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In this article, we will discuss standard output and standard error in Linux. We will see how to redirect standard output and standard error in Bash.

Let us start by understanding the terms standard output and standard error in Linux.

Standard output and standard error in Linux

In Linux, commands take some input from the user which can be a file or any attribute. When we execute these commands, they give us some output of our input which is called standard output.

This standard output can be either success or error. If the output we get is an error, we call it standard error.

Both will be displayed on our terminal screen. Sometimes, we want to store standard output and standard error in files for testing or debugging code.

In Linux, we can redirect both the output into a file, the process of storing it is called redirection.

In Linux, for every process we have three data streams, standard input (stdin), standard output (stdout) and standard error (stderr).

Standard input (stdin) takes input from the user through the keyboard. Standard output (stdout) displays output on the terminal screen.

Standard error (stderr) displays error messages on the screen. By default, both standard output and standard error are printed on the terminal screen.

In Linux, each data stream has a unique numeric ID:

• For standard input (stdin), the numeric ID is 0.
• For standard output (stdout), the numeric id is 1.
• For standard error (stderr), the numeric ID is 2.

Let's explain the redirection of standard output and standard error in more detail.

Redirecting standard output to a file in Bash

In Linux, we can redirect the standard output to a file using numeric ID. To redirect the standard output of any command, we use the redirection operation 1 and the > symbol.

In our example, we will use the ls command. We will redirect the output of the ls command to a file.

We will then look at the text file to see the output of the ls command stored in the file.

Sample code:

$ ls 1> stdout.txt

To view the file, we use the following command:

$ cat stdout.txt

As shown in the above terminal screen, we have used ls 1> stdout.txt command in the terminal. This command stores the output of the ls command into a file named stdout.txt.

We use ls command in Linux to list all the files and directories. So we redirect the output of ls command into a file.

When we view the file using cat command, we can see that the standard output of ls command is stored in the file. This is how we can redirect the standard output of any command to a file using its numeric id.

Redirecting standard error to a file in Bash

We can also redirect standard error to a file using its numeric id. To redirect standard error to a file, we use its numeric id 2 with the redirection operator >.

We can do this using the following command in the terminal.

Sample code:

$ cat file.txt 2> stderr.txt

To view a file called stderr.txt, we use the following command:

$ cat stderr.txt

As we can see, we have used cat file.txt 2> stderr.txt command in the terminal. Since there is no file named file.txt in the current directory, the cat command will give the error which we have appended in the stderr.txt file.

When we view the stderr.txt file using the cat command, we can see that it displays the error No such file or directory. This means that we have redirected the standard error of the cat command to a file named stderr.txt.

This is how we redirect the standard error of any command to a file.

Redirecting standard output and standard error to files in Bash

We can also redirect both the outputs into a file using a single command. For this, we use the following command.

Sample code:

$ ls 1> stdout.txt 2> stderr.txt

To view the stdout.txt file, we use the following command:

$ cat stdout.txt

To view the stderr.txt file, we use the following command:

$ cat stderr.txt

As we can see, we redirected both standard output and standard error to files in a single command. The output of ls command will be stored in the file stdout.txt while stderr.txt fill will remain empty as no error will occur.

This is how we redirect both standard output and standard error to a file with one command.

We hope this Linux article helped you understand how to redirect standard output and standard error of any command to a file in Bash.