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— tutorial:linux_tutorial_five [2016/05/20 11:48] (current)
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 +=====Linux Tutorial Five=====
 +====5.1 File system security (access rights)====
 +
 +In your testdir directory, type
 +
 +<code>compsci-user@tim:~$ ls -l (l for long listing!)</code>
 +
 +You will see that you now get lots of details about the contents of your directory, similar to the example below.
 +
 +File and directory access rights
 +
 +Each file (and directory) has associated access rights, which may be found by typing ls -l. Also, ls -lg gives additional information as to which group owns the file (beng95 in the following example):
 +
 +  * -rwxrw-r-- 1 ee51ab beng95 2450 Sept29 11:52 file1
 +
 +In the left-hand column is a 10 symbol string consisting of the symbols d, r, w, x, -, and, occasionally, s or S. If d is present, it will be at the left hand end of the string, and indicates a directory: otherwise - will be the starting symbol of the string.
 +
 +The 9 remaining symbols indicate the permissions, or access rights, and are taken as three groups of 3.
 +
 +  * The left group of 3 gives the file permissions for the user that owns the file (or directory) (ee51ab in the above example); 
 +  * the middle group gives the permissions for the group of people to whom the file (or directory) belongs (eebeng95 in the above example);
 +  * the rightmost group gives the permissions for all others.
 +
 +The symbols r, w, etc., have slightly different meanings depending on whether they refer to a simple file or to a directory.
 +
 +===Access rights on files.===
 +  * r (or -), indicates read permission (or otherwise), that is, the presence or absence of permission to read and copy the file 
 +  * w (or -), indicates write permission (or otherwise), that is, the permission (or otherwise) to change a file 
 +  * x (or -), indicates execution permission (or otherwise), that is, the permission to execute a file, where appropriate
 +
 +===Access rights on directories.===
 +  * r allows users to list files in the directory;
 +  * w means that users may delete files from the directory or move files into it;
 +  * x means the right to access files in the directory. This implies that you may read files in the directory provided you have read permission on the individual files.
 +
 +So, in order to read a file, you must have execute permission on the directory containing that file, and hence on any directory containing that directory as a subdirectory, and so on, up the tree.
 +
 +===Some examples===
 +| -rwxrwxrwx | a file that everyone can read, write and execute (and delete). |
 +| -rw------- | a file that only the owner can read and write - no-one else can read or write and no-one has execution rights (e.g. your mailbox file). |
 +
 +====5.2 Changing access rights====
 +
 +===chmod (changing a file mode)===
 +
 +Only the owner of a file can use chmod to change the permissions of a file. The options of chmod are as follows
 +
 +^ Symbol  ^ Meaning                         ^
 +|  u       | user                            |
 +|  g       | group                           |
 +|  o       | other                           |
 +|  a       | all                             |
 +|  r       | read                            |
 +|  w       | write (and delete)              |
 +|  x       | execute (and access directory)  |
 +|  +       | add permission                  |
 +|  -       | take away permission            |
 +
 +
 +For example, to remove read write and execute permissions on the file biglist for the group and others, type
 +
 +% chmod go-rwx biglist
 +
 +This will leave the other permissions unaffected.
 +
 +To give read and write permissions on the file biglist to all,
 +
 +<code>compsci-user@tim:~$ chmod a+rw biglist</code>
 +
 +===Exercise 5a===
 +
 +Try changing access permissions on the file science.txt and on the directory backups
 +
 +Use **ls -l** to check that the permissions have changed.
 +
 +====5.3 Processes and Jobs====
 +
 +A process is an executing program identified by a unique PID (process identifier). To see information about your processes, with their associated PID and status, type
 +
 +<code>compsci-user@tim:~$ ps</code>
 +
 +A process may be in the foreground, in the background, or be suspended. In general the shell does not return the UNIX prompt until the current process has finished executing.
 +
 +Some processes take a long time to run and hold up the terminal. Backgrounding a long process has the effect that the UNIX prompt is returned immediately, and other tasks can be carried out while the original process continues executing.
 +
 +===Running background processes===
 +
 +To background a process, type an & at the end of the command line. For example, the command sleep waits a given number of seconds before continuing. Type
 +
 +<code>compsci-user@tim:~$ sleep 10</code>
 +
 +This will wait 10 seconds before returning the command prompt %. Until the command prompt is returned, you can do nothing except wait.
 +
 +To run sleep in the background, type
 +
 +<code>compsci-user@tim:~$ sleep 10 &</code>
 +
 +[1] 6259
 +
 +The **&** runs the job in the background and returns the prompt straight away, allowing you do run other programs while waiting for that one to finish.
 +
 +The first line in the above example is typed in by the user; the next line, indicating job number and PID, is returned by the machine. The user is be notified of a job number (numbered from 1) enclosed in square brackets, together with a PID and is notified when a background process is finished. Backgrounding is useful for jobs which will take a long time to complete.
 +
 +===Backgrounding a current foreground process===
 +
 +At the prompt, type
 +
 +<code>compsci-user@tim:~$ sleep 100</code>
 +
 +You can suspend the process running in the foreground by holding down the [control] key and typing [z] (written as ^Z) Then to put it in the background, type
 +
 +<code>compsci-user@tim:~$ bg</code>
 +
 +<note>Note: do not background programs that require user interaction e.g. pine</note>
 +
 +====5.4 Listing suspended and background processes====
 +
 +When a process is running, backgrounded or suspended, it will be entered onto a list along with a job number. To examine this list, type
 +
 +<code>compsci-user@tim:~$ jobs</code>
 +
 +An example of a job list could be
 +
 +  * [1] Suspended sleep 100
 +  * [2] Running netscape
 +  * [3] Running nedit
 +
 +To restart (foreground) a suspended processes, type
 +
 +<code>compsci-user@tim:~$ fg %jobnumber</code>
 +
 +For example, to restart sleep 100, type
 +
 +<code>compsci-user@tim:~$ fg %1</code>
 +
 +Typing **fg** with no job number foregrounds the last suspended process.
 +
 +====5.5 Killing a process====
 +===kill (terminate or signal a process)===
 +
 +It is sometimes necessary to kill a process (for example, when an executing program is in an infinite loop)
 +
 +To kill a job running in the foreground, type ^C (control c). For example, run
 +
 +<code>compsci-user@tim:~$ sleep 100
 +^C</code>
 +
 +To kill a suspended or background process, type
 +
 +<code>compsci-user@tim:~$ kill %jobnumber</code>
 +
 +For example, run
 +
 +<code>compsci-user@tim:~$ sleep 100 &
 +compsci-user@tim:~$ jobs</code>
 +
 +If it is job number 4, type
 +
 +<code>compsci-user@tim:~$ kill %4</code>
 +
 +To check whether this has worked, examine the job list again to see if the process has been removed.
 +
 +===ps (process status)===
 +
 +Alternatively, processes can be killed by finding their process numbers (PIDs) and using kill PID_number
 +
 +<code>
 +compsci-user@tim:~$ sleep 100 &
 +compsci-user@tim:~$ ps
 +</code>
 +
 +  * PID TT S TIME COMMAND
 +  * 20077 pts/5 S 0:05 sleep 100
 +  * 21563 pts/5 T 0:00 netscape
 +  * 21873 pts/5 S 0:25 nedit
 +
 +To kill off the process sleep 100, type
 +
 +<code>compsci-user@tim:~$ kill 20077</code>
 +
 +and then type **ps** again to see if it has been removed from the list.
 +
 +If a process refuses to be killed, uses the -9 option, i.e. type
 +
 +<code>compsci-user@tim:~$ kill -9 20077</code>
 +
 +<note>Note: It is not possible to kill off other users' processes !!!</note>
 +
 +====Summary====
 +^ Command                   ^ Meaning                                    ^
 +| **ls -lag**               | list access rights for all files           |
 +| **chmod [options] file**  | change access rights for named file        |
 +| **command &**             | run command in background                  |
 +| **^C**                    | kill the job running in the foreground     |
 +| **^Z**                    | suspend the job running in the foreground  |
 +| **bg**                    | background the suspended job               |
 +| **jobs**                  | list current jobs                          |
 +| **fg %1**                 | foreground job number 1                    |
 +| **kill %1**               | kill job number 1                          |
 +| **ps**                    | list current processes                     |
 +| **kill 26152**            | kill process number 26152                  |