CodingHowTo

Using a While Loop

The most common and efficient way to read a file line by line in Bash is by using a while loop with the `read` command. Here's how you can do it:

                
                    #!/bin/bash

                    while IFS= read -r line
                    do
                        echo "$line"
                    done < filename.txt
                
            

Explanation:

• `while IFS= read -r line`: This loop reads each line of the file into the variable `line`. The `-r` option prevents backslashes from being interpreted as escape characters, and `IFS=` ensures that leading/trailing whitespace is preserved.
• `done < filename.txt`: Redirects the input of the loop to `filename.txt`, making it read line by line.

Using a While Loop with cat

Another method is to use a while loop in combination with the `cat` command:

                
                    #!/bin/bash

                    cat filename.txt | while IFS= read -r line
                    do
                        echo "$line"
                    done
                
            

Explanation:

• `cat filename.txt`: Outputs the contents of `filename.txt` to standard output.
• `while IFS= read -r line`: Reads each line from the standard input (which is piped from `cat`).

Performance Considerations

Both methods above are efficient, but using a while loop directly with the file redirection (`<`) is generally faster and more straightforward. It avoids unnecessary pipe operations and can be slightly more readable.

Handling Empty Lines

If you want to ignore empty lines, you can add a conditional check inside the loop:

                
                    #!/bin/bash

                    while IFS= read -r line
                    do
                        if [[ -n "$line" ]]; then
                            echo "$line"
                        fi
                    done < filename.txt
                
            

Explanation:

• `if [[ -n "$line" ]]`: Checks if the line is not empty before processing it.