CS 497C – Introduction to UNIX Lecture 16: - File Attributes

1. CS 497C – Introduction to UNIXLecture 16: - File Attributes Chin-Chih Changchang@cs.twsu.edu

2. File Modification and Access Times • A UNIX file has three time stamps associated with it. • Time of last file modification • Time of last access • Time of last inode modification • When a file’s contents are changed, its last modification time is updated by the kernel. • The –t option of ls present files in order of their modification time instead of the ASCII collating sequence.

3. File Modification and Access Times • The –u option of ls displays a file’s access time. • If you view the contents of the file with the cat command, you’ll update its access time but not the modification time. • The –r (reverse) option of ls lists files in a reverse order. • If a file is incorrectly stamped when extracting it, you can use touch to rest the times.

4. Touch: Changing the Time Stamps • The touch command changes these times and is used in the following manner: touch options expression filename(s) • When touch is used without options or an expression, both times are set to the current time. • When touch is used without options but with an expression, it changes both time.

5. Touch: Changing the Time Stamps • The expression consists of an eight-digit number – using the format MMDDhhmm (month, day, hour, and minute). • Here is an example: $ ls -l chap3 -rw-r--r-- 1 cs497c grader 306 Sep 26 11:55 chap3 $ ls -lu chap3 -rw-r--r-- 1 cs497c grader 306 Sep 27 23:39 chap3 • The first one shows the modification time. The second one shows the access time.

6. Touch: Changing the Time Stamps • touch changes both modification and access time: $ date Sun Sep 30 20:57:56 CDT 2001 $ touch 09302057 chap3; ls -l chap3 -rw-r--r-- 1 cs497c grader 306 Sep 30 20:57 chap3 $ ls -lu chap3 -rw-r--r-- 1 cs497c grader 306 Sep 30 20:57 chap3 • With the –m (modification) option, you can alter the modification time alone:

7. Touch: Changing the Time Stamps • With the –a (access) option, you can alter the modification time alone. • The system administrator often uses touch to touch up these times so that a file may be included in or excluded from an incremental backup.

8. File Systems and Inodes • All the attributes that we have discussed are stored in a table called the inode. • Every file has one inode and is accessed by a number called the inode number. • The inode number for a file is unique in a single file system. • Every file system has a separate area earmarked for holding inodes. ls fetches the attributes of files from their inodes.

9. ln: Links • UNIX allows a file to have more than one name and yet maintain a single copy in the disk. This file is then said to have more than one link. • A file can have as many names as you want to give it, but they all have the same inode number.

10. ln: Links • A file is linked with the ln (link) command which takes two filenames as arguments. The first one is the existing file. The second one is the alias we provide. • The following command links the existing file display.sh with print.sh: ln display.sh print.sh • The –i option of ls displays the inode number of a file.

11. ln: Links • When we use rm to remove files, rm actually removes a link from the file. It bring the link count down to one. • These are three applications when we need to link a file: • Use a link to “notionally place” a file in a specific directory where many programs expect to find it.

12. ln: Links • C and the shell programming language support a feature which lets a program know the name by which it is called. • Link useful files in a common directory. • The ordinary links have two serious drawbacks: • You can’t link a file across two file systems. • You can’t line a directory even within the same file system.

13. Symbolic Links • A symbolic link overcomes both these problems. • Unlike the other link, a symbolic link is a directory entry that points to the file or directory that actually has the contents. • A symbolic link is also known as a soft link. The ordinary link is called a hard link.

14. Symbolic Links • You can identify the symbolic links by the ls –l command. • Use the –i (node) option of ls shows the inode number of the symbolic link. $ ln -s chap3 chap4 $ ls -li chap3 chap4 612393 -rw-r--r-- 1 cs497c grader 306 Sep 30 20:57 chap3 612388 lrwxrwxrwx 1 cs497c grader 5 Sep 30 22:14 chap4 -> chap3 • Use the –F (classiFy) option of ls shows the symbolic link with a trailing @ symbol.

15. find: Locating Files • find is one the power tools of the system. It recursively examines a directory tree to look for files either by name or by matching one or more file attributes. • find can be broken up into three components: find path_list selection_criteria action • Let’s check the following example: find /home –name index.html –print

16. find: Locating Files • It recursively examines all files in the /home directory, selects the file with the name index.html, and takes the action –print to display the found files. find . –name “*.lst” –print find . –name ‘[A-Z]*’ -print • The –type operator followed by the letter f, d, or l selects files of the ordinary, directory, and symbolic link type.

17. find: Locating Files $ find /home/henry –type d –print • You can also use the –perm operator to locate files having a specified set of permission. find . –perm 666 -print • If a file is symbolic link pointing to a directory, then find by default won’t into that directory. We can use –follow operator, find locates the file.

18. find: Locating Files $ find . –name night.gif –follow -print • You can use the –mtime operator to match a file’s modification time. (-atime matches the access time.) • If you want to check the files that have been modified in less than two days, use: $ find . –mtime –2 print

19. find: Locating Files • find uses the –ls action component to display a special listing of those files that were found. • find uses the operators –a and –o to signify the AND and OR conditions. • You can take action on selected files with -exec and –ok operator. • The command is executed and terminated with sequence {} \; where {} represents filename.

20. find: Locating Files $ find /var/preserve –mtime +30 –exec rm –rf {} \; • You can use the –ok operator instead of -exec for interactive deletion. $ find /home –size +2000 –atime +180 –ok rm {} \; • GNU find doesn’t need –print option; it prints by default. $ find –name “*.java”