How to Replace String in Bash [6 Best Ways]

Akshat

UX/UI Designer at - Adobe

Akshat is a software engineer, product designer and the co-founder of Scrutify. He's an experienced Linux professional and the senior editor of this blog. He...Read more

Bash is a popular Unix shell that offers powerful tools for string manipulation, such as modifying, extracting, or replacing parts of a string in a text file. This is useful when Linux programmers need to find and replace string in Bash in a variety of scenarios, such as data processing, system administration, log analysis, automation tasks, and more. So, in this comprehensive guide, you’ll get to know various techniques to replace strings in Bash that may come in handy to you as a Linux programmer.

How to Replace String in Bash [6 Easy Ways]

To replace string in Bash, you can use sed, awk, tr, and regex methods. Additionally, built-in string manipulation functions and Bash scripting can be used for automated string replacement. Here, I’ll provide step-by-step instructions for each of these methods:

1. sed Command

sed (stream editor) is a powerful text processing tool in Unix-based systems, primarily used for text substitution. Here’s how to use it:

1. Launch the Terminal app and execute the command below to read the text file:

cat filename

2. Now, run the command below to replace string in Bash.

sed 's/original/replacement/g' input-file > output-file

3. This command will replace all occurrences of “original” with “replacement”. Now, you’ll see the following output using the cat command:

2. awk Command

awk is a versatile text processing tool that operates with patterns and actions. You can use this command to replace string in Bash in the following way:

1. Execute the command below to read the text file in the Terminal window:

cat filename

2. Run the following command in the Linux command prompt:

awk '{gsub(/original/, "replacement")}1' input-file > output-file

2. Now, you’ll see all occurrences of “string 1” replaced with “string 2”:

3. tr Command

tr (translate) is a command for character-level translations and deletions. Follow these steps to use it to replace string in Bash:

1. Run the cat command in the command prompt to read the file.

cat filename

2. Enter the following command in the Terminal window:

tr 'original' 'replacement' < input-file > output-file

2. tr 'aeiou' 'AEIOU' < text.txt > text_modified.txt will find and replace all lowercase vowels with their uppercase counterparts. Here is the output:

4. Bash Built-In Functions

Bash provides built-in string manipulation functions using parameter expansion, substring replacement, and case modification. To replace string in Bash, try these built-in functions:

1. Save the string “The quick brown fox jumps over the lazy dog” in a variable named “myString” with the following command:

myString="The quick brown fox jumps over the lazy dog"

2. Now, you can use the variable “myString” in other commands to manipulate or display its contents. To confirm the variable value, run the echo $myString command in the Linux command prompt:

3. To replace only the first occurrence of a substring “brown” with “red” in the string, you can use the following command:

echo ${myString/brown/red}

2. The output would be:

3. You can also run the following command to replace all occurrences of a substring:

echo ${myString//e/a}

3. If done correctly, it’ll replace string in Bash, and you’ll get the output in Terminal:

4. You can also convert a string to uppercase with the following command:

echo ${myString^^}

5. You’ll get the following output in the Terminal window.

6. Similarly, if you want to replace string in Bash with its lowercase, run the following command:

echo ${myString,,}

7. The output of this command would be:

5. Regular Expressions

Regular expressions (regex) are a powerful way to define search patterns in text. It is a sequence of characters that specifies a search pattern. It can be used to match, find, or replace specific strings within a text. Here’s how you can use it:

1. Create and save the text file with the following content:

123456789
The quick brown fox jumps over the 9 lazy dogs.

2. Launch the Terminal app and execute the cat command to confirm the text file content.

3. Now, use the sed command below to replace all digits with an ‘X’:

sed 's/[0-9]/X/g' input-file > output-file

4. Use the cat command to see the output in your Terminal window.

5. Alternatively, you can use the awk command to replace all digits with an ‘X’:

awk '{gsub(/[0-9]/, "X")}1' input-file > output-file

6. You’ll see the following output using the cat command:

6. Bash Scripting and Automation

You can create reusable scripts and automate the string replacement tasks to replace string in Bash. This can save time and reduce errors. To do so, follow these steps:

1. Create a Bash script file:

touch replace_string.sh

2. Execute the nano command with the filename.

nano replace_string.sh

3. Add the following content to the script:

#!/bin/bash
input_file="$1"
search_string="$2"
replacement_string="$3" 
output_file="$4"
sed "s/$search_string/$replacement_string/g" "$input_file" > "$output_file"  

This script takes four arguments: the input file name, the search string, the replacement string, and the output file name. It uses the sed command to replace all occurrences of the search string with the replacement string in the input file and saves the output to the specified output file.

3. Press Ctrl + O and Enter to save. Then, press Ctrl + X to exit the Nano text editor.

4. Make the script executable:

chmod +x replace_string.sh

4. Execute the cat command to read the input file.

cat filename

5. Run the script in the Terminal window:

./replace_string.sh input-file search-string replacement-string output-file

6. Run the cat command to see the output:

4 Quick Tips to Replace String in Bash Efficiently

To replace string in Bash efficiently, use optimized techniques to do it. This is especially important when handling large datasets and complex automation processes. Consider the following four tips to optimize your string replacement techniques and improve your Bash scripting skills:

• 🚀 Speed considerations: For simple string replacement tasks, use built-in Bash functions, as they are generally faster than external tools like sed and awk. However, you may still have to use sed or awk over Bash built-in functions for large files, as they handle the memory more efficiently.
• 🧪 Test the string replacement: Always test your string replacement commands and scripts on sample data before applying them to critical files or production environments. Keep your scripts modular and reusable to reduce the likelihood of errors and improve maintainability.
• 💾 Memory usage: Be mindful of memory usage when working with large files or data sets. sed and awk can be more memory-efficient than built-in Bash functions for such tasks.
• 📄 Print specific lines: Use the -n option with sed to disable automatic printing of output, and then use the p command to print only specific lines that match your search pattern. This can save time and resources when working with large files.

In Conclusion

By now, you should have a solid understanding of various techniques to replace string in Bash using the sed, awk, tr, and regex methods. You can further explore Bash string manipulation to help you automate tasks and streamline your workflow.

However, your learning journey in Bash scripting doesn’t end here. There are many other aspects to explore, such as improving code readability with echo and using set x for debugging. You can also look into multiple ways to clear Bash history to experience efficient Bash performance. Continuously learning and growing will lead to success in the world of Unix shell scripting.

Frequently Asked Questions

AkshatUX/UI Designer at - Adobe

Akshat is a software engineer, product designer and the co-founder of Scrutify. He's an experienced Linux professional and the senior editor of this blog. He is also an open-source contributor to many projects on Github and has written several technical guides on Linux. Apart from that, he’s also actively sharing his ideas and tutorials on Medium and Attirer. As the editor of this blog, Akshat brings his wealth of knowledge and experience to provide readers with valuable insights and advice on a wide range of Linux-related topics.

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