How to Read File Line by Line in Bash

Quick Answer: Read File Line by Line in Bash

To read a file line by line in Bash, use a while loop with the read command: while IFS= read -r line; do ... done < filename.txt. The IFS= preserves whitespace, and -r prevents interpretation of backslashes.

Quick Comparison: File Reading Methods

Method	Speed	Best For	Complexity
while read loop	Very fast	Processing lines	Simple
mapfile	Fastest	Loading entire file	Simple
while read (no IFS=)	Very fast	Standard parsing	Simple
cat + loop	Slower	Simple scripts	Moderate
sed/awk	Fast	Pattern-based	Moderate

Bottom line: Use while IFS= read -r for reliable line processing.

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Read files line by line in Bash for processing.

Method 1: Using while read Loop (Recommended)

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

Method 2: Using read without IFS Modification

while read -r line; do
  echo "Processing: $line"
done < data.txt

Method 3: Reading Specific Columns

while IFS=',' read -r name age email; do
  echo "Name: $name, Age: $age"
done < data.csv

Method 4: Using cat and pipe

cat filename.txt | while read -r line; do
  echo "Line: $line"
done

Practical Examples

Count Lines in File

count=0
while IFS= read -r line; do
  count=$((count + 1))
done < file.txt
echo "Total lines: $count"

Process CSV File

while IFS=',' read -r id name email; do
  echo "User: $name ($email)"
done < users.csv

Skip Header Line

while IFS= read -r line; do
  # Skip first iteration
  [ -z "$header" ] && { header=1; continue; }
  echo "$line"
done < data.csv

Filter Lines

while IFS= read -r line; do
  # Skip empty lines
  [ -z "$line" ] && continue
  # Skip comments
  [[ "$line" =~ ^# ]] && continue
  echo "$line"
done < config.txt

Modify and Write

while IFS= read -r line; do
  # Convert to uppercase
  echo "${line^^}"
done < input.txt > output.txt

Line with Whitespace

# Preserve leading/trailing whitespace
while IFS= read -r line; do
  echo "[$line]"
done < file.txt

Performance Comparison

Method	Speed	Notes
while read	Fast	No subshell
cat | while	Slower	Subshell created
for loop	Medium	Loads entire file

Use while IFS= read -r for best performance.

Debugging

# Show what's being read
while IFS= read -r line; do
  echo "DEBUG: '$line'"
done < file.txt

Key Points

• Use IFS= to preserve whitespace
• Use -r to prevent backslash interpretation
• Use < filename to redirect file into loop
• Avoid pipes (slower, uses subshell)

Recommended Pattern

while IFS= read -r line; do
  # Process $line
  echo "$line"
done < "$input_file"