Basic Operations
In this section we will discuss how to accomplish basic operations from the command line. These include how to log in and log out from the system, restart or shutdown the system, locate applications, access directories, identify the absolute and relative paths, and explore the filesystem
工具/原料Ubuntu 14.04 OS
Logging In and Out
Logging In and Out
An available text terminal will prompt for a username (with the string login:) and password. When typing your password, nothing is displayed on the terminal (not even a * to indicate that you typed in something) to prevent others from seeing your password. After you have logged in to the system, you can perform basic operations.
Once your session is started (either by logging in to a text terminal or via a graphical terminal program) you can also connect and log in to remote systems via the Secure Shell (SSH) utility. For example, by typing ssh username@remote-server.com, SSH would connect securely to the remote machine and give you a command line terminal window, using passwords (as with regular logins) or cryptographic keys (a topic we won't discuss) to prove your identity.Rebooting and Shutting Down
The preferred method to shut down or reboot the system is to use the shutdown command. This sends a warning message and then prevents further users from logging in. The init process will then control shutting down or rebooting the system. It is important to always shut down properly; failure to do so can result in damage to the system and/or loss of data.
The halt and poweroff commands issue shutdown -h to halt the system; reboot issues shutdown -r and causes the machine to reboot instead of just shutting down. Both rebooting and shutting down from the command line requires superuser (root) access.
When administering a multiuser system, you have the option of notifying all users prior to shutdown as in:
$ sudo shutdown -h 10:00 "Shutting down for scheduled maintenance."Locating Applications
Depending on the specifics of your particular distribution's policy, programs and software packages can be installed in various directories. In general, executable programs should live in the /bin, /usr/bin,/sbin,/usr/sbin directories or under /opt.
One way to locate programs is to employ the which utility. For example, to find out exactly where the diff program resides on the filesystem:
$ which diff
If which does not find the program, whereis is a good alternative because it looks for packages in a broader range of system directories:
$ whereis diffAccessing Directories
When you first log into a system or open a terminal, the default directory should be your home directory; you can print the exact path of this by typing echo $HOME. (Note that some Linux distributions actually open new graphical terminals in $HOME/Desktop.) The following commands are useful for directory navigation:Understanding Absolute and Relative Paths
There are two ways to identify paths:
Absolute pathname:
An absolute pathname begins with the root directory and follows the tree, branch by branch, until it reaches the desired directory or file. Absolute paths always start with /.
Relative pathname:
A relative pathname starts from the present working directory. Relative paths never start with /.
Multiple slashes (/) between directories and files are allowed, but all but one slash between elements in the pathname is ignored by the system. ////usr//bin is valid, but seen as /usr/bin by the system.
Most of the time it is most convenient to use relative paths, which require less typing. Usually you take advantage of the shortcuts provided by: . (present directory), .. (parent directory) and ~ (your home directory).
For example, suppose you are currently working in your home directory and wish to move to the /usr/bin directory. The following two ways will bring you to the same directory from your home directory:
1、Absolute pathname method: $ cd /usr/bin
2、Relative pathname method: $ cd ../../usr/bin
In this case, the absolute pathname method is less typing.Exploring the Filesystem
Traversing up and down the filesystem tree can get tedious. The tree command is a good way to get a bird’s-eye view of the filesystem tree. Use tree -d to view just the directories and to suppress listing file names.
The following commands can help in exploring the filesystem:Hard and Soft (Symbolic) Links
ln can be used to create hard links and (with the -s option) soft links, also known as symbolic links or symlinks. These two kinds of links are very useful in UNIX-based operating systems. The advantages of symbolic links are discusssed on the following screen.
Suppose that file1 already exists. A hard link, called file2, is created with the command:
$ ln file1 file2
Note that two files now appear to exist. However, a closer inspection of the file listing shows that this is not quite true.
$ ls -li file1 file2
The -i option to ls prints out in the first column the inode number, which is a unique quantity for each file object. This field is the same for both of these files; what is really going on here is that it is only one file but it has more than one name associated with it, as is indicated by the 3 that appears in the ls output. Thus, there already was another object linked to file1 before the command was executed.
Click the image to view an enlarged version.Symbolic Links
Symbolic (or Soft) links are created with the -s option as in:
$ ln -s file1 file4 $ ls -li file1 file4
Notice file4 no longer appears to be a regular file, and it clearly points to file1 and has a different inode number.
Symbolic links take no extra space on the filesystem (unless their names are very long). They are extremely convenient as they can easily be modified to point to different places. An easy way to create a shortcut from your home directory to long pathnames is to create a symbolic link.
Unlike hard links, soft links can point to objects even on different filesystems (or partitions) which may or may not be currently available or even exist. In the case where the link does not point to a currently available or existing object, you obtain a dangling link.
Hard links are very useful and they save space, but you have to be careful with their use, sometimes in subtle ways. For one thing if you remove either file1 or file2 in the example on the previous screen, the inode object (and the remaining file name) will remain, which might be undesirable as it may lead to subtle errors later if you recreate a file of that name.
If you edit one of the files, exactly what happens depends on your editor; most editors including vi and gedit will retain the link by default but it is possible that modifying one of the names may break the link and result in the creation of two objects.Navigating the Directory History
The cd command remembers where you were last, and lets you get back there with cd -. For remembering more than just the last directory visited, use pushd to change the directory instead of cd; this pushes your starting directory onto a list. Using popd will then send you back to those directories, walking in reverse order (the most recent directory will be the first one retrieved with popd). The list of directories is displayed with the dirs command.
