How to Remove Subsequent Lines in a File After a Matching Pattern

1. Introduction

When working on large log files or data sets, we often come across irrelevant information that adds no value and can slow down the analysis process. To address this issue, we can remove subsequent lines after finding a matching pattern to focus solely on the relevant data. For example, when checking error logs, we may want to focus on the context around a particular error message. This approach not only streamlines the analysis but also enables us to identify the root cause more efficiently.

In this tutorial, we’ll learn multiple methods for removing subsequent lines in a file after finding a matching pattern. These methods include awk, sed, and Python code.

2. Understanding the Scenario

For this tutorial, we use a text file named input_file:

$ cat input_file
This is line1
This is line2
desired error
This is line4
This is line5

Notably, we set this as the sample content for consistency in the output of each method.

The third line of the output_file file, i.e., desired error, is used as our matching criteria:

$ cat output_file
This is line1 
This is line2 
desired error

In this sample output, we can see that all the subsequent lines have been removed after a matching pattern is found.

3. Using the awk Command

The awk command is a powerful text-processing tool that we can use in Linux and other Unix-like operating systems. Additionally, we can employ it to analyze text files for data extraction and reporting.

The awk tool is also proficient in manipulating data, such as removing subsequent lines after finding a matching pattern:

$ awk '/desired error/{print; exit} 1' input_file > output_file

Now, let’s understand the command:

• /desired error/: pattern to be matched, enclosed in forward slashes (/)
• {print; exit} 1: this block runs if the matching pattern is found and prints the content up to the matching line, exiting directly after
• input_file: awk works on the contents of input_file
• > output_file: the output goes (>) to output_file

In summary, we process input_file and save the new changes to output_file, so we can ensure the original file remains intact.

If we want to make the changes in the original file, we can use a temporary copy:

$ awk '/desired error/{print; exit} 1' > tmp && mv tmp input_file

However, we should be cautious when writing changes back to the same file to avoid potential issues.

4. Using the sed Command

The sed command is a handy tool for changing or manipulating text in different ways like finding, replacing adding, or deleting specific lines. Moreover, one notable feature of sed is its ability to replace or remove text based on patterns. This makes sed very useful for processing text quickly and efficiently.

For example, using sed enables us to delete subsequent lines after finding a matching pattern:

$ sed '/desired error/q' input_file > output_file

Here, the sed command looks for each line one by one for the matching pattern text from the input_file. Once it finds the matching pattern, q stops the processing for the remaining lines. Thus, the result, which only includes all the lines up to the matching pattern, is stored in output_file.

If we want to save the changes in the original file, we can employ the -i flag:

$ sed '/desired error/q' -i input_file

This flag can modify input_file in place, thereby potentially changing its contents.

5. Using Python

Python stands out as one of the most widely used programming languages. Moreover, Python comes with extensive libraries making it an ideal choice for operating systems like Linux.

To run Python code, we first install Python in our Linux distribution. For example, if we want to use Python on Ubuntu, we can install it using the apt package manager:

$ sudo apt install python3

Next, we use any text editor to write our Python code:

$ nano python_script.py

In this case, we use the nano text editor, whereas python.code.py is the Python code name.

5.1. Writing the Code

Now, we write the Python code within python.code.py to remove the subsequent lines from the input_file after finding a matching pattern:

def print_lines_until_pattern(input_file_path, pattern, output_file_path):
    with open(input_file_path, 'r') as file:
        lines = file.readlines()
    
    with open(output_file_path, 'w') as output_file:
        for line in lines:
            output_file.write(line)
            if pattern in line:
                break

input_file_path = '/home/ubuntu/Desktop/input_file'
output_file_path = '/home/ubuntu/Desktop/output_file'
matching_pattern = 'desired error'
print_lines_until_pattern(input_file_path, matching_pattern, output_file_path)

To understand the Python code, we divided it into three sections.

5.2. Understanding the Python Code

First, we define a function called print_lines_until_pattern, which takes three arguments:

• input_file_path: path of the file to be read
• pattern: the pattern to look for
• output_file_path: path of file that holds the changes

Let’s see its prototype:

def print_lines_until_pattern(input_file_path, pattern, output_file_path):

Inside the function, we open the input file in read (r) mode to read all its lines using file.readlines(), and store them inside a new variable called lines:

with open(input_file_path, 'r') as file:
    lines = file.readlines()

Next, we specify the file path for output_file in write (w) mode:

with open(output_file_path, 'w') as output_file:

Using a for loop, we iterate over each line and write it to the output file using output_file.write(line). In addition, the if condition checks for the matching pattern. If found, the loop exits using the break statement:

for line in lines:
    output_file.write(line)
    if pattern in line:
        break

Lastly, we set the file paths and matching patterns as parameters. Then, we call the function we defined with these parameters.

Subsequently, the code prints lines from the file until it finds the specified pattern:

input_file_path = '/home/ubuntu/Desktop/input_file'
output_file_path = '/home/ubuntu/Desktop/output_file'
matching_pattern = 'desired error'
print_lines_until_pattern(input_file_path, matching_pattern, output_file_path)

Finally, we run the Python code after saving it:

$ python3 python_script.py

After we run the Python code, we can see output_file contains all the lines from the original file up to the specified pattern.

Consequently, we can make changes in the original file by providing the same path inside output_file_path as input_file_path.

6. Conclusion

In this article, we discussed sed and awk commands along with some Python code for removing lines that follow a matching pattern.

The awk and sed commands provide a concise solution with a one-liner making them fairly easy to understand. On the other hand, the Python code requires a more extended code that may require a deeper understanding, but provides more flexibility.