Reading files line by line (while read) in Bash Scripting

Introduction

Reading a file line by line helps you process each line separately. This is useful when you want to handle big files or extract information step by step.

You want to count or analyze each line in a log file.

You need to extract specific data from a file line by line.

You want to automate tasks that depend on each line of a configuration file.

You want to read user input saved in a file and process it one line at a time.

Syntax

Bash Scripting

while IFS= read -r line; do
  # commands using "$line"
done < filename

IFS= prevents trimming leading/trailing spaces.

-r stops backslash escapes from being interpreted.

Examples

Reads each line and prints it. Simple but may trim spaces or interpret backslashes.

Bash Scripting

while read line; do
  echo "$line"
done < file.txt

Reads each line exactly as is, preserving spaces and backslashes.

Bash Scripting

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

Adds a label before printing each line.

Bash Scripting

while IFS= read -r line; do
  echo "Line: $line"
done < file.txt

Sample Program

This script reads a file named sample.txt line by line. It prints each line with its number. It also checks if the file exists before reading.

Bash Scripting

#!/bin/bash

# Read file line by line and print each line with line number
filename="sample.txt"

if [[ ! -f "$filename" ]]; then
  echo "File not found: $filename"
  exit 1
fi

count=1
while IFS= read -r line; do
  echo "Line $count: $line"
  ((count++))
done < "$filename"

Important Notes

Always quote variables like "$line" to preserve spaces and special characters.

Use IFS= and -r to avoid common pitfalls with spaces and backslashes.

Redirect the file into the loop using done < filename to read from the file.

Summary

Use while IFS= read -r line to read files line by line safely.

Process each line inside the loop for automation or data extraction.

Always check if the file exists before reading to avoid errors.