How to Read a File Line by Line in Bash Script

In Bash, you can read a file line by line using a while loop combined with the read command like this: while IFS= read -r line; do ... done < filename. This reads each line safely, preserving spaces and special characters.

Syntax

The basic syntax to read a file line by line in Bash uses a while loop with the read command. Here is the pattern:

• while: starts the loop that runs until all lines are read.
• IFS=: sets the Internal Field Separator to empty to preserve leading/trailing spaces.
• read -r line: reads one line into the variable line, -r prevents backslash escapes.
• do ... done < filename: executes commands for each line, reading from the file.

bash

while IFS= read -r line; do
  # process each $line here
  echo "$line"
done < filename

Example

This example reads a file named example.txt line by line and prints each line with a prefix.

bash

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

Output

Line read: Hello, world!
Line read: This is a test.
Line read: Bash scripting is fun!

Common Pitfalls

Common mistakes when reading files line by line in Bash include:

• Not setting IFS=, which can trim leading/trailing spaces.
• Omitting -r in read, causing backslashes to be interpreted.
• Using a pipe to feed the file, which runs the loop in a subshell and can cause variable scope issues.

Correct approach uses input redirection < filename instead of a pipe.

bash

wrong way:
cat filename | while read line; do
  echo "$line"
done

right way:
while IFS= read -r line; do
  echo "$line"
done < filename

Quick Reference

Command	Description
while IFS= read -r line; do ... done < filename	Read file line by line safely
IFS=	Preserves spaces in lines
read -r	Reads line without interpreting backslashes
done < filename	Redirects file input to the loop
Avoid: cat file | while read ...	Runs loop in subshell, variable scope issues

Key Takeaways

Use 'while IFS= read -r line; do ... done < filename' to read lines safely.

Set IFS= to keep leading and trailing spaces intact.

Use the '-r' option with read to prevent backslash escapes.

Avoid piping file content into while loops to prevent subshell issues.

Input redirection '< filename' is the preferred way to feed the file.