Adv. UNIX: FileStr/11 Advanced UNIX v Objectives –to supplement the “Introduction to UNIX” slides with extra information on files Special Topics in Comp. Eng. 1 Semester 2, The File Structure (Ch.4, Sobell)

Adv. UNIX: FileStr/12 Overview 1.Access Permissions 2.Links

Adv. UNIX: FileStr/13 1. Access Permissions 1.1Types of Users and Access 1.2More File Information 1.3Access Permission Characters 1.4Changing Permissions

Adv. UNIX: FileStr/ Types of User and Access v Types of user: –creator / owneru –groupg –otherso v Types of access: –readr –writew –executex

Adv. UNIX: FileStr/ More File Information $ ls -lF -rwxr-xr-x 1 ad users Sep 26 15:02 BLP.gz* -rwxr-xr-x 2 ad users 852 May 5 14:03 check_spell drwxr-xr-x 2 ad users 1024 Sep 26 16:04 curses/ -rw-r--r-- 1 ad users 3355 May letter.txt v Meaning (left to right): –file type (first char) –file access permission (9 chars) –number of links –owner’s name –group name - byte size of file - creation time/ last modified - name, with / or * ending

Adv. UNIX: FileStr/ Access Permission Chars v First 3 chars: –refer to creator/owner (u) v Middle 3 chars: –refer to group (g) v Last 3 chars: –refer to everyone else (o)

Adv. UNIX: FileStr/17 v The 3 characters for u, g, and o have the same format: –First character indicates whether the file can be read (r). For a directory, this means you can do a ls. –Second character indicates whether the file can be written to (w). For a directory, you can add/remove files. –Third character indicates whether the file can be executed (x). For a directory, you can do a cd. A bit hard to remember

Adv. UNIX: FileStr/ Changing Permissions chmod who+what file // add a permission chmod who-what file // remove who:u, g, o, a (all) what:r, w, x and combinations v Examples: chmod u+x foo-p chmod a+rw letter.txt chmod o-rx check_spell

Adv. UNIX: FileStr/19 $ chmod a+rw letter.txt $ ls -lg letter.txt -rw-rw-rw- 1 ad staff 3355 May 2 10:52 letter.txt $ chmod o-rx check_spell $ ls -lg check_spell -rwxr-x--- 2 ad staff 852 May 5 14:03 check_spell

Adv. UNIX: FileStr/110 Warning v Your files and directories are automatically protected correctly. v Don’t change their permissions unless you really know what you are doing.

Adv. UNIX: FileStr/111 Directory Access  Let everyone ls, add/remove and cd to my info directory: $ chmod a+rwx /home/ad/info v Check permissions: $ ls -ldF /home/ad/info drwxrwxrwx 3 ad staff 112 Apr 15 11:05 /home/ad/info

Adv. UNIX: FileStr/ Links 2.1What is a Link? 2.2Creating a Link 2.3Seeing Links 2.4Removing a Link 2.5Symbolic Links

Adv. UNIX: FileStr/ What is a Link? v A link is a pointer to a file. v Useful for sharing files: –a file can be shared by giving each person their own link (pointer) to it.

Adv. UNIX: FileStr/ Creating a Link ln existing-file new-pointer v Jenny types: ln draft /home/ad/letter / home adjenny memoplanning /home/jenny/draft and /home/ad/letter

Adv. UNIX: FileStr/115 v Changes to a file affects every link: $ cat file_a This is file A. $ ln file_a file_b $ cat file_b This is file A. $ vi file_b : $ cat file_b This is file B after the change. $ cat file_a This is file B after the change.

Adv. UNIX: FileStr/ Seeing Links v Compare status information: $ ls -l file_a file_b file_c file_d -rw-r--r-- 2 ad 33 May 24 10:52 file_a -rw-r--r-- 2 ad 33 May 24 10:52 file_b -rw-r--r-- 1 ad 16 May 24 10:55 file_c -rw-r--r-- 1 ad 33 May 24 10:57 file_d v Look at inode number: $ ls -i file_a file_b file_c file_d 3534 file_a 3534 file_b 5800 file_c 7328 file_d

Adv. UNIX: FileStr/117 v Directories may appear to have many links: drwxr-xr-x 23 ad users 1024 Jan BLP/  This is because subdirectories (e.g. directories inside BLP/ ) have a link back to their parent.

Adv. UNIX: FileStr/ Removing a Link v Deleting a link does not remove the file. v Only when the file and every link is gone will the file be removed.

Adv. UNIX: FileStr/ Symbolic Links v The links described so far are often called hard links –a hard link is a pointer to a file which must be on the same filesystem v A symbolic link is an indirect pointer to a file –it stores the pathname of the pointed-to file –it can link across filesystems

Adv. UNIX: FileStr/120 v Jenny types: ln -s shared /home/ad/project / home adjenny memoplanning /home/jenny/shared and /home/ad/project separate filesystem

Adv. UNIX: FileStr/121 v Symbolic links are listed differently: $ ln -s pics /home/ad/images $ ls -lF pics /home/ad/images drw-r--r-- 1 ad staff 981 May 24 10:55 pics lrwxrwxrxw 1 ad staff 4 May 24 10:57 /home/ad/images --> pics

Adv. UNIX: FileStr/122 v Symbolic links can confuse: $ ln -s /home/ad/grades /tmp/grades-old $ cd /tmp/grades-old $ pwd /home/ad/grades $ echo $cwd (C Shell only) /tmp/grades-old $ cd.. $ echo $cwd /home/ad