What is Output Redirection in Bash [With 4 Practical Cases]

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In Bash, Output redirection is a powerful feature that allows you to control where the output of commands or scripts is sent. By default, any command output (standard output or stdout) is displayed on the terminal. However, with output redirection, you can send the output to a file, append it to an existing file, or even merge it into a file. In this article, I will discuss the basics of Bash output redirection & then show some practical Bash script examples of it for further explanation.

Key Takeaways

• Learning basics of Bash output redirection.
• Getting a practical overview of the process with Bash script examples.
• Learning the importance of the output redirection process in Bash scripting.

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4 Practical Cases of Bash Output Redirection

The fundamental method for altering the source or destination of a program’s file descriptors is through Bash redirection. In the following article, I will show how you can use output redirection in different practical scenarios while generating Bash scripts.

Case 1: Redirecting Output to a File in Bash

You can redirect the stdout of a command to a file using the output redirection procedure. Now if the file doesn’t exist, it will be created. If it exists, its contents will be overwritten. Please follow the below steps to check the entire process yourself:

Steps to Follow >

❶ At first, open your Ubuntu Terminal application.

❷ Now, open a file, let’s say, named ‘file.sh’ in the nano editor by using the following command:

nano file.sh

➌ Next, write the following script inside the Bash file in the nano text editor:

#! /bin/bash

echo “Hello World!” > Output.txt

➍ Then, press CTRL+S to save the file & press CTRL+X to exit the nano editor.

➎ After that, use the following command to make the script executable:

chmod u+x file.sh

➏ Finally, run the script by the following command:

./file.sh

From the output image, you can see after executing the script it displays no output as it is redirected to the ‘Output.txt’ file. However, later I viewed that file using the cat command. And you can see the output string ‘Hello World!’ inside it.

Case 2: Appending Output to a File in Bash

Normally, when you redirect a stdout to a file with the ‘>’ redirection operator, it overwrites the file. But if you want, you can append the stdout to the existing file contents while redirecting by using the ‘>>’ redirection operators. Check out the following script:

Script (append.sh) >

#! /bin/bash

echo "Hello World!" >> Output.txt

Now, run the script by the following command:

./append.sh

The script appends the output of the script to the ‘Output.txt’ file. As you can see from the output of the ‘cat command’.

Case 3: Redirecting Output to “/dev/null”

While working in Bash scripts, you may need to suppress your certain command output instead of redirecting it inside a file. Just to check if the command works or maybe for a cleaner terminal you want to discard the output. Now the simplest and most common approach to suppress output in Bash is by redirecting it to the null device, ‘/dev/null’. This special file in Unix-like systems discards all data written to it.

Check out the following script, where I will perform the execution of the curl command in silence while suppressing its output to the null device.

Script (discard.sh) >

#! /bin/bash

url="https://linuxsimply.com"

if curl -s "$url" > /dev/null; then #Checks the URL request validity
   echo "Request succeeded"
else
   echo "Request failed"
fi

Now, run the script by the following command:

./discard.sh

Executing the script only gives the “Request succeeded” message. While it suppresses all the output messages of the curl command.

Case 4: Redirecting Output & Input Simultaneously

Redirecting input is a process where the standard input (stdin) of a command is changed from the keyboard to a  file. By, using the ‘<’ symbol you can take input data from a file instead of typing it interactively. In this case, let’s see how input & output redirection works concurrently.

Script (combo.sh) >

#! /bin/bash

sort <input.txt> sorted_output.txt

Now, run the script by the following command:

./combo.sh

From the image, first, you can see the contents of the input.txt file, and after executing the script the sorted output of the input.txt file which was redirected to the sorted_output.txt file.

Importance of Bash Output Redirection

Bash output redirection is a fundamental concept in shell scripting and command-line usage that provides several benefits and use cases. Here are some key reasons highlighting the importance of Bash output redirection:

Conclusion

To sum up, output redirection is a powerful technique in Bash that allows you to control where command outputs are directed. By using various redirection operators and techniques, you can save, discard, or manage command output as needed. This writing covered the basics of output redirection and provided practical cases to help you get started!

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