Which of the following symbols could be used to redirect stdout?

Learn how to process the output of shell commands within a script and send it to files, devices, or other commands or scripts.

Posted: April 21, 2022 |%t min read| by Roberto Nozaki (Sudoer, Red Hat)

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Photo by Isaque Pereira from Pexels

Shell scripts provide a very powerful feature: the ability to redirect the output from commands and scripts and send it to files, devices, or even as input to other commands or scripts.

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This article focuses on command and script output.

Types of output

Commands and scripts in a shell can generate two basic types of outputs:

• STDOUT: The normal output from a command/script (file descriptor 1)
• STDERR: The error output from a command/script (file descriptor 2)

By default, STDOUT and STDERR are sent to your terminal's screen.

In terms of input, STDIN by default reads input from the keyboard (file descriptor 0). A file descriptor is a unique identifier for a file or other I/O resource.

How to redirect shell output

There are multiple ways to redirect output from shell scripts and commands.

1. Redirect STDOUT

For the following examples, I will use this simple set of files:

$ls -la file*
-rw-r--r--. 1 admin2 admin2  7 Mar 27 15:34 file1.txt
-rw-r--r--. 1 admin2 admin2 10 Mar 27 15:34 file2.txt
-rw-r--r--. 1 admin2 admin2 13 Mar 27 15:34 file3.txt

I'm running a simple ls commands to illustrate STDOUT vs. STDERR, but the same principle applies to most of the commands you execute from a shell.

I can redirect the standard output to a file using

$ls file* > my_stdout.txt

$ cat my_stdout.txt 
file1.txt
file2.txt
file3.txt

0:

$ls file* > my_stdout.txt

$ cat my_stdout.txt 
file1.txt
file2.txt
file3.txt

Next, I run a similar command, but with a

$ls file* > my_stdout.txt

$ cat my_stdout.txt 
file1.txt
file2.txt
file3.txt

1 before

$ls file* > my_stdout.txt

$ cat my_stdout.txt 
file1.txt
file2.txt
file3.txt

2. Redirecting using the

$ls file* > my_stdout.txt

$ cat my_stdout.txt 
file1.txt
file2.txt
file3.txt

2 signal is the same as using

$ls file* > my_stdout.txt

$ cat my_stdout.txt 
file1.txt
file2.txt
file3.txt

4 to do so: I'm telling the shell to redirect the STDOUT to that file. If I omit the file descriptor, STDOUT is used by default. I can prove this by running the

$ls file* > my_stdout.txt

$ cat my_stdout.txt 
file1.txt
file2.txt
file3.txt

5 command to show the output of both commands side by side:

$ls file* 1> my_other_stdout.txt

$sdiff my_stdout.txt my_other_stdout.txt
file1.txt			             file1.txt
file2.txt			             file2.txt
file3.txt			             file3.txt

As you can see, both outputs have the same content.

2. Redirect STDERR

Now, what is special about STDERR? To demonstrate, I will introduce an error condition to the previous example with

$ls file* > my_stdout.txt

$ cat my_stdout.txt 
file1.txt
file2.txt
file3.txt

6:

Here's the result:

Image

(Roberto Nozaki, CC BY-SA 4.0)

Here are some observations from the above test:

1. The output about the existing files is sent correctly to the destination file.
2. The error (which displays when I try to list something that does not exist) is sent to the screen. This is the default place errors are sent unless you redirect them.

[ Download a Bash Shell Scripting Cheat Sheet. ]

Next, I'll redirect the error output by referencing file descriptor 2 explicitly with  

$ls file* > my_stdout.txt

$ cat my_stdout.txt 
file1.txt
file2.txt
file3.txt

7

Image

(Roberto Nozaki, CC BY-SA 4.0)

In the example above:

1. The ls command does not display the error message on the screen like before.
2. The normal output contains what I expect.
3. The error message is sent to the

   $ls file* > my_stdout.txt
   
   $ cat my_stdout.txt 
   file1.txt
   file2.txt
   file3.txt

   9 file.

3. Send STDOUT and STDERR to the same file

Another common situation is to send both STDOUT and STDERR to the same file:

$ls file* non-existing-file* > my_consolidated_output.txt 2>&1

$ cat my_consolidated_output.txt 
ls: cannot access 'non-existing-file*': No such file or directory
file1.txt
file2.txt
file3.txt

In this example, all output (normal and error) is sent to the same file.

The

$ls file* 1> my_other_stdout.txt

$sdiff my_stdout.txt my_other_stdout.txt
file1.txt			             file1.txt
file2.txt			             file2.txt
file3.txt			             file3.txt

0 construction means "send the STDERR to the same place you are sending the STDOUT."

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4. Redirect output, but append the file

In all the previous examples, whenever I redirected some output, I used a single

$ls file* > my_stdout.txt

$ cat my_stdout.txt 
file1.txt
file2.txt
file3.txt

2, which means "send something to this file, and start the file from scratch." As a result, if the destination file exists, it is overwritten.

If I want to append to an existing file, I need to use

$ls file* 1> my_other_stdout.txt

$sdiff my_stdout.txt my_other_stdout.txt
file1.txt			             file1.txt
file2.txt			             file2.txt
file3.txt			             file3.txt

2. If the file doesn't already exist, it will be created:

$echo "Adding stuff to the end of a file" >> my_output.txt

$cat my_output.txt 
file1.txt
file2.txt
file3.txt
Adding stuff to the end of a file

5. Redirect to another process or to nowhere

The examples above cover redirecting output to a file, but you can also redirect outputs to other processes or 

$ls file* 1> my_other_stdout.txt

$sdiff my_stdout.txt my_other_stdout.txt
file1.txt			             file1.txt
file2.txt			             file2.txt
file3.txt			             file3.txt

3.

Sending output to other processes is one of the most powerful features of a shell. For this task, use the

$ls file* 1> my_other_stdout.txt

$sdiff my_stdout.txt my_other_stdout.txt
file1.txt			             file1.txt
file2.txt			             file2.txt
file3.txt			             file3.txt

4 (pipe) symbol, which sends the output from one command to the input of the next command:

ps -ef | grep chrome | grep -v grep | wc -l
21

The above example lists my processes, filters any that contain the string chrome, ignores the line about my

$ls file* 1> my_other_stdout.txt

$sdiff my_stdout.txt my_other_stdout.txt
file1.txt			             file1.txt
file2.txt			             file2.txt
file3.txt			             file3.txt

5 command, and counts the resulting lines. If I want to send the output to a file, I add 

$ls file* > my_stdout.txt

$ cat my_stdout.txt 
file1.txt
file2.txt
file3.txt

2 and a file name to the end of the chain.

Finally, here is an example where I want to ignore one of the outputs, the STDERR:

$tar cvf my_files.tar file* more-non-existing*
file1.txt
file2.txt
file3.txt
tar: more-non-existing*: Cannot stat: No such file or directory
tar: Exiting with failure status due to previous errors

Because the

$ls file* 1> my_other_stdout.txt

$sdiff my_stdout.txt my_other_stdout.txt
file1.txt			             file1.txt
file2.txt			             file2.txt
file3.txt			             file3.txt

7 command did not find any files with names starting with

$ls file* 1> my_other_stdout.txt

$sdiff my_stdout.txt my_other_stdout.txt
file1.txt			             file1.txt
file2.txt			             file2.txt
file3.txt			             file3.txt

8, some error messages display at the end.

Suppose I create some scripts and don't care about seeing or capturing these errors (I know, in real life, you should prevent and handle the errors, not just ignore them):

$tar cvf my_files.tar file* more-non-existing* 2> /dev/null
file1.txt
file2.txt
file3.txt

The

$ls file* 1> my_other_stdout.txt

$sdiff my_stdout.txt my_other_stdout.txt
file1.txt			             file1.txt
file2.txt			             file2.txt
file3.txt			             file3.txt

9 is a special device file that is like a "black hole": What you send there just disappears.

[ Download this guide to installing applications on Linux. ]

6. Use redirection in a script

Image

(Roberto Nozaki, CC BY-SA 4.0)

=== SUMMARY OF INVESTIGATION OF chrome ===
Date/Time of the execution: 2022-03-25 18:05:50
Number of processes found.: 5
PIDs:
1245475
1249558
1316941
1382460
1384452

More Linux resources

• Advanced Linux commands cheat sheet
• Download RHEL 9 at no charge through the Red Hat Developer program
• A guide to installing applications on Linux
• Linux system administration skills assessment

This very simple script does the following:

• Line 3: Executes a command in the operating system and saves to the variable DATE_TIME.
• Line 6: Runs the

  $ls file* non-existing-file* > my_consolidated_output.txt 2>&1
  
  $ cat my_consolidated_output.txt 
  ls: cannot access 'non-existing-file*': No such file or directory
  file1.txt
  file2.txt
  file3.txt

  0 command and redirects to

  $ls file* 1> my_other_stdout.txt
  
  $sdiff my_stdout.txt my_other_stdout.txt
  file1.txt			             file1.txt
  file2.txt			             file2.txt
  file3.txt			             file3.txt

  5 and to a file.
  • Instead of sending the output to a file, I could send it to a variable (as in line 3), but in this case, I want to run other actions using the same output, so I capture it. In a more realistic situation, having the file could also be useful during development or troubleshooting, so I can more easily investigate what the command generates.
• Line 8: Runs additional commands, redirects the outputs to

  $ls file* non-existing-file* > my_consolidated_output.txt 2>&1
  
  $ cat my_consolidated_output.txt 
  ls: cannot access 'non-existing-file*': No such file or directory
  file1.txt
  file2.txt
  file3.txt

  2, and assigns the result to a variable.
• Line 9: Uses

  $ls file* non-existing-file* > my_consolidated_output.txt 2>&1
  
  $ cat my_consolidated_output.txt 
  ls: cannot access 'non-existing-file*': No such file or directory
  file1.txt
  file2.txt
  file3.txt

  3 to select only column 2 from the output and sorts it in descending order (just for the sake of adding another pipe).

Wrap up

Those were some examples of redirecting STDOUT and STDERR. Putting all this together, you realize how powerful redirection can be. By chaining individual commands, manipulating their output, and using the result as the input for the next command, you can perform tasks that otherwise could require you to develop a script or program. You could also incorporate the technique into other scripts, using everything as building blocks.

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Roberto Nozaki

Roberto Nozaki (RHCSA/RHCE/RHCA) is an Automation Principal Consultant at Red Hat Canada where he specializes in IT automation with Ansible. More about me

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Which symbol is used for redirect?

Redirection is done using either the ">" (greater-than symbol), or using the "|" (pipe) operator which sends the standard output of one command to another command as standard input.

What is the meaning of 2 >& 1?

The expression 2>&1 copies file descriptor 1 to location 2 , so any output written to 2 ("standard error") in the execution environment goes to the same file originally described by 1 ("standard output").

Which symbol is used for standards output stdout redirection?

Redirecting Standard Error and Other Output Generally, when a command starts, three files are already open: stdin (standard input), stdout (standard output), and stderr (standard error). If you want to redirect standard input or standard output, you can use the <, >, or > > symbols.

Which of the following is not a redirection symbol A B C * D >>?

* is not a redirection symbol. In general, a redirection operator is a special character that can be used with a command, like DOS command or a Command Prompt command to either redirect the input to the command as well as the output from the command. Command inputs as well as outputs are called command handles.