1. Overview

Bash, the Bourne Again Shell, and utilities like awk and sed represent powerful tools widely utilized across Unix-like operating systems. In particular, these command-line interpreters and text-processing utilities are renowned for their scripting capabilities, making them essential choices for automating tasks and crafting efficient scripts across diverse Linux environments.

For example, we might sometimes need to add quotations around each line of a file or multi-line string, and Bash provides several methods to achieve this. In this tutorial, we’ll explore different ways to accomplish this task, complete with code examples and explanations.

2. Why Wrap Quotes Around Each Line?

Before we delve into the techniques, let’s understand why we might want to wrap quotes around each line of text in a file or a variable:

  1. data integrity: without proper quoting, special characters such as commas and semicolons, can wreak havoc when processing data in formats like CSV
  2. consistent formatting: wrapping quotes around each line provides a standardized structure that enhances readability and simplifies downstream data processing, whether we’re generating reports, logs, or any other textual content
  3. avoid ambiguity: wrapping quotes around each line helps mitigate ambiguity by providing clear boundaries for each piece of information
  4. enhanced scripting flexibility: process lines as discrete units or strings, making it easier to apply transformations, filters, and conditional operations

Now, let’s explore various methods to add quotes to the beginning and end of a line in Bash.

3. Using a while Loop With read and Redirection

One straightforward approach is to use a while loop to read each line from a file or variable and wrap quotes around it.

Let’s check an example using a file named sample.txt:

$ cat sample.txt
Bash is great

Here, we used the cat command to view the contents of the sample.txt file.

Now, let’s create a while loop and use the read command with redirection to wrap quotes around each line:

$ input_file="sample.txt"
$ output_file="quoted_sample.txt"
while IFS= read -r line; do
  echo "\"$line\""
done < "$input_file" > "$output_file"

Now, let’s elaborate on the above code snippet:

  • input_file and output_file store the input and output file names, respectively
  • we use a while loop to read each line from the input file by redirecting its content via <
  • IFS= ensures that leading and trailing whitespace is preserved
  • -r prevents backslashes from being treated as escape characters
  • echo “\”$line\”” wraps double quotes around each line and echoes it
  • we overwrite the quoted_sample.txt file with the contents that the loop produces using the > redirection operator

After running the script, the quoted_sample.txt file changes:

$ cat quoted_sample.txt
"Bash is great"

Finally, we confirm that each line is wrapped with quotes as intended.

4. Using the sed Command

The sed command is a versatile tool for text manipulation. As such, we can use it to wrap quotes around each line of text within a file:

$ input_file="sample.txt"
$ output_file="quoted_sample.txt"
$ sed 's/.*/"&"/' "$input_file" > "$output_file"

Now, let’s break down the sed ‘s/.*/”&”/’ substitution command above:

  • .* matches the entire line, i.e., any number of any character
  • & represents the matched text, i.e., the entire line
  • “&” wraps double quotes around the matched text

To validate this method, we check the content of the quoted_sample.txt file:

$ cat quoted_sample.txt
"Bash is great"

Again, quotes are now added where we want them.

5. Using the awk Command

awk operates on a line-by-line basis, processing input data and performing actions based on patterns and instructions defined by the user. It’s particularly well-suited for tasks like data extraction, transformation, and reporting.

Let’s check how to use awk to wrap quotes around each line in the sample.txt file:

$ input_file="sample.txt"
$ output_file="quoted_sample.txt"
$ awk '{print "\"" $0 "\""}' "$input_file" > "$output_file"

Next, let’s verify the results from this method:

$ cat quoted_sample.txt
"Bash is great"

In this case, we used print “\”” $0 “\”” to print each line with double quotes added. Notably, we had to escape the quotes we want to add to avoid terminating the print statement.

6. Using mapfile After Bash Version 4

Starting from Bash version 4, we can use the mapfile command to read lines from a file into an array and wrap quotes around each line.

6.1. Theoretical Understanding

We employ the mapfile command in Bash to read lines of text from a file or standard input (stdin) into an array variable. Particularly, its primary role is to efficiently read and store lines of text from an input source, such as a file, into an array variable for further processing.

This command is particularly useful when we need to work with lines of text one by one, manipulate them, or perform operations on them within a Bash script.

6.2. Practical Illustration

Let’s see how to meet our objective using mapfile:

$ input_file="sample.txt"
$ output_file="quoted_sample.txt"
$ mapfile -t lines < "$input_file"
$ quoted_lines=()
for line in "${lines[@]}"; do
$ printf "%s\n" "${quoted_lines[@]}" > "$output_file"

The snippet above uses several techniques:

  • mapfile -t lines < “$input_file” reads lines from the input file into the lines array
  • “${lines[@]}” expands to the array contents, so we can iterate over them with a for loop
  • quoted_lines+=(“\”$line\””) wraps the current line within quotes and stores it in the quoted_lines array
  • printf “%s\n” prints the quoted lines to the output file

Finally, we’ll verify the result via quoted_sample.txt:

$ cat quoted_sample.txt
"Bash is great"

Now, let’s move on to some considerations.

7. Remarks

While wrapping quotes around each line in Bash, there are some important conditions to consider:

  1. input source: determine whether we’re working with a file or a variable, as the methods may vary, i.e., whether we use interpolation, parameter expansion, process substitution, and similar
  2. text content: depending on any special characters and formatting, we may need to adjust the methods accordingly
  3. Bash version: some methods, like mapfile, require Bash version 4 or later. Hence, we might want to check our Bash version using bash –version if we encounter issues.

Wrapping quotes around each line is more than just a cosmetic transformation of text. Therefore, it’s usually good to consider the above conditions before we proceed with taking action.

8. Conclusion

Wrapping quotes around each line in Bash is a common text processing task, often required when preparing data for various applications or when manipulating text files.

In this article, we explored several methods to achieve this task, including using while loops, sed, awk, and mapfile. Moreover, each method has its advantages and may be more suitable in different scenarios.

Furthermore, by understanding these techniques and the conditions to consider, we should be well-equipped to handle even more tasks in our Bash scripting endeavors.

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