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<?xml version="1.0" encoding="iso-8859-1"?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head> <title> UNIX Tutorial Five</title> <link href="unixtut1.css" rel="stylesheet" type="text/css" /><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /><meta name="Copyright" content="Michael Stonebank, 1995" /></head> <body><h1>UNIX Tutorial Five </h1><h2>5.1 File system security (access rights) </h2><p>In your unixstuff directory, type </p><p class="command"> % ls -l (l for long listing!)</p><p>You will see that you now get lots of details about the contents of your directory, similar to the example below. </p><p> <img src="file1.gif" alt="File and directory access rights" /> </p><p>Each file (and directory) has associated access rights, which may be found by typing <samp>ls -l</samp>. Also, <samp>ls -lg</samp> gives additional information as to which group owns the file (beng95 in the following example): </p><p class="stdout"> -rwxrw-r-- 1 ee51ab beng95 2450 Sept29 11:52 file1</p><p>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. </p><p>The 9 remaining symbols indicate the permissions, or access rights, and are taken as three groups of 3. </p><ul> <li> The left group of 3 gives the file permissions for the user that owns the file (or directory) (ee51ab in the above example); <br /> </li> <li> the middle group gives the permissions for the group of people to whom the file (or directory) belongs (eebeng95 in the above example);<br /> </li> <li> the rightmost group gives the permissions for all others.</li></ul><p>The symbols r, w, etc., have slightly different meanings depending on whether they refer to a simple file or to a directory. </p><h3>Access rights on files.</h3><ul> <li> r (or -), indicates read permission (or otherwise), that is, the presence or absence of permission to read and copy the file <br /> </li> <li> w (or -), indicates write permission (or otherwise), that is, the permission (or otherwise) to change a file <br /> </li> <li> x (or -), indicates execution permission (or otherwise), that is, the permission to execute a file, where appropriate </li></ul><h3>Access rights on directories.</h3><ul> <li> r allows users to list files in the directory; </li> <li> w means that users may delete files from the directory or move files into it; </li> <li> 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. </li></ul><p>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. </p><h3>Some examples</h3><table border="1" align="center" cellpadding="5" cellspacing="0"> <tr> <td><samp>-rwxrwxrwx</samp></td> <td>a file that everyone can read, write and execute (and delete).</td> </tr> <tr> <td><samp>-rw-------</samp></td> <td>a file that only the owner can read and write - no-one else <br /> can read or write and no-one has execution rights (e.g. your <br /> mailbox file).</td> </tr></table><h2>5.2 Changing access rights</h2><h3>chmod (changing a file mode)</h3><p>Only the owner of a file can use <samp>chmod</samp> to change the permissions of a file. The options of <samp>chmod</samp> are as follows </p><table border="1" align="center" cellpadding="3" cellspacing="0"> <tr> <th>Symbol</th> <th>Meaning</th> </tr> <tr> <td><div align="center">u</div></td> <td>user</td> </tr> <tr> <td><div align="center">g</div></td> <td>group</td> </tr> <tr> <td><div align="center">o</div></td> <td>other</td> </tr> <tr> <td><div align="center">a</div></td> <td>all</td> </tr> <tr> <td><div align="center">r</div></td> <td>read</td> </tr> <tr> <td><div align="center">w</div></td> <td>write (and delete)</td> </tr> <tr> <td><div align="center">x</div></td> <td>execute (and access directory) </td> </tr> <tr> <td><div align="center">+</div></td> <td>add permission</td> </tr> <tr> <td><div align="center">-</div></td> <td>take away permission</td> </tr></table><p> For example, to remove read write and execute permissions on the file <strong>biglist</strong> for the group and others, type </p><p class="command"> % chmod go-rwx biglist </p><p>This will leave the other permissions unaffected. </p><p>To give read and write permissions on the file <strong>biglist</strong> to all, </p><p class="command"> % chmod a+rw biglist </p><h3>Exercise 5a</h3><p>Try changing access permissions on the file <strong>science.txt</strong> and on the directory <strong>backups</strong></p><p> Use <samp>ls -l</samp> to check that the permissions have changed. </p><h2>5.3 Processes and Jobs </h2><p>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 </p><p class="command"> % ps </p><p>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. </p><p> 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. </p><h3>Running background processes</h3><p>To background a process, type an <strong>&</strong> at the end of the command line. For example, the command <samp>sleep</samp> waits a given number of seconds before continuing. Type </p><p class="command"> % sleep 10 </p><p>This will wait 10 seconds before returning the command prompt %. Until the command prompt is returned, you can do nothing except wait. </p><p> To run <samp>sleep</samp> in the background, type </p><p class="command"> % sleep 10 &</p><p class="stdout">[1] 6259 </p><p>The <strong>&</strong> 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. </p><p>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. </p><h3>Backgrounding a current foreground process</h3><p>At the prompt, type </p><p class="command"> % sleep 100 </p><p>You can suspend the process running in the foreground by holding down the <code>[control]</code> key and typing <code>[z]</code> (written as <strong>^Z</strong>) Then to put it in the background, type </p><p class="command"> % bg </p><p class="hint">Note: do not background programs that require user interaction e.g. pine </p><h2>5.4 Listing suspended and background processes </h2><p>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 </p><p class="command"> % jobs </p><p>An example of a job list could be </p><p class="stdout"> [1] Suspended sleep 100<br /> [2] Running netscape<br /> [3] Running nedit</p><p>To restart (foreground) a suspended processes, type </p><p class="command"> % fg %jobnumber </p><p>For example, to restart <samp>sleep 100</samp>, type </p><p class="command"> % fg %1 </p><p>Typing <samp>fg</samp> with no job number foregrounds the last suspended process. </p> <p> </p><h2>5.5 Killing a process </h2><h3>kill (terminate or signal a process)</h3><p>It is sometimes necessary to kill a process (for example, when an executing program is in an infinite loop) </p><p> To kill a job running in the foreground, type <samp>^C</samp> (control c). For example, run </p><p class="command">% sleep 100<br /> ^C </p><p> To kill a suspended or background process, type </p><p class="command"> % kill %jobnumber </p><p>For example, run </p><p class="command">% sleep 100 &<br /> % jobs</p><p> If it is job number 4, type </p><p class="command"> % kill %4 </p><p>To check whether this has worked, examine the job list again to see if the process has been removed. </p><h3>ps (process status)</h3><p> Alternatively, processes can be killed by finding their process numbers (PIDs) and using <samp>kill <var>PID_number</var></samp></p><p class="command"> % sleep 100 &<br /> % ps</p><p class="stdout">PID TT S TIME COMMAND<br /> 20077 pts/5 S 0:05 sleep 100<br /> 21563 pts/5 T 0:00 netscape<br /> 21873 pts/5 S 0:25 nedit </p><p>To kill off the process <samp>sleep 100</samp>, type </p><p class="command"> % kill 20077 </p><p>and then type <samp>ps</samp> again to see if it has been removed from the list. </p><p> If a process refuses to be killed, uses the <strong>-9</strong> option, i.e. type </p><p class="command"> % kill -9 20077 </p><p class="hint">Note: It is not possible to kill off other users' processes !!!</p><h2>Summary </h2><table border="1" align="center" cellpadding="3" cellspacing="0"> <tr> <td><code>ls -lag</code></td> <td>list access rights for all files</td> </tr> <tr> <td><code>chmod [<var>options</var>] <var>file</var></code></td> <td> change access rights for named file</td> </tr> <tr> <td><code><var>command</var> &</code></td> <td>run command in background</td> </tr> <tr> <td><code>^C</code></td> <td>kill the job running in the foreground</td> </tr> <tr> <td><code>^Z</code></td> <td>suspend the job running in the foreground</td> </tr> <tr> <td><code>bg</code></td> <td>background the suspended job</td> </tr> <tr> <td><code>jobs</code></td> <td>list current jobs</td> </tr> <tr> <td><code>fg %1</code></td> <td>foreground job number 1</td> </tr> <tr> <td><code>kill %1</code></td> <td> kill job number 1</td> </tr> <tr> <td><code>ps</code></td> <td>list current processes</td> </tr> <tr> <td><code>kill 26152</code></td> <td>kill process number 26152</td> </tr></table> <p> </p><p class="navbar"><a href="unix4.html"><img src="media/left.gif" alt="Previous" width="37" height="39" border="0" /></a> <a href="index.html"><img src="media/home.gif" alt="Home" width="81" height="39" border="0" /></a><a href="unix6.html"><img src="media/right.gif" alt="Next" width="37" height="39" border="0" /></a> </p><p class="date"> M.Stonebank@surrey.ac.uk, 9th October 2000 </p></body></html>⌨️ 快捷键说明
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