How to Grep and Replace a String in Linux Source Code Using Command Line Tools

When working with source code, there are times when you need to search for a specific string or pattern across multiple files and replace it efficiently. Linux offers powerful command-line tools like grep and sed that allow you to do this quickly and effectively. These tools are widely used by developers, sysadmins, and power users because they provide a fast and reliable way to manipulate files and code.

In this post, we’ll show you how to use grep to find strings in your source code and how to use sed to replace those strings, all directly from the Linux terminal. We’ll keep things simple, with clear examples that you can easily apply to your projects.

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Why Use Grep and Sed?

• Grep: This command is used for searching text or patterns within files. It’s perfect for finding occurrences of a string in your source code.
• Sed: This is a stream editor, ideal for transforming text in files. You can use sed to search and replace strings within multiple files.

Together, these commands form a powerful duo for managing source code in Linux, especially when you need to make changes across large projects.

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Step 1: Search for a String in Your Source Code Using Grep

The first step is to search for the string or pattern you want to replace. To do this, we use the grep command. Let’s say you want to find all occurrences of the string foo in your source code directory.

Syntax:

grep -r "foo" /path/to/source

Example:

grep -r "foo" ~/projects/myapp/src

• -r: Recursively searches through all files and directories.
• "foo": The string or pattern you are searching for.
• /path/to/source: The directory containing your source code.

This will output a list of all the files and the lines where the string foo is found.

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Step 2: Replace a String Using Sed

Once you’ve identified where the string appears, you can replace it using sed. Here’s how you can use sed to search for a string and replace it with another string.

Syntax:

sed -i 's/old_string/new_string/g' filename

• -i: Edits the file in place.
• s/old_string/new_string/g: Substitutes every occurrence of old_string with new_string globally (i.e., throughout the entire file).
• filename: The file in which the replacement will occur.

Example:
Let’s say you want to replace all occurrences of foo with bar in a specific file:

sed -i 's/foo/bar/g' ~/projects/myapp/src/main.c

This will replace all instances of foo with bar in the file main.c.

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Step 3: Search and Replace Across Multiple Files

If you need to replace a string in multiple files, you can combine grep and sed. First, use grep to find all the files that contain the string, and then pipe the results into sed to perform the replacement.

Example:
To replace the string foo with bar in all .c files in your project:

grep -rl "foo" ~/projects/myapp/src | xargs sed -i 's/foo/bar/g'

• -r: Recursively searches all files and directories.
• -l: Outputs only the file names that contain the string.
• xargs: Takes the list of files output by grep and passes them to sed.

This command will search for foo in all the files in the src directory, and replace it with bar in each file that contains it.

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Step 4: Replace a String in Multiple File Types

If your project contains different file types (e.g., .c, .h, .cpp), you can use grep to search and replace in specific file types. Here’s how you can replace a string in all .c and .h files:

Example:

grep -rl --include=\*.{c,h} "foo" ~/projects/myapp/src | xargs sed -i 's/foo/bar/g'

• --include=\*.{c,h}: Limits the search to .c and .h files.
• sed -i 's/foo/bar/g': Replaces foo with bar.

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Step 5: Perform a Dry Run to Test the Command

Before making actual changes to your files, it’s a good idea to perform a “dry run” using sed without the -i flag. This will show the changes without editing the files.

Example:

grep -rl "foo" ~/projects/myapp/src | xargs sed 's/foo/bar/g'

This will display the potential changes in the terminal without altering the files. Once you’re confident with the output, you can rerun the command with the -i flag to apply the changes.

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Example Use Case: Updating Deprecated Function Names

Imagine you have a large project, and a certain function oldFunction() has been deprecated. You need to update all instances of oldFunction() to newFunction() across your codebase.

Step 1: Find All Instances:

grep -r "oldFunction()" ~/projects/myapp/src

Step 2: Replace with New Function:

grep -rl "oldFunction()" ~/projects/myapp/src | xargs sed -i 's/oldFunction()/newFunction()/g'

This will ensure that all instances of oldFunction() are replaced with newFunction().

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Using grep and sed in combination is a powerful way to manage and manipulate strings in your source code. These commands allow you to search for specific patterns and replace them across multiple files quickly and efficiently. Whether you’re cleaning up your code, updating function names, or fixing typos, mastering these tools can save you significant time and effort.