File system security

$ ls -l (l for long listing!)

This command will let you get lots of detail about the contents of your directory, similar to the example below.

For example, here is a directory's details:

drwxr-xr-x   8 imac  staff     272  7 12 10:48 test

最左边的这一串10个字符的解释：

• This column is a 10 symbol string consistingof the symbols d,r,w,x,-,and,occasionally, s or S. 这串字符由d、r、w、x、- 以及s、S构成的十个字符组成。
• If it starts with d, then this indicates a directory.Otherwise, - will be the starting symbol of the string.如果这串字符以d开头，则说明这是一个目录，否则其他是以 - 开头。
• The 9 remaining symbols indicate the permission, or access rights, and are taken as three groups of 3.
  剩下的9个字符，分别表示了允许进入权限等信息，3个为一组分成3组数。
  1.The left group of 3 givs the file permission for the user that owns the file( or directory);最左边的3个字符表示给文件的主人用户对文件进行读写执行的权限。这里的ownner是imac。
  2.The middle group gives the permissions for the group of people to whom the file(or directory) belongs; 中间的3个字符表示对拥有该文件的用户授予的文件读写执行的权限。这里是staff。
  3.The mostright group gives the permissions for all others. 最右边的三个表示对其他用户的权限支持。

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

Changing access rights

Using chmod to change the access rights of the file testfile.
The options of chmod are as follows:

$ chmod go-rwx testfile
　This will remove read write and execute permissions on the file testfile for the group and others.取消（收回）组内和其他人的读写、执行testfile文件的权限。

$ chmod a+rw testfile
This will give read and write permissions on the file testfile to all.给予所有人读写testfile的权限。

Processes and Jobs

A process is an executing program identified by a unique PID (process identifer).
$ ps
This command will help you to see information about your processes, like their associated PID and status.查看进程的状态信息，PID、运行状态等。

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.进程的状态有三种：前台、后台、被挂起。

把长进程放在后台，可以让UNIX prompt立刻返回结果，同时其他任务还在继续执行。

Running background processes

To background a process, type an & at the end of command line.
For example, the command sleep waits a given number of seconds before continuing.
$ sleep 10
This will wait 10 seconds before returning the command prompt %. Until the command prompt is returned, you can do nothing except wait.让进程睡眠十秒钟，这段时间你什么操作都不能做。
$ sleep 10 &
The & runs the job in the background and returns the prompt straight away, allowing you to do run other programs while waiting for that one to finish. &符号在后台执行sleep命令，在窗口会立刻提示，你可以做其他操作。

Backgrounding a current foreground process

At the prompt, type
$ sleep 1000
Then you can suspend the process runing in the foreground by typing ^Z, Then to put it in the background:
$ bg
这样能够让在前台执行中的程序挂起。

Listing suspended and background processes

$ jobs
This can list processes are running,backgrounded or suspended with their job numbers.
把所有执行中、后台的、挂起的程序列出，并且有一个job number。

$ fg $jobnumber
To restart(foreground) a suspended process.再次执行挂起的进程。

Killing a process

To kill a job runing in the foreground, type ^C

   $ sleep 100
   ^C

结束一个正在前台执行的进程。

To kill a suspended or background process :
$ kill $jobnumber
结束一个被挂起或在后台的进程。

To kill the processes by finding their process numbers(PIDS)
$ kill PID_number
Remember that using ps to show the process status to find out the PID.
通过PID来结束一个进程。

Quotes

UNIX Tutorial for Beginners