ils 1 Single user mode Z WNLiciwsexe wWaliclacwie NES The same as 3 if you do not have networking 3 Pull imuliciwuser moce 4 unused 5 Xl 6 ielooer De WOW sete shnstiveleiceilhe ico iwlasS A simple description of the UNIX system also applicable to Linux is this On a UNIX system everything is a file if something is not a file it is a process This statement is true because there are special files that are more than just files named pipes and sockets for instance but
8. kernel decrements the links count and deallocates the inode if this count becomes zero This type of link is called a hard link and can only be used within a single filesystem it is impossible to create cross filesystem hard links Moreover hard links can only point on files a directory hard link cannot be created to prevent the apparition of a cycle in the directory tree Another kind of links exists in most Unix filesystems Symbolic links are simply files which contain a filename When the kernel encounters a symbolic link during a pathname to inode conversion it replaces the name of the link by its contents i e the name of the target file and restarts the pathname interpretation Since a symbolic link does not point to an inode it is possible to create cross filesystems symbolic links Symbolic links can point to any type of file even on nonexistent files Symbolic links are very useful because they don t have the limitations associated to hard links However they use some disk space allocated for their inode and their data blocks and cause an overhead in the pathname to inode conversion because the kernel has to restart the name interpretation when it encounters a symbolic link Q What is the minimum link count of a directory in Linux Unix Device special files In Unix like operating systems devices can be accessed via special files A device special file does not use any space on the filesystem It is only an access point to the dev
9. using the symbols u user g group o other a all r read w write x execute add permission take away permission and assign permission For example the command chmod ug rw o rw a x txt sets the permissions on all files ending in txt to rw rw i e the owner and users in the file s group can read and write to the file while the general public do not have any sort of access chmod also supports a R option which can be used to recursively modify file permissions e g chmod R go r play will grant group and other read rights to the directory play and all of the files and directories within play e chgrp change group chgrp group files can be used to change the group that a file or directory belongs to It also supports a R option Besides cat there are several other useful utilities for investigating the contents of files e file filename s file analyzes a file s contents for you and reports a high level description of what type of file it appears to be file myprog c letter txt webpage html myprog c C program text letter txt English text webpage html HTML document text file can identify a wide range of files but sometimes gets understandably confused e g when trying to automatically detect the difference between C and Java code e head tail filename head and tail display the first and last few lines in a file respectively You can specify the number of lines as an option e g
10. Operating Systems Unix Linux Unix Linux Unix Filesystem The UNIX operating system is built around the concept of a filesystem which is used to store all of the information that constitutes the long term state of the system This state includes the operating system kernel itself the executable files for the commands supported by the operating system configuration information temporary workfiles user data and various special files that are used to give controlled access to system hardware and operating system functions Every item stored in a UNIX filesystem belongs to one of four types Ordinary files Ordinary files can contain text data or program information Files cannot contain other files or directories Unlike other operating systems UNIX filenames are not broken into a name part and an extension part although extensions are still frequently used as a means to classify files Instead they can contain any keyboard character except for and be up to 256 characters long note however that characters such as and amp have special meaning in most shells and should not therefore be used in filenames Putting spaces in filenames also makes them difficult to manipulate rather use the underscore _ Directories Directories are containers or folders that hold files and other directories Devices To provide applications with easy access to hardware devices UNIX allows them to be used in much the same way as ordinary files Th
11. access control e Named pipes act more or less like sockets and form a way for processes to communicate with each other without using network socket semantics The l option to Is displays the file type using the first character of each input line jaime Documents gt Is 1 total 80 rw tw r 1 psdg faculty 31744 Feb 21 17 56 intro Linux doc rw tw r 1 psdg faculty 41472 Feb 21 17 56 Linux doc drwxrwxr x 2psdg faculty 4096 Feb 25 11 50 itenv This table gives an overview of the characters determining the file type Table File types in a long list Symbol Meaning Regular file d Directory 1 Link c Special file s Socket p Named pipe b Block device In order not to always have to perform a long listing for seeing the file type a lot of systems by default don t issue just Is but Is F which suffixes file names with one of the characters to indicate the file type As a user one only needs to deal directly with plain files executable files directories and links The special file types are there for making your system do what you demand from it and are dealt with by system administrators and programmers Now before we look at the important files and directories we need to know more about partitions About partitioning Why partition Most people have a vague knowledge of what partitions are since every operating system has the ability to create or remove them It may seem strange that Linux uses more than
12. append only Linux Swap The Linux swap partition is something you generally create once and then forget about This is an amount of disk space in which Linux temporarily writes data from RAM to free up memory for other processes The swap partition is different from all others in that it is not used to store files in so it won t be dealt with in any further detail here Windows and OS 2 partitions FAT partitions vfat FAT16 was the standard partition type up to Windows 95 and FAT32 partitions are the standard with Windows 98 FAT 16 partitions are supported by all versions of Windows while FAT32 is supported by Windows95 second edition onwards and Windows 2000 onwards but is not supported in Windows NT Linux supports both reading from and writing to FAT16 and FAT32 partitions Their main use in a Linux context is to share files between Linux and Windows on a dual boot system However note that these filesystems are unable to hold information such as file owners and permissions FAT 16 partitions are limited to a maximum of 2Gb Although the theoretical maximum size for FAT32 is 8 TB Windows98 s scandisk only supports 128Gb and Windows2000 does not permit the creation of FAT32 disks larger than 32Gb NTFS NTFS was introduced with Windows NT v4 0 and aimed at removing the limitations in the old FAT16 filesystems while adding new features not found in OS 2 s HPFS Under Linux this filesystem is currently supported in read only mode note that
13. at has the same contents as the source file If the destination is a directory many source files can be specified each of which will be copied into the destination directory To copy entire directories including their contents use a recursive copy cp rd source directories destination directory e mv move rename mv is used to rename files directories and or move them from one directory into another Exactly one source and one destination must be specified mv source destination If destination is an existing directory the new name for source whether it be a file or a directory will be destination source If source and destination are both files source is renamed destination N B if destination is an existing file it will be destroyed and overwritten by source you can use the i option if you would like to be asked for confirmation before a file is overwritten in this way e rm remove delete rm target file s removes the specified files Unlike other operating systems it is almost impossible to recover a deleted file unless you have a backup there is no recycle bin so use this command with care If you would like to be asked before files are deleted use the i option rm i myfile rm remove myfile rm can also be used to delete directories along with all of their contents including any subdirectories they contain To do this use the r option To avoid rm from asking any questions or giving errors e g if th
14. e file doesn t exist you used the f force option Extreme care needs to be taken when using this option consider what would happen if a system administrator was trying to delete user will s home directory and accidentally typed rm rf home will 4 instead of rm rf home will e cat catenate type cat target file s displays the contents of target file s on the screen one after the other You can also use it to create files from keyboard input as follows gt is the output redirection operator cat gt hello txt hello world 4 ctrl d Is hello txt hello txt cat hello txt 7 hello world e more and less catenate with pause more target file s displays the contents of target file s on the screen pausing at the end of each screenful and asking the user to press a key useful for long files It also incorporates a searching facility press and then type a phrase that you want to look for You can also use more to break up the output of commands that produce more than one screenful of output as follows lis the pipe operator Is 1 more 7 less is just like more except that has a few extra features such as allowing users to scroll backwards and forwards through the displayed file less not a standard utility however and may not be present on all UNIX systems Direct hard and indirect soft or symbolic links from one file or directory to another can be created using the In command
15. edundant Array of Inexpensive Disks solution Partition layout and types There are two kinds of major partitions on a Linux system e data partition normal Linux system data including the root partition containing all the data to start up and run the system and e swap partition expansion of the computer s physical memory extra memory on hard disk Most systems contain a root partition one or more data partitions and one or more swap partitions Systems in mixed environments may contain partitions for other system data such as a partition with a FAT or VFAT file system for MS Windows data Most Linux systems use fdisk at installation time to set the partition type This usually happens automatically In some cases one will need to select the partition type manually and even manually do the actual partitioning The standard Linux partitions have number 82 for swap and 83 for data which can be journaled ext3 or normal ext2 on older systems The fdisk utility has built in help should you forget these values Apart from these two Linux supports a variety of other file system types such as the relatively new Reiser file system JFS NFS FATxx and many other file systems natively available on other proprietary operating systems The standard root partition indicated with a single forward slash is about 100 500 MB and contains the system configuration files most basic commands and server programs system libraries some tempo
16. eleted without affecting the actual scripts that kill or start the services The symbolic links to the various scripts are numbered in a particular order so that they start in that order You can change the order in which the services start up or are killed by changing the name of the symbolic link that refers to the script that actually controls the service You can use the same number multiple times if you want a particular service started or stopped right before or after another service as in the example below listing the content of etc rc5 d where crond and xfs are both started from a linkname starting with S90 In this case the scripts are started in alphabetical order After init has progressed through the run levels to get to the default run level the etc inittab script forks a getty process for each virtual console login prompt in text mode getty opens tty lines sets their modes prints the login prompt gets the user s name and then initiates a login process for that user This allows users to authenticate themselves to the system and use it By default most systems offer 6 virtual consoles but as you can see from the inittab file this is configurable The idea behind operating different services at different run levels essentially revolves around the fact that different systems can be used in different ways Some services cannot be used until the system is in a particular state or mode such as being ready for more than one user or
17. elpful so some users prefer to the use the non standard info utility if it is installed info Is e cd change current working directory cd path changes your current working directory to path which can be an absolute or a relative path One of the most common relative paths to use is i e the parent directory of the current directory Used without any target directory cd resets your current working directory to your home directory useful if you get lost If you change into a directory and you subsequently want to return to your original directory use cd e mkdir make directory mkdir directory creates a subdirectory called directoryin the current working directory You can only create subdirectories in a directory if you have write permission on that directory e rmdir remove directory rmdir directory removes the subdirectory directory from the current working directory You can only remove subdirectories if they are completely empty i e of all entries besides the and directories e cp copy cp is used to make copies of files or entire directories To copy files use cp source file s destination where source file s and destination specify the source and destination of the copy respectively The behaviour of cp depends on whether the destination is a file or a directory If the destination is a file only one source file is allowed and cp makes a new file called destination th
18. ere are two types of devices in UNIX block oriented devices which transfer data in blocks e g hard disks and character oriented devices that transfer data on a byte by byte basis e g modems and dumb terminals Links A link is a pointer to another file There are two types of links a hard link to a file is indistinguishable from the file itself A soft link or symbolic link provides an indirect pointer or shortcut to a file A soft link is implemented as a directory file entry containing a pathname Typical Unix Directory Structure The UNIX filesystem is laid out as a hierarchical tree structure which is anchored at a special top level directory known as the root designated by a slash Because of the tree structure a directory can have many child directories but only one parent directory Figure below illustrates this layout A PR To specify a location in the directory hierarchy we must specify a path through the tree The path to a location can be defined by an absolute path from the root or as a relative path from the current working directory To specify a path each directory along the route from the source to the destination must be included in the path with each directory in the sequence being separated by a slash To help with the specification of relative paths UNIX provides the shorthand for the current directory and for the parent directory For example the absolute path to the direct
19. f your system If this is the case one will find that there is a boot partition holding your kernel s and accompanying data files The rest of the hard disk s is generally divided in data partitions although it may be that all of the non system critical data resides on one partition for example when you perform a standard workstation installation When non critical data is separated on different partitions it usually happens following a set pattern e apartition for user programs usr e a partition containing the users personal data home e qa partition to store temporary data like print and mail queues var e qa partition for third party and extra software opt Once the partitions are made you can only add more Changing sizes or properties of existing partitions is possible but not advisable The division of hard disks into partitions is determined by the system administrator On larger systems he or she may even spread one partition over several hard disks using the appropriate software Most distributions allow for standard setups optimized for workstations average users and for general server purposes but also accept customized partitions During the installation process you can define your own partition layout using either your distribution specific tool which is usually a straight forward graphical interface or fdisk a text based tool for creating partitions and setting their properties A workstation or client installation is
20. file file type access rights owners timestamps size pointers to data blocks The addresses of data blocks allocated to a file are stored in its inode When a user requests an I O operation on the file the kernel code converts the current offset to a block number uses this number as an index in the block addresses table and reads or writes the physical block This figure represents the structure of an inode Direct blocks Double indirect blocks Indirect blocks Directories Directories are structured in a hierarchical tree Each directory can contain files and subdirectories Directories are implemented as a special type of files Actually a directory is a file containing a list of entries Each entry contains an inode number and a file name When a process uses a pathname the kernel code searchs in the directories to find the corresponding inode number After the name has been converted to an inode number the inode is loaded into memory and is used by subsequent requests node table This figure represents a directory Links Unix filesystems implement the concept of link Several names can be associated with a inode The inode contains a field containing the number associated with the file Adding a link simply consists in creating a directory entry where the inode number points to the inode and in incrementing the links count in the inode When a link is deleted i e when one uses the rm command to remove a filename the
21. gt So alex play is another way for user psdg to specify an absolute path to the directory homes alex play User alex may refer to the directory as play e pwd print current working directory pwd displays the full absolute path to the current location in the filesystem So pwd usr bin implies that usr bin is the current working directory e Js list directory Is lists the contents of a directory If no target directory is given then the contents of the current working directory are displayed So if the current working directory is Is bin dev home mnt share usr var boot etc lib proc sbin tmp vol Actually ls doesn t show all the entries in a directory files and directories that begin with a dot are hidden this includes the directories and which are always present The reason for this is that files that begin with a usually contain important configuration information and should not be changed under normal circumstances If you want to see all files ls supports the a option Is a Even this listing is not that helpful there are no hints to properties such as the size type and ownership of files just their names To see more detailed information use the l option long listing which can be combined with the a option as follows Is a 14 or equivalently Is al 4 Each line of the output looks like this permissions owner type links size where e type is a single charac
22. he default filesystem for Linux It supports partitions of up to 4 Terabytes in size 1 Terabyte is 1024 Gigabytes while a single file can be up to 2 Gigabytes However many kernels only support block devices up to 2TB so the practical limit is 2TB Filenames can be up to 255 characters long In the case of a power outage or a similar situation in which the system is switched off without a proper shutdown the disks can be left in an unclean state where some data is only partly written or where data has been written without the directory entry being updated In that situation Linux performs a filesystem check The ext2 filesystem has a tried and tested filesystem checker e2fsck In order to ensure that everything is well the system also runs this program once every so many boot ups even when correctly shut down The ext2 filesystem also offers the possibility of undeleting files which were deleted by mistake This is unrelated to the Move to Trash method of deletion which is common with many graphical environments like KDE In the latter case the files are not really deleted but moved to a special folder from which they are deleted after a set limit is reached However with ext2 even when a file is actually deleted it is possible to restore it unless it has been overwritten The ext2 filesystem also allows secure deletion which ensures that a confidential file cannot be undeleted Special attributes allow files to be marked as unmodifiable or
23. ice driver Two types of special files exist character and block special files The former allows I O operations in character mode while the later requires data to be written in block mode via the buffer cache functions When an I O request is made on a special file it is forwarded to a pseudo device driver A special file is referenced by a major number which identifies the device type and a minor number which identifies the unit The Virtual File System Principle The Linux kernel contains a Virtual File System layer which is used during system calls acting on files The VFS is an indirection layer which handles the file oriented system calls and calls the necessary functions in the physical filesystem code to do the I O This indirection mechanism is frequently used in Unix like operating systems to ease the integration and the use of several filesystem types When a process issues a file oriented system call the kernel calls a function contained in the VFS This function handles the structure independent manipulations and redirects the call to a function contained in the physical filesystem code which is responsible for handling the structure dependent operations Filesystem code uses the buffer cache functions to request I O on devices This scheme is illustrated in this figure User process Syster call rap System calls interface Linux Kemel Buffer Cache IO request Dist controler Hardware The VES structure The
24. ing Do not remove lib Library files includes files for all kinds of programs needed by the system and the users lost found Every partition has a lost found in its upper directory Files that were saved during failures are here misc For miscellaneous purposes mnt Standard mount point for external file systems e g a CD ROM or a digital camera net Standard mount point for entire remote file systems opt Typically contains extra and third party software A virtual file system containing information about system resources More information about the proc meaning of the files in proc is obtained by entering the command man proc in a terminal window The file proc txt discusses the virtual file system in detail The administrative user s home directory Mind the difference between the root directory and root ioo the home directory of the root user sbin Programs for use by the system and the system administrator tmp Temporary space for use by the system cleaned upon reboot so don t use this for saving any work usr Programs libraries documentation etc for all user related programs Storage for all variable files and temporary files created by users such as log files the mail queue the var print spooler area space for temporary storage of files downloaded from the Internet or to keep an image of a CD before burning it How can you find out which partition a directory is on Using the df command with a dot as an
25. ins with a letter from m to z and ends in a letter from a to T usr bin lib file expands to usr bin file usr lib file bin file and lib file A PWN m Interpretation of permissions for files and directories Every file or directory on a UNIX system has three types of permissions describing what operations can be performed on it by various categories of users The permissions are read r write w and execute x and the three categories of users are user owner u group g and others 0 Because files and directories are different entities the interpretation of the permissions assigned to each differs slightly File and directory permissions can only be modified by their owners or by the superuser root by using the chmod system utility e chmod change file or directory mode chmod options files chmod accepts options in two forms Firstly permissions may be specified as a sequence of 3 octal digits octal is like decimal except that the digit range is 0 to 7 instead of 0 to 9 Each octal digit represents the access permissions for the user owner group and others respectively The mappings of permissions onto their corresponding octal digits is as follows 0 X 1 W 2 WX 3 r 4 r x 5 rw 6 rwx 7 For example the command chmod 600 private txt sets the permissions on private txt to rw i e only the owner can read and write to the file Permissions may be specified symbolically
26. ks without risks of damaging the system or user data Naturally in this configuration we don t need to offer user services so they will all be disabled Another run level is the reboot run level or run level 6 which shuts down all running services according to the appropriate procedures and then restarts the system Commonly run level 3 is configured to be text mode on a Linux machine and run level 5 initializes the graphical login and environment After having determined the default run level for your system init starts all of the background processes necessary for the system to run by looking in the appropriate rc directory for that run level init runs each of the kill scripts their file names start with a K with a stop parameter It then runs all of the start scripts their file names start with an S in the appropriate run level directory so that all services and applications are started correctly In fact you can execute these same scripts manually after the system is finished booting with a command like etc init d httpd stop or service httpd stop logged in as root in this case stopping the web server None of the scripts that actually start and stop the services are located in etc rc lt x gt d Rather all of the files in etc rc lt x gt d are symbolic links that point to the actual scripts located in etc init d A symbolic link is nothing more than a file that points to another file and is used in this case because it can be created and d
27. ng the shell does when it sees the gt operator is to create an empty file ready for the output One last point to note is that we can pass standard output to system utilities that require filenames as Introduction A filesystem is the method used to organise data on a disk It controls the allocation of disk space to files and associates each file with a filename directory permissions and other information Unlike most other operating systems Linux supports a wide variety of filesystems This is possible because in its infancy Linux got a virtual file system layer that allowed any filesystem to be plugged in Its own standard filesystem is ext2 which is a very well established stable and mature filesystem However Linux also allows you to access Windows FAT partitions NTFS partitions etc Now recent versions of Linux have added new journaling filesystems which offer a number of advantages over the traditional ext2 The choice of filesystem is an important one since it affects performance recovery from errors compatibility with other OS s limitations on partition and file sizes and so on With Linux distributions such as Mandrake or SuSE every one of the filesystems described are available on installation so one is spoilt for choice This article explains some of the differences between the different filesystem types to help you make the best choice for your Linux system Standard partitions ext2 The ext2 filesystem is t
28. nnel called standard error where processes write their error messages by default error messages are also sent to the screen To redirect standard output to a file instead of the screen we use the gt operator echo hello hello echo hello gt output 4 cat output hello In this case the contents of the file output will be destroyed if the file already exists If instead we want to append the output of the echo command to the file we can use the gt gt operator echo bye gt gt output cat output hello bye To capture standard error prefix the gt operator with a 2 in UNIX the file numbers 0 1 and 2 are assigned to standard input standard output and standard error respectively e g cat nonexistent 2 gt errors 4 cat errors 47 cat nonexistent No such file or directory You can redirect standard error and standard output to two different files find print 1 gt errors 2 gt files 7 or to the same file find print 1 gt output 2 gt output 4 or find print gt amp output Standard input can also be redirected using the lt operator so that input is read from a file instead of the keyboard cat lt output 4 hello bye You can combine input redirection with output redirection but be careful not to use the same filename in both places For example cat lt output gt output will destroy the contents of the file output This is because the first thi
29. not all Linux distributions enable this feature Read write access to this disk will probably be added in future NTFS partitions are not accessible from Windows 95 or 98 Journaled filesystems Introduction Whenever a computer is switched off without a proper shutdown there is the possibility that data on the disk becomes corrupted that is some of the data will have been written while some has not leaving files or even internal filesystem data in a half finished state Whenever that happens the system goes through a routine to check the disk for errors fsck in Linux and scandisk in Windows This is time consuming especially on today s very large disks This check is also forced once every so many boot ups to make sure everything is working properly Journaling filesystems get rid of these problems Instead of writing modified files directly onto their area on the disk the system maintains a journal on the disk which describes all the changes which must be made to disk Then a background process takes each journal entry makes the change and marks it as completed If the system is halted without a shutdown any pending changes are performed when it is restarted and the system is ready to continue running in seconds Incomplete entries in the journal are discarded This guarantees consistency and removes the need for a long and complex filesystem check on bootup ext3 Ext3 is the descendant of ext2 as its name implies In fact it is
30. one partition on the same disk even when using the standard installation procedure so some explanation is called for One of the goals of having different partitions is to achieve higher data security in case of disaster By dividing the hard disk in partitions data can be grouped and separated When an accident occurs only the data in the partition that got the hit will be damaged while the data on the other partitions will most likely survive This principle dates from the days when Linux didn t have journaled file systems and power failures might have lead to disaster The use of partitions remains for security and robustness reasons so a breach on one part of the system doesn t automatically mean that the whole computer is in danger This is currently the most important reason for partitioning A simple example a user creates a script a program or a web application that starts filling up the disk If the disk contains only one big partition the entire system will stop functioning if the disk is full If the user stores the data on a separate partition then only that data partition will be affected while the system partitions and possible other data partitions keep functioning Having a journaled file system only provides data security in case of power failure and sudden disconnection of storage devices This does not protect your data against bad blocks and logical errors in the file system In those cases you should use a RAID R
31. option shows the partition the current directory belongs to and informs about the amount of space used on this partition sandra lib gt df h Filesystem Size Used Avail Use Mounted on dev hda7 980M 163M 767M 18 As a general rule every directory under the root directory is on the root partition unless it has a separate entry in the full listing from df or df h with no other options The file system in reality For most users and for most common system administration tasks it is enough to accept that files and directories are ordered in a tree like structure The computer however doesn t understand a thing about trees or tree structures Every partition has its own file system By imagining all those file systems together we can form an idea of the tree structure of the entire system but it is not as simple as that In a file system a file is represented by an inode a kind of serial number containing information about the actual data that makes up the file to whom this file belongs and where is it located on the hard disk Every partition has its own set of inodes throughout a system with multiple partitions files with the same inode number can exist Each inode describes a data structure on the hard disk storing the properties of a file including the physical location of the file data When a hard disk is initialized to accept data storage usually during the initial system installation process or when adding extra disks to
32. ory play is home alex play while the relative path to this directory from psdg is alex play Note that these although these subdirectories appear as part of a seamless logical filesystem they do not need be present on the same hard disk device some may even be located on a remote machine and accessed across a network Q Compare and contrast the relative and absolute pathnames Directory Typical Contents The root directory Join Essential low level system utilities usr bin Higher level system utilities and application programs Superuser system utilities for performing system sbin oer administration tasks Program libraries collections of system calls that can be Alib included in programs by a compiler for low level system utilities fuse Vals Program libraries for higher level user programs tmp Temporary file storage space can be used by any user home or User home directories containing personal file space for each homes user Each directory is named after the login of the user etc UNIX system configuration and information files dev Hardware devices A pseudo filesystem which is used as an interface to the proc kernel Includes a sub directory for each active program or Processi Typical UNIX directories When one logs into UNIX the current working directory is the user home directory The home directory can be referred to as and the home directory of other users as lt login
33. rary space and the home directory of the administrative user A standard installation requires about 250 MB for the root partition Swap space indicated with swap is only accessible for the system itself and is hidden from view during normal operation Swap is the system that ensures like on normal UNIX systems that one can keep on working whatever happens The swap or virtual memory procedure has long been adopted by operating systems outside the UNIX world by now Using memory on a hard disk is naturally slower than using the real memory chips of a computer but having this little extra is a great comfort Linux generally counts on having twice the amount of physical memory in the form of swap space on the hard disk When installing a system you have to know how you are going to do this An example on a system with 512 MB of RAM 1st possibility one swap partition of 1 GB 2nd possibility two swap partitions of 512 MB 3rd possibility with two hard disks 1 partition of 512 MB on each disk The last option will give the best results when a lot of I O is to be expected Read the software documentation for specific guidelines Some applications such as databases might require more swap space Others such as some handheld systems might not have any swap at all by lack of a hard disk Swap space may also depend on your kernel version The kernel is on a separate partition as well in many distributions because it is the most important file o
34. running system information about the partitions and their mount points can be displayed using the df command which stands for disk full or disk free In Linux df is the GNU version and supports the h or human readable option which greatly improves readability Note that commercial UNIX machines commonly have their own versions of df and many other commands Their behavior is usually the same though GNU versions of common tools often have more and better features The df command only displays information about active non swap partitions These can include partitions from other networked systems like in the example below where the home directories are mounted from a file server on the network a situation often encountered in corporate environments psdg gt df h Filesystem Size Used Avail Use Mounted on dev hda8 496M 183M 288M 39 dev hdal 124M 8 4M 109M 8 boot dev hda5 19G 15G 2 7G 85 opt dev hda6 7 0G 5 4G 1 2G 81 usr dev hda7 3 7G 2 7G 867M 77 Ivar fs1 home 8 9G 3 7G 4 7G 44 automount fs 1 root home More file system layout Visual For convenience the Linux file system is usually thought of in a tree structure On a standard Linux system the following layout generally follows the scheme presented below This is a layout from a RedHat system Depending on the system admin the operating system and the mission of the UNIX machine the structure may vary and directories may be left out or added at will The names
35. s to functions that can be used to act on any file e g create unlink the file descriptors contains pointer to functions which can only act on open files e g read write The Second Extended File System Motivations The Second Extended File System has been designed and implemented to fix some problems present in the first Extended File System Our goal was to provide a powerful filesystem which implements Unix file semantics and offers advanced features Of course we wanted to Ext2fs to have excellent performance We also wanted to provide a very robust filesystem in order to reduce the risk of data loss in intensive use Last but not least Ext2fs had to include provision for extensions to allow users to benefit from new features without reformatting their filesystem System calls System calls using using VFS user file system X Disk I O calls interface interface Mapping function to file system X File Files on g system X storage file syste Linux VFS User process VFS system calls e In the VFS a file may be created read from written to or deleted For any file system a mapping module is needed to transform the characteristics of the real file system to the characteristics expected by the VFS e The figure above shows the key ingredients of the Linux file system strategy A user process issues a file system call e g OPEN using the VFS VFS then converts this to an internal kernel file sy
36. stem call that is passed to a mapping function for a specific file system e g NTFS e The target file system software is then invoked to perform the requested function on a file or a directory under its control and secondary storage e Results of the operation are then communicated back to the user in a similar way
37. system via a mount point The mount point defines the place of a particular data set in the file system Usually all partitions are connected through the root partition On this partition which is indicated with the slash directories are created These empty directories will be the starting point of the partitions that are attached to them An example given a partition that holds the following directories videos cd images pictures We want to attach this partition in the filesystem in a directory called opt media In order to do this the system administrator has to make sure that the directory opt media exists on the system Preferably it should be an empty directory Then using the mount command the administrator can attach the partition to the system The formerly empty directory opt media it will contain the files and directories that are on the mounted medium hard disk or partition of a hard disk CD DVD flash card USB or other storage device During system startup all the partitions are thus mounted as described in the file etc fstab Some partitions are not mounted by default for instance if they are not constantly connected to the system such like the storage used by your digital camera If well configured the device will be mounted as soon as the system notices that it is connected or it can be user mountable i e you don t need to be system administrator to attach and detach the device to and from the system On a
38. tail 20 messages txt 47 head 5 messages txt tail includes a useful f option that can be used to continuously monitor the last few lines of a possibly changing file This can be used to monitor log files for example tail f var log messages continuously outputs the latest additions to the system log file Linux Virtual File System What is its utility Linux can be run as a process under Windows etc e Versatile and powerful file handling facility designed to support a wide variety of file management systems and file structures VES presents a single uniform file system interface to user processes e It defines a common file model that is capable of representing any conceivable file system s general feature and behavior e Assumes files are objects in a computer s mass storage that share basic properties regardless of the target file system or the underlying processor hardware e Files have symbolic names that allow them to be uniquely identified within a specific directory in the file system e A file has several properties including an owner protection against unauthorized access or modification The output from programs is usually written to the screen while their input usually comes from the keyboard if no file arguments are given In technical terms we say that processes usually write to standard output the screen and take their input from standard input the keyboard There is in fact another output cha
39. ter which is either d directory ordinary file l symbolic link b block oriented device or c character oriented device e permissions is a set of characters describing access rights There are 9 permission characters describing 3 access types given to 3 user categories The three access types are read r write w and execute x and the three users categories are the user who owns the file users in the group that the file belongs to and other users the general public An r w or x character means the corresponding permission is present a means it is absent links refers to the number of filesystem links pointing to the file directory owner is usually the user who created the file or directory e group denotes a collection of users who are allowed to access the file according to the group access rights specified in the permissions field e size is the length of a file or the number of bytes used by the operating system to store the list of files in a directory e date is the date when the file or directory was last modified written to The u option display the time when the file was last accessed read e name is the name of the file or directory ls supports more options To find out what they are type man Is man is the online UNIX user manual and you can use it to get help with commands and find out about what options are supported It has quite a terse style which is often not that h
40. to keep things simple saying that everything is a file is an acceptable generalization A Linux system just like UNIX makes no difference between a file and a directory since a directory is just a file containing names of other files Programs services texts images and so forth are all files Input and output devices and generally all devices are considered to be files according to the system In order to manage all those files in an orderly fashion man likes to think of them in an ordered tree like structure on the hard disk as we know from MS DOS Disk Operating System for instance The large branches contain more branches and the branches at the end contain the tree s leaves or normal files Q Is the directory hierarchy a tree or a network 3 1 1 2 Sorts of files Most files are just files called regular files they contain normal data for example text files executable files or programs input for or output from a program and so on While it is reasonably safe to suppose that everything you encounter on a Linux system is a file there are some exceptions e Directories files that are lists of other files e Special files the mechanism used for input and output Most special files are in dev e Links a system to make a file or directory visible in multiple parts of the system s file tree e Domain sockets a special file type similar to TCP IP sockets providing inter process networking protected by the file system s
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