How to Read a File Line by Line Using Bash

We may face several situations in Bash, where we need to process the data stored in a file line by line. In such cases, we need to read the contents of the file. We use the read command in Bash to read a file line by line.

Read the File Line by Line in Bash

Syntax

while IFS= read -r line
do
  echo "$line"
done < file_name

It reads the content of the file file_name one line at a time and prints the lines one by one in the terminal. The loop is executed until we reach the end of the file. The IFS is set to the null string, which helps to retain leading and trailing whitespaces.

Alternatively, the above command can also be replaced by the following command within a single line:

while IFS= read -r line; do echo $line; done < file_name

Example: Read the File Line by Line in Bash

In the example, we will read the file file.txt, which contains numbers in each line and then find the sum of all numbers in the file.

Content of file.txt

1
5
6
7
8
10

#!/bin/bash

sum=0
echo "The numbers in the file are:"
while IFS= read -r line
do
  echo "$line"
  sum=$(( $sum + $line ))
done < file.txt
echo "The sum of the numbers in the file is:$sum"

Output:

The numbers in the file are:
1
5
6
7
8
The sum of the numbers in the file is:27

It reads the numbers line by line from a file named file.txt and then sums up all those numbers and finally echoes the sum.

Example: Set Fields in Files to Variables

We can set fields in the file to variables by passing multiple variables to the read command, which will separate fields within a line based on the value of IFS.

Content of file.txt

Rohit-10
Harish-30
Manish-50
Kapil-10
Anish-20

#!/bin/bash

while IFS=- read -r name earnings
do
    echo "$name" has made earnings of "$earnings" pounds today!
done < file.txt

Output:

Rohit has made earnings of 10 pounds today!
Harish has made earnings of 30 pounds today!
Manish has made earnings of 50 pounds today!
Kapil has made earnings of 10 pounds today!

Here, each line in the file is divided into two segments as we have passed two variables to the read command. The first segment will be assigned to the name variable, which extends from the beginning of the line until the first -, and the remaining portion will be assigned to the earnings variable.

Alternative Methods to Read Files in Bash

#!/bin/bash

while IFS=- read -r name earnings
do
    echo "$name" has made earnings of "$earnings" pounds today!
done < <(cat file.txt )

Output:

Rohit has made earnings of 10 pounds today!
Harish has made earnings of 30 pounds today!
Manish has made earnings of 50 pounds today!
Kapil has made earnings of 10 pounds today!

Here, the filename file.txt is passed to the program as an output of the cat command.