Add Comma to End of Each Line in Bash

1. Overview

Adding characters like commas to the end of each line in a text file is a common operation in file processing and data manipulation. This task can be especially useful in formatting data files for CSV conversion or similar purposes.

2. Introduction to Problem Statement

Imagine we have a text file named input.txt containing the following 3 lines:
For example:

Our goal is to add comma to the end of each line. Expected output is:

We will also see how to add comma at the end of each line in a file, except for the last line.

Now, let’s continue learning various methods to add a comma at the end of each file line.

3. Using sed Command

The sed (Stream Editor) is a powerful text processing tool ideal for pattern-based transformations.

Let’s use sed command to add comma to each line of the file:

On MAC OS, we can use following command:

In the above code, the sed command performs a substitution operation (s) on each input file line (input.txt); the expression $ matches the end of each line, and the comma is added with /,/. -i option edits file in-place.

To add a comma to each line except the last one, we can use following command:

$!: This is an address specifier in sed. The dollar sign ($) represents the last line of the file. The exclamation mark (!) negates the address, meaning the subsequent command will be executed on all lines except the last line.
s/$/,/: This subsequent command is similar to adding comma to each line of the file.

4. Using awk Command

The awk is a programming language designed for text processing and offers robust capabilities for manipulating file content.

Let’s use awk to achieve our goal:

The command awk in the above code reads input data from a file and executes operations on each input line following the user-specified rules. For example, the'{print $0","}' prints each input file line, followed by a comma.

To add comma to each line except last one, we can use the following awk command:

{printf "%s%s",sep,$0; sep=",\n"}: This is the main action block of the script, executed for each line of the input file.
Let’s deep dive into this main block:

• printf "%s%s",sep,$0;: Here, printf is used for formatted printing. %s is a format specifier for a string. It prints two strings: sep (initially empty) and $0 (the entire current line from the file).
• sep=",\n": After printing the first line, sep is set to a comma followed by a newline character. This means that for the subsequent lines, a comma and a newline will be printed before the line content.
• END{print ""}: This is executed after all lines of the file have been processed. It simply prints a newline character. This ensures that the output ends with a newline, which is a good practice in Unix-like systems to properly end the last line.

5. Using Bash Loop

For more control, a Bash script can read and write each line of the file, appending a comma.

The above script reads each line of the file and appends a comma, and writes the result to a temporary file, then renames it.

6. Using vim editor

If we want to do it using vim editor, we can use expression :%s/$/,/g.
Here are steps:

• Open the file using vim. vi intput.txt
• Use expression :%s/$/,/g and press enter.

It will add a comma to the end of every line in bash.

7. Conclusion

Appending a comma to the end of each line in a file can be efficiently done using sed, awk, or a Bash loop. sed is generally the fastest and most efficient, especially for larger files. awk offers a balance between speed and flexibility, providing additional text processing options if needed.

While Bash loops offer the most control, they are less efficient and are better suited for smaller files or less performance-critical tasks.