Unix Tutorial

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Two commands are available to change the owner and the group of files: 1. chown: The chown command stands for "change owner" and is used to change the owner of a file. 2. chgrp: The chgrp command stands for "change group" and is used to change the group of a file. Changing Ownership: The chown command changes the ownership of a file. The basic syntax is as follows: $ chown user filelist The value of user can be either the name of a user on the system or the user id (uid) of a user on the system. Following example: $ chown amrood testfile $ Changes the owner of the given file to the user amrood. NOTE: The super user, root, has the unrestricted capability to change the ownership of a any file but normal users can change only the owner of files they own. Changing Group Ownership: The chrgp command changes the group ownership of a file. The basic syntax is as follows: $ chgrp group filelist The value of group can be the name of a group on the system or the group ID (GID) of a group on the system. Following example: $ chgrp special testfile $ Changes the group of the given file to special group. SUID and SGID File Permission: Often when a command is executed, it will have to be executed with special privileges in order to accomplish its task. As an example, when you change your password with the passwd command, your new password is stored in the file /etc/shadow. As a regular user, you do not have read or write access to this file for security reasons, but when you change your password, you need to have write permission to this file. This means that the passwd program has to give you additional permissions so that you can write to the file /etc/shadow. Additional permissions are given to programs via a mechanism known as the Set User ID ( SUID) and Set Group ID ( SGID) bits. When you execute a program that has the SUID bit enabled, you inherit the permissions of that program's owner. Programs that do not have the SUID bit set are run with the permissions of the user who started the program. This is true for SGID as well. Normally programs execute with your group permissions, but instead your group will be changed just for this program to the group owner of the program. TUTORIALS POINT Simply Easy Learning
The SUID and SGID bits will appear as the letter "s" if the permission is available. The SUID "s" bit will be located in the permission bits where the owners execute permission would normally reside. For example, the command $ ls -l /usr/bin/passwd -r-sr-xr-x 1 root bin 19031 Feb 7 13:47 /usr/bin/passwd* $ Which shows that the SUID bit is set and that the command is owned by the root. A capital letter S in the execute position instead of a lowercase s indicates that the execute bit is not set. If the sticky bit is enabled on the directory, files can only be removed if you are one of the following users: The owner of the sticky directory The owner of the file being removed The super user, root To set the SUID and SGID bits for any directory try the following: $ chmod ug+s dirname $ ls -l drwsr-sr-x 2 root root 4096 Jun 19 06:45 dirname $ TUTORIALS POINT Simply Easy Learning
Page 1 and 2: Unix Tutorial
Page 3 and 4: ABOUT THE TUTORIAL Unix Tutorial UN
Page 5 and 6: Renaming Directories: .............
Page 7 and 8: Extended Shell Scripts: ...........
Page 9 and 10: Example: ..........................
Page 11 and 12: Trapping Signals: .................
Page 13 and 14: The main concept that unites all ve
Page 15 and 16: New UNIX password:******* Retype ne
Page 17 and 18: CHAPTER 2 Unix File Management All
Page 19 and 20: Displays all the files whose name s
Page 21 and 22: 4. Fourth Column: represents file n
Page 23 and 24: CHAPTER 3 Unix Directories Adirecto
Page 25 and 26: Creates the directories docs and pu
Page 27 and 28: CHAPTER 4 Unix File Permission Setu
Page 29: $ls -l testfile -rw-rwxrwx 1 amrood
Page 33 and 34: Note - The shell initialization pro
Page 35 and 36: The default secondary prompt is > (
Page 37 and 38: CHAPTER 6 Java Basic Utilities So f
Page 39 and 40: laserp-574 grace 82744 Nov 7 11:28
Page 41 and 42: CHAPTER 7 Unix Pipes and Filters Yo
Page 43 and 44: This pipe sorts all files in your d
Page 45 and 46: The process runs in the foreground,
Page 47 and 48: $kill -9 6738 Terminated Parent and
Page 49 and 50: CHAPTER 9 Unix Communication When y
Page 51 and 52: prompt on Turns prompt on. Dir List
Page 53 and 54: No Plan. Check all the logged in us
Page 55 and 56: ~ ~ ~ ~ ~ ~ ~ ~ "testfile" [New Fil
Page 57 and 58: Command CTRL+d CTRL+d CTRL+f CTRL+u
Page 59 and 60: ^G U U J Press CNTRL and G keys at
Page 61 and 62: IMPORTANT: Here are the key points
Page 63 and 64: POSIX shell ( sh) The different C-t
Page 65 and 66: CHAPTER 12 Unix- Using Variables Av
Page 67 and 68: NAME="Zara Ali" unset NAME echo $NA
Page 69 and 70: Command-Line Arguments: The command
Page 71 and 72: CHAPTER 14 Unix - Using Arrays Ashe
Page 73 and 74: CHAPTER 15 Unix - Basic Operators T
Page 75 and 76: val=`expr $b / $a` echo "b / a : $v
Page 77 and 78: else echo "$a -ge $b: a is not grea
Page 79 and 80: != Checks if the value of two opera
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] is true. -x file Check if file is
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Right shift == String comparison fo
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-r file Checks if file is readable
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Example: #!/bin/sh a=10 b=20 if [ $
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There is only one form of case...es
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CHAPTER 17 Unix - Shell Loops Loops
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done This will produce following re
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4) juice 5) appe 6) all 7) none #?
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CHAPTER 18 Unix - Loop Control So f
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1 0 1 5 The continue statement: The
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\n new line \r carriage return \t h
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CHAPTER 20 Unix - Quoting Mechanism
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The Double Quotes: Try to execute t
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CHAPTER 21 Unix - IO Redirections M
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EOF 3 $ This is a simple lookup pro
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CHAPTER 22 Unix - Shell Functions F
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echo "Return value is $ret" This wo
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CHAPTER 23 Unix - Manpage Help All
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CHAPTER 24 Unix - Regular Expressio
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Try out the following address range
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sys:x:3:3:sys:/dev:/bin/sh sync:x:4
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[0-9] Matches a single number [a-zA
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(555)555-1213 (555)555-1214 (666)55
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dev /etc /lib /boot /home /mnt /pro
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$du -h /etc 5k /etc/cron.d 63k /etc
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CHAPTER 26 Unix - User Administrati
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$ groupdel developer This removes o
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CHAPTER 27 Unix - System Performanc
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CHAPTER 28 Unix - System Logging Un
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The combination of facilities and l
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Unix - Signals and Traps CHAPTER 29
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Trapping these signals is quite eas
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CHAPTER 30 Unix - Useful Commands T
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Ispell Emacs ex, edit Emacs Emacs C
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Command Dbx Gdb Make Nm Size Strip
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Reset Reset terminal mode Script Ke
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