How to Read File Line by Line in Linux Shell Script

Often you may need to read file line by line in Linux shell script. In this article, we will look at different use cases for reading file in shell script.

Suppose you have a file /home/ubuntu/data.txt with the following information

Today
is
a
beautiful
day

How to Read File Line by Line in Linux Shell Script

Here are the different ways to read the above file line by line.

1. Read File from Command Line

If you don’t want to write a shell script or even use cat command to view your file contents, you can use a while loop in terminal prompt to simply go through the file line by line and display its contents.

$ while read line; do echo $line; done < /home/ubuntu/data.txt

In the above code, the while loop goes through the file. Each line is temporarily stored in $line variable. We use echo command to display the line. Please note, if you don’t use the full path below, it will look for the file in your present working directory.

$ while read line; do echo $line; done < /home/ubuntu/data.txt
Today
is
a
beautiful
day

2. Read File using Shell Script

Here is a simple shell script to read the file and display its contents line by line.

$ sudo vi read_file.sh

Once you have created an empty shell script with the above command, add the following lines to it.

#!/bin/bash
filename='/home/ubuntu/data.txt'
n=1
while read line; do
# reading each line
echo "Line No. $n : $line"
n=$((n+1))
done < $filename

In the above code, we store file path in $filename variable. Then we run a while loop through the file, storing each line in $line variable. We also use $n variable to keep count of line number.

Make the shell script executable

$ sudo chmod +x read_file.sh

Run the shell script using the following command.

$ ./read_file.sh

If you want to pass the filepath via command line argument, replace the second line of your script as

filename=$1

In the above code, $1 stores the first command line argument. Now your shell script will be

#!/bin/bash
filename=$1
n=1
while read line; do
# reading each line
echo "Line No. $n : $line"
n=$((n+1))
done < $filename

Now you can run your shell script file as

$ ./read_file.sh /home/ubuntu/data.txt

3. Omit backslash escape character

Please note, if your file contains backslash then the above code will treat it as escape character. If you want to omit backslash escape, then you need to use -r option with read command in your while loop, as shown below.

#!/bin/bash
while read -r line; do
# Reading each line
echo $line
done < /home/ubuntu/data.txt

In this article, we have learnt different ways to read file line by line shell script. Reading files is a very common requirement in Linux and can be used in many bigger scripts. If you want to perform any operation on each read line, then you can include those statements between while..done statements.

You can use these commands with any shell. In case you use any of the above shell scripts with other shell, please update the first line to point to the right shell.

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