Navigation IO in Unix / Linux

Most Unix system commands receive input from your terminal and send the output back to your terminal. A command usually reads its input from a location called standard input, which happens to your terminal by default. In a similar way, a command usually outputs to standard output (standard output), which also happens to your terminal by default.

Navigate the output in Unix / Linux

Output from a command that tends to wait for standard output can easily be directed to a file. This ability is known as re-navigating the output.

If the> file symbol is assigned to any command, it usually writes its output to standard output, the output of that command will be written to the file instead of your terminal.

We check the who command, which will redirect the entire output of the command in files users:

 $ who > users 

Notice that no output appears on the terminal. That's because the output has been redirected from the default standard output device (terminal) into a specific file. If you check the file users then that file will have full content:

 $ cat users oko tty01 Sep 12 07 : 30 ai tty15 Sep 12 13 : 32 ruth tty21 Sep 12 10 : 10 pat tty24 Sep 12 13 : 07 steve tty25 Sep 12 $ 13 : 03 

If a command with output is redirected to a file and the file already contains some data, that data will be lost. Consider the following example:

 $ echo line 1 > users $ cat users line 1 $ 

You can use the >> operator to assign the output to an existing file as follows:

 $ echo line 2 >> users $ cat users line 1 line 2 $ 

Redirecting input in Unix / Linux

Similar to the output of the command, the command input can also redirect from a file. When the larger character> is used for input navigation, the smaller character

Commands that often receive their input from standard input can have input from a file according to the user's operation. For example, to calculate the number of lines in a users file, you can run the following command:

 $ wc - l users 2 users $ 

Here it will produce 2 lines. You can calculate the number of lines in the file by redirecting the standard input of wc command from users.

 $ wc - l < users 2 $ 

Remember, there is a difference in output by 2 patterns of wc command. In the first case, the name of the users file is listed with the line number, but in the second case it is not.

In the first case, wc knows that it is reading its input from users. In the second case, it only knows that it is its input from the standard input so it does not display the file name.

Here document in Unix / Linux

Here a document is used to redirect input into an interactive shell script or program.

We can run an interactive program in a shell script without the user's manipulation providing the required input for the program or the interactive shell script.

The general template for this document is:

 << command delimiter document delimiter 

Here, the shell interprets the << operator as an instruction to read the input until it finds a line containing the specified limit. All input that is above the line containing the limit is then given as the standard input of the command.

This limit tells the shell that here the document is done. Without it, the shell continues to read the input forever. The limit must be a single word that does not contain spaces or tabs.

Below is the input of the wc -l command to calculate the total number of lines.

 $ wc - l << EOF This is a simple lookup program cho good ( and sai ) restaurants trong Cape Town . $ 3 EOF 

You can use here document to print multiple lines using your script as follows:

 #! / bin / sh cat << EOF This is a simple lookup program cho good ( và sai ) restaurants trong Cape Town . EOF 

This code will produce the following result:

 Đây là một Lookup đơn chương trình này cho good ( và sai ) restaurants trong Cape Town . 

The following script runs an area with the Micro Text Editor and stores the input into the test.txt file.

 #! / bin / sh filename = test . txt vi $ filename << EndOfCommands i This file được tạo được tự động từ một script này ^ [ ZZ EndOfCommands 

If you run this script with vim that works similarly, then you will see the output like this:

 $ sh test . sh Vim : Cảnh báo : Đầu vào không phải từ một dòng $ 

After running the script, you will see the following output added to the test.txt file.

 $ cat test . txt tập tin này được tạo được tự động từ một $ shell script 

Remove the output in Unix / Linux

Sometimes you will need to run a command, but you do not want the output displayed on the screen. In such cases, you can remove the output by redirecting it into the file / dev / null:

 $ command > / dev / null 

Here, command is the name of the command you want to run. File / dev / null is a special file that automatically removes all its input.

To remove both the output of a command and the faulty output of the command, you use standard redirection to redirect STDERR to STDOUT.

 $ command > / dev / null 2 > & 1 

Here 2 represents STDERR and 1 represents STDOUT. You can display a message on STDERR by redirecting STDIN to STDERR as follows:

 $ echo message 1 > & 2 

The commands are redirected in Unix / Linux

Below is a complete list of commands that you can use to redirect:

Command Descriptionpgm> file The output of pgm is directed to the file. pgm pgm >> file The output of pgm is assigned to the file. n> file Output from the stream with the n sign being directed to the file. n >> file Output from stream with n assigned to file. n> & m Merging output from stream n with stream m. n <& m Merging input from stream n with stream m. << standard input tag comes from here via the next tag at the beginning of the line. | Get output from a program or a process and send it to another program .

Note that the 0 file signature is usually the standard input (STDIN), 1 is usually the standard output (STDOUT), and 2 is usually the standard error output (STAERR).

According to Tutorialspoint

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