到先前的課程為止，我們在用 command lines 跑了很多 commands，然後收到 terminal 的 output。這堂課我們主要會講 input and output (I/O) redirection.

Through redirection you can direct the input and output of a command to and from other files and programs, and chain commands together in a pipeline. Let's try it out.

Echo Command

$ echo "Hello"
Hello

The echocommand accepts the string "Hello" as standard input, and echoes the string "Hello" back to the terminal as

standard output.

Let's learn more about standard input, standard output, and standard error:

1. standard input, abbreviated as stdin, is information inputted into the terminal through the keyboard or input device.

2. standard output, abbreviated as stdout, is the information outputted after a process is run.

3. standard error, abbreviated as stderr, is an error message outputted by a failed process.

Redirection reroutes standard input, standard output, and standard error to or from a different location.

How does redirection work?

The Cat Command

• The cat (short for “concatenate“)
• 很常用!!
• 其實 Cat 有一堆用法。這邊有講到下面幾種
• Use Standard Output with Redirection Operator. (像 $ echo "Hello" > hello.txt)
• 觀看檔案內容(Display Contents of File

類型1

$ cat hello.txt

The catcommand outputs the contents of a file to the terminal. When you type cat hello.txt, the contents of hello.txt

are displayed.

類型2

$ echo "Hello" > hello.txt

The>command redirects the standard output to a file.

Here,"Hello"is entered as the standard input. The standard output"Hello"is redirected by>to the file hello.txt.

$ cat oceans.txt > continents.txt

>takes the standard output of the command on the left, and redirects it to the file on the right. Here the standard output of

cat oceans.txtis redirected to continents.txt.

Note that>overwrites all original content in continents.txt. When you view the output data by typing caton continents.txt

, you will see only the contents of oceans.txt.

• 會把原來的檔案覆蓋掉

類型3

$ cat glaciers.txt >> rivers.txt

• 添加的功能，rivers 這樣就會有 glaciers 的檔案

>>takes the standard output of the command on the left and appends(adds) it to the file on the right. You can view the output data of the file with catand the filename.

Here, the output data of rivers.txt will contain the original contents of rivers.txt with the content of glaciers.txt appended to it.

類型4

$ cat < lakes.txt

<takes the standard input from the file on the right and inputs it into the program on the left. Here,

lakes.txt is the standard input for the catcommand. The standard output appears in the terminal.

WC (Word Count) Command 和 Pipe

$ cat volcanoes.txt | wc

|is a "pipe".The|takes the standard output of the command on the left, and pipes it as standard input to the command on the right. You can think of this as "command to command" redirection.

Here the output of cat volcanoes.txtis the standard input of wc. in turn, the wccommand outputs the number of lines, words, and characters in volcanoes.txt, respectively.

$ cat volcanoes.txt | wc | cat > islands.txt