Linux 学习笔记（1）: Linux Shell Powerful Tools

Linux Shell Powerful Tools：


Connecting and expanding commands
A truly powerful feature of the shell is the capability to redirect the input and output of commands to and from other commands and files. To allow commands to be strung together,the shell uses metacharacters. As noted earlier, a metacharacter is a typed character that has special meaning to the shell for connecting commands or requesting expansion.

Piping commands


The pipe (|) metacharacter connects the output from one command to the input of another command. This lets you have one command work on some data, and then have the next command deal with the results. Here is an example of a command line that includes pipes:


$ cat /etc/passwd | sort | cut -f1,5 -d: | less


This command lists the contents of the /etc/passwd file and pipes the output to the sort command. The sort command takes the user names that begin each line of the /etc/passwd file, sorts them alphabetically, and pipes the output to the cut command. The cut command takes fields 1 and 5, with the fields delimited by a colon (:), then pipes the output to the less command. The less command displays the output one page at a time, so that you can go through page by page.


Using file-matching metacharacters
To save you some keystrokes and to be able to refer easily to a group of files, the bash shell lets you use metacharacters. Anytime you need to refer to a file or directory, such as to list it, open it, or remove it, you can use metacharacters to match the files you want. Here are some useful metacharacters for matching filenames:

• * — This matches any number of characters.
• ? — This matches any one character.
• [...] — This matches any one of the characters between the brackets, which can
include a dash-separated range of letters or numbers.

Sample Examples:

Using file-redirection metacharacters
Commands receive data from standard input and send it to standard output. Standard input is normally user input from the keyboard, and standard output is normally displayed on the screen. Using pipes (described earlier), you can direct standard output from one command to the standard input of another. With files, you can use less than (<) and greater than (>) signs to direct data to and from files. Here are the file redirection characters:
• < — Direct the contents of a file as input to the command (because many commands take a file name as an option, the < key is not usually needed).
• > — Direct the output of a command to a file, overwriting any existing file.
• >> — Direct the output of a command to a file, adding the output to the end of the existing file.
Here are some examples of command lines where information is directed to and from files.

Managing background and foreground processes
If you are using Linux over a network or from a dumb terminal (a monitor that allows only text input with no GUI support), your shell may be all that you have. You may be used to a windowing environment where you have a lot of programs active at the same time so that you can switch among them as needed. This shell thing can seem pretty limited.


There are several ways to place an active program in the background. One mentioned earlier is to add an ampersand (&) to the end of a command line. Another way is to use the at command to run commands in a way in which they are not connected to the shell. (See Chapter 12 for more information about the at command.)
To stop a running command and put it in the background, press Ctrl+z. After the command is stopped, you can either bring it to the foreground to run (the fg command) or start it running in the background (the bg command).