What is stdin and stdout in linux

Understanding stdin, stderr and stdout in Linux

There is a decent chance that if you have used Linux operating systems then you might have encountered the three famous data streams known as stdin, stderr and stdout. All these are different in their functions and have their own uses but one thing common between all three of them is that they are data streams that bash creates.

Let’s understand more about what data streams actually mean and how they are beneficial. In terms of computing, a data stream is something that gives us the ability to transfer data from a source to an outflow and vice versa. The source and the outflow are the two end points of the data stream. It might be interesting for you to know that whatever command you are running in your Linux terminal, it will either be at one of these end points.

Now we know a little about the data streams, let’s know more about the three famous data streams.

• stdin − It stands for standard input, and is used for taking text as an input.
• stdout − It stands for standard output, and is used to text output of any command you type in the terminal, and then that output is stored in the stdout stream.
• stderr − It stands for standard error. It is invoked whenever a command faces an error, then that error message gets stored in this data stream.

It should be noted that in Linux all these streams are treated as if they were files. Also, linux assigns unique values to each of these data streams.

Now let’s consider a few examples of these three data streams.

The example shown below depicts a typical stdin stream.

Command

Output

mmukul@192 Docs-Linux % read This is to stdin

In the above command, we are providing an input to the stream and the read tool is getting the input from stdin.

Now, an example of stdout is shown below −

Command

Output

immukul@192 Downloads % ls -ltr total 1085456 drwxr-xr-x@ 13 immukul staff 416 Dec 7 2019 source-code-pro-release -rw-r--r--@ 1 immukul staff 350337 Dec 22 2019 messi.jpg -rw-r--r--@ 1 immukul staff 5953321 Dec 22 2019 927225.png -rw-r--r--@ 1 immukul staff 601852 Dec 22 2019 238870.jpg . . .

We know that we make use of the ls command with the -ltr flag to list all the files in a certain sorted fashion, where the lastly updated file is shown at the bottom. The list is sent to the stdout data stream and the terminal then simply prints it out.

Now, an example of stderr is shown below −

Command

The above command is invalid as I don’t have any directory named printit and it will generate an error message that will be sent to stderr and then the terminal will print it.

Output

immukul@192 Downloads % ls -ltr printit ls: printit: No such file or directory

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How to Use the Stdin, Stderr, and Stdout Streams in Bash

Three streams are opened when the Linux operating system starts. These are stdin, stdout, and stderr. The full form of stdin is standard input which is used to take an input from the user. The full form of stdout is the standard output which is used to store the output of the command into the stdout stream. The full form of stderr is the standard error which is used to store the error message that is generated from any command into the data stream. The uses of these streams are shown in this tutorial. The corresponding numerical identifier values of stdin, stdout, and stderr are 0, 1, and 2.

Redirection Operators of Stdin, Stdout, and Stderr

Uses of Stdin, Stdout, and Stderr

The uses of stdin, stdout, and stderr are shown in this part of the tutorial using multiple examples.

Example 1: Use of Stdin

The method of taking the content of a file and printing it in the terminal is shown in this example.

Run the following “cat” command to create a text file named testdata.txt with some content:

Run the following “cat” command to append some content into the testdata.txt file:

Run the following “cat” command to take an input from the testdata.txt file and print it into the terminal:

The following output appears after executing the previous commands after adding the string, “linuxhint.com”, and “Scripting Language” into the testdata.txt file:

Example 2: Use of Stdout

The method of creating a file using pipe (|) and redirection operator is shown in this example.

Run the following command to write a string data into the text file named testdata2.txt by piping. The output of the “echo” command is sent to the input of the “cat” command using the pipe (|) operator:

Run the following “cat” command to check the content of the testdata2.txt file:

The following output appears after executing the previous commands. According to the output, the output of the “echo” command is written into the testdata2.txt file:

Run the following command to write the output of the “ls –l” command into a text file named list.txt using the redirection operator (‘>’):

Run the following “cat” command to check the content of the list.txt file:

The following output appears after executing the previous commands. According to the output, the output of the “ls –l” command is written into the list.txt file:

Example 3: Use of Stdin and Stdout

The method of using both stdin and stdout to take an input from a file and write it into a file is shown in this example.

Run the following “cat” command to take the content of the testdata.txt file and write it into the testfile.txt file and the terminal:

Run the following “cat” command to print the content of the testdata.txt file:

Run the following “cat” command to print the content of the testfile.txt file:

The following output appears after executing the previous commands. According to the output, the content of the testdata.txt file is written into the testfile.txt file and printed in the terminal:

Example 4: Use of Stderr

The content of the standard error can be printed in the terminal or redirected into a file or sent to the /dev/null that works like the recycle bin. The different ways to pass the standard error are shown in this example.

The following command is valid and it prints the “Hello” string with the newline. So, no standard error is generated for the following command:

The following command is invalid because there is no command named “pirntf”. So, a standard error is generated and the error is printed in the terminal:

The following output appears after executing the previous command. According to the output, the standard error is printed in the terminal:

Sometimes, it requires printing the custom error by hiding the standard error to make the error more understandable for the users. This task can be done by redirecting the error into the /dev/null. The “2>” is used here to redirect the error into /dev/null.

Run the following command to redirect the error into the /dev/null which will not display any error if any error exists:

The following output appears after executing the previous command. According to the output, the standard error is not printed in the terminal to redirect to /dev/null:

Sometimes, the standard error requires storing into a file for future use. This task can be done by redirecting the error into a file using the “2>” operator.

Run the following command to redirect the standard error into a file named errorfile.txt:

Run the following command to check the content of the errorfile.txt file:

The following output appears after executing the previous commands. According to the output, the standard error is written properly into the errorfile.txt file:

The standard error can be redirected to both /dev/null and the errorfile.txt file using the following command:

Conclusion

The uses of stdin, stdout, and stderr are explained in this tutorial using multiple examples that will help the Linux users to understand the concept of these streams and use them properly when required.

About the author

Fahmida Yesmin

I am a trainer of web programming courses. I like to write article or tutorial on various IT topics. I have a YouTube channel where many types of tutorials based on Ubuntu, Windows, Word, Excel, WordPress, Magento, Laravel etc. are published: Tutorials4u Help.

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What are stdin, stderr and stdout in Bash

stdin: Stands for standard input. It takes text as input. stdout: Stands for standard output. The text output of a command is stored in the stdout stream. stderr: Stands for standard error.

1. What is difference between stdout stdin and stderr?
2. What is stdout and stderr in bash?
3. What is bash stderr?
4. What are stdin stdout and stderr in C?
5. What is stdout in bash?
6. Where is stdout file in Linux?
7. How do I redirect stderr and stdout in bash?
8. How do I redirect stderr?
9. How do I redirect a bash error?
10. What is the stdout in Linux?
11. What is fprintf stderr?
12. How do I redirect stderr to a variable in bash?

What is difference between stdout stdin and stderr?

If my understanding is correct, stdin is the file in which a program writes into its requests to run a task in the process, stdout is the file into which the kernel writes its output and the process requesting it accesses the information from, and stderr is the file into which all the exceptions are entered.

What is stdout and stderr in bash?

The Linux Standard Streams

Text output from the command to the shell is delivered via the stdout (standard out) stream. Error messages from the command are sent through the stderr (standard error) stream.

What is bash stderr?

Stderr, also known as standard error, is the default file descriptor where a process can write error messages. In Unix-like operating systems, such as Linux, macOS X, and BSD, stderr is defined by the POSIX standard. . In the terminal, standard error defaults to the user’s screen.

What are stdin stdout and stderr in C?

In computer programming, standard streams are interconnected input and output communication channels between a computer program and its environment when it begins execution. The three input/output (I/O) connections are called standard input (stdin), standard output (stdout) and standard error (stderr).

What is stdout in bash?

stdout: Stands for standard output. The text output of a command is stored in the stdout stream. stderr: Stands for standard error. Whenever a command faces an error, the error message is stored in this stream.

Where is stdout file in Linux?

In Linux, you can generally find stdin through the /proc file system in /proc/self/fd/0 , and stdout is /proc/self/fd/1 .

How do I redirect stderr and stdout in bash?

To redirect stderr and stdout , use the 2>&1 or &> constructs.

How do I redirect stderr?

1. Redirect stdout to one file and stderr to another file: command > out 2>error.
2. Redirect stdout to a file ( >out ), and then redirect stderr to stdout ( 2>&1 ): command >out 2>&1.

How do I redirect a bash error?

1. To redirect stderr (standard error) to a file: command 2> errors.txt.
2. Let us redirect both stderr and stdout (standard output): command &> output.txt.
3. Finally, we can redirect stdout to a file named myoutput.txt, and then redirect stderr to stdout using 2>&1 (errors.txt):

What is the stdout in Linux?

Stdout, also known as standard output, is the default file descriptor where a process can write output. In Unix-like operating systems, such as Linux, macOS X, and BSD, stdout is defined by the POSIX standard. Its default file descriptor number is 1. In the terminal, standard output defaults to the user’s screen.

What is fprintf stderr?

fprintf(stderr,»»); Prints whatever is provided within the quotes, to the console. Where, stdout and stderr are both output streams. stdout is a stream where the program writes output data. stderr is an output stream typically used by programs to output error messages or diagnostics to the console.

How do I redirect stderr to a variable in bash?

To store stderr into a variable we need to use command substitution. But, by default, command substitution only catches the standard output(stdout). To capture stderr we need to use 2>&1 redirector. Following example, will store both stdout and stderr into the $VAR variable.

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