What is a file system Link? Hard & Symbolic Links. What is a file system Link? What is a file system Link? Murray Saul Seneca College

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1 What is a file system Lik? Hard & Symbolic Liks Murray Saul Seeca College Adapted by Dr. Adrew Vardy Memorial Uiversity A lik is a poiter to a file. This poiter associates a file ame with a umber called a i-ode umber A i-ode is the cotrol structure for a file (o a UNIX/Liux file system) If two file ames have the same i-ode umber, they are liks to the same file What is a file system Lik? Use the commad ls -i to prit i-ode umber of each file: [ray@localhost week8]$ ls -i lab3a.html lab3b.html lab3.zip Use the commad ls -il for log listig [ray@localhost week8]$ ls -il total rw-r--r-- 1 ray ray 1097 Sep 13 08:53 lab3a.html rw-r--r-- 1 ray ray 6582 Sep 13 08:53 lab3b.html rw-rw-r-- 1 ray ray 6218 Sep 14 00:05 lab3.zip What is a file system Lik? There are two kids of liks: 1. Hard Liks 2. Soft or Symbolic Liks i-ode 1

2 Hard Liks Hard lik is a referece to the physical data o a file system All regular files are actually hard liks More tha oe ame ca be associated with the same physical data Hard liks ca oly refer to data that exists o the same file system You ca ot create hard lik to a directory Hard Liks Example: Assume you used a editor to create a ew file, you ve just created the first hard lik To Create the 2 d, 3 rd ad etc. hard liks, use the commad: l myfile lik-ame Display Hard Liks ifo Create a ew file called myfile Ru the commad ls -il to display the i-ode umber ad lik couter rw-rw-r-- 1 uli uli 29 Oct 29 08:47 myfile -- iode # -- lik couter (oe lik) Display Hard Lik Ifo Create a 2 d lik to the same data: l myfile mylik Ru the commad ls -il : rw-rw-r-- 2 uli uli 29 Oct 29 08:47 myfile rw-rw-r-- 2 uli uli 29 Oct 29 08:47 mylik -- iode # --lik couter (2 liks) 2

3 Add the 3 rd Lik Create a 3 rd lik to the same data: l myfile ewlik Ru the commad ls -il : rw-rw-r-- 3 uli uli 29 Oct 29 08:47 myfile rw-rw-r-- 3 uli uli 29 Oct 29 08:47 mylik rw-rw-r-- 3 uli uli 29 Oct 29 08:47 ewlik -- iode # --lik couter (3 liks) Removig a Hard Lik Whe a file has more tha oe lik, you ca remove ay oe lik ad still be able to access the file through the remaiig liks. Symbolic Liks Also Kow As (a.k.a.): Soft liks or Symliks A Symbolic Lik is a idirect poiter to a file a poiter to the hard lik to the file You ca create a symbolic lik to a directory Not so for hard liks A symbolic lik ca poit to a file o a differet file system Not so for hard liks A symbolic lik ca poit to a oexistet file (referred to as a broke lik ) Differet i-ode Symbolic Liks To create a symbolic lik to the file myfile, use l -s myfile symlik or l --symbolic myfile symlik [uli@seeca courses] ls -li myfile rw-rw-r-- 1 uli uli 49 Oct 29 14:33 myfile [uli@seeca courses] l -s myfile symlik [uli@seeca courses] ls -li myfile symlik rw-rw-r-- 1 uli uli 49 Oct 29 14:33 myfile lrwxrwxrwx 1 uli uli 6 Oct 29 14:33 symlik -> myfile File type: (symbolic lik) Lik couter: (1 lik) 3

4 Properties of Symbolic Liks Create Symbolic Lik Directory The i-ode umber is differet from the poited to file The lik couter of the ew symbolic lik file is 1 Symbolic lik file does ot affect the lik couter of the poited to file The type field of symblic file cotais the letter l The symbolic lik file ad the poited to file have differet status iformatio (e.g. file size, last modificatio time etc.) The sytax is the same as likig to a file l -s target_directory lik_directory l --symbolic target_directory lik_directory [uli@seeca week8]$ ls -li drwxrwxr-x 7 uli uli 168 Oct 29 13:32 courses [uli@seeca week8]$ l courses mydir l: `courses': hard lik ot allowed for directory [uli@seeca week8]$ l -s courses mydir [uli@seeca week8]$ ls -li drwxrwxr-x 7 uli uli 168 Oct 29 13:32 courses lrwxrwxrwx 1 uli uli 7 Oct 29 15:41 mydir -> courses Directory Listig Delete lik to a directory To display the cotets i a directory, we ormally use the commad ls -l directory_ame Compare the followig two commads [uli@seeca week8]$ ls -l mydir lrwxrwxrwx 1 uli uli 7 Oct 29 15:41 mydir -> courses [uli@seeca week8]$ ls -l courses drwxrwxr-x 2 uli uli 72 Oct 29 11:15 ica101 drwxrwxr-x 2 uli uli 72 Oct 29 11:16 ios110 drwxrwxr-x 2 uli uli 120 Oct 29 11:20 to_do drwxrwxr-x 2 uli uli 72 Oct 29 11:14 uli101 To delete a lik to a directory, simply use the rm commad: [uli@seeca week8]$ ls -l drwxrwxr-x 7 uli uli 168 Oct 29 13:32 courses lrwxrwxrwx 1 uli uli 7 Oct 29 15:41 mydir -> courses [uli@seeca week8]$ rm mydir [uli@seeca week8]$ ls -l drwxrwxr-x 7 uli uli 168 Oct 29 13:32 courses 4

5 Properties of Symbolic Lik Directory The Symbolic lik to a directory has a file type of l (the first letter of the permissio field). The permissios o the lik are set to rwx for all. The chmod commad applied o the lik applies to the actual directory (or file), the permissios o the lik stay the same Ca poit to a oexistet directory Drawbacks of Symbolic Liks If the origial file is moved to a differet locatio, it ca o loger be accessed via the symbolic lik (daglig lik) Extra space o disk ad extra iode to store the lik file (miimal) Extra time required for access to the origial file: the lik file has to be read first, the path followed to target file (miimal) 5