输出和重定向（echo、cat、>）

In Linux, almost all commands read data from an input stream and write results to an output stream. These channels are called standard streams:

• Standard Input (stdin): The stream from which the command reads its input (default is the keyboard).
• Standard Output (stdout): The stream to which the command writes its successful output (default is the terminal screen).
• Standard Error (stderr): The stream to which the command writes its error messages (default is the screen).

Using the command line, we can redirect these streams to save results directly into files.

Printing Text: echo

The echo command simply writes its arguments to its standard output (the screen):

echo "Hello everyone"

Redirecting Output (Overwrite): >

If you want to save the output of a command to a file instead of displaying it on the screen, you can use the greater-than redirect operator >:

echo "Hello everyone" > greetings.txt

This command runs echo and redirects its output to the file greetings.txt.

[!IMPORTANT] If the file greetings.txt does not exist, it is created. If it already exists, its existing content is completely deleted and overwritten with the new output.

Reading File Contents: cat

To view the content of one or more text files on the fly, use the cat command (short for concatenate):

cat greetings.txt

Appending Output: >>

If you want to append new lines to the end of an existing file rather than overwriting it, you must use the append operator >>:

echo "New line of text" >> greetings.txt

Now, reading the file with cat greetings.txt will show both the first line and the second line.

Usage Examples and Common Errors

You can pass multiple files to cat at the same time to display them sequentially (concatenated):

cat notes.txt info.txt
# Output:
# Keep learning Linux!
# Linux is awesome.

You can redirect the combined output to a third file using the > operator:

cat notes.txt info.txt > combined.txt

If you try to read a file that does not exist, cat will write an error message:

cat nonexistent_file.txt
# Output:
# cat: nonexistent_file.txt: No such file or directory

Try it yourself

Exercise 1: Create a file with a greeting

Exercise 2: Append notes to a file

Exercise 3: Concatenate files