Shell Scripting Tutorial: Shell Script File Operations

Most of the times, shell script is used to interact with files.

File Example 1:

Below is a program to check if a file exists or not

In this example, we take file name input from the user. Note the “\c” option, this is used to keep the cursor in the same line. Then we use “-e” option to check if the user entered file is present or not. This will check the presence of the file in the same directory.

#!/bin/bash
 
echo -e "Enter the file name \c"
read file_name
 
if [ -e $file_name ]
then
        echo "File found"
else
        echo "File not found"
fi

Output:

sh file.sh
Enter the file name hello.txt
File not found

As initially the file is not present, it will display file not found.
We shall create a file by using touch command, then run the program again.

touch hello.txt

sh file.sh
Enter the file name hello.txt
File found

File Example 2:

“-f” option is used to check if the file exist and it is a regular file or not.

#!/bin/bash
 
echo -e "Enter the file name \c"
read file_name
 
if [ -f $file_name ]
then
        echo "File found and is a regular file"
else
        echo "File not found"
fi

Output:

sh file.sh
Enter the file name hello.txt
File found and is a regular file

File Example 3:

“-d” option is used to check if the file exist and it is a directory file or not.

#!/bin/bash
 
echo -e "Enter the file name \c"
read file_name
 
if [ -d $file_name ]
then
        echo "File found and is a directory file"
else
        echo "File not found"
fi

sh file.sh

Enter the file name mydir
File found and is a directory file

File Example 4:

#!/bin/bash
 
echo -e "Enter the file name \c"
read file_name
 
if [ -f $file_name ]
then
        echo "File found"
        if [ -w $file_name ]
        then
                echo "File is having write permission. Write text below. Ctrl +
                cat >> $file_name
        fi
else
        echo "File not found"
fi

Output:

sh file.sh
Enter the file name hello.txt
File found
File is having write permission. Write text below. Ctrl + D to exit
Hello world from prodevelopertutorial.com

Now the text “Hello world from prodevelopertutorial.com” will be written in “hello.txt” file.

File Example 5:

In this example we shall look how to read files using while loop:

Using “read” command.

Using read command, we shall use read command along with a variable, then echo that variable to output. We use “<” input redirection to input the file.

#!/bin/bash
 
echo "Reading the file hello.txt"
 
while read r
do
        echo $r
done < hello.txt

Output:

sh 2_file.sh
Reading the file hello.txt
Hello world from prodevelopertutorial.com

Using “read” and pipe command.

In the previous example, we used “<” command. In this example we use “|” command to input file name.

#!/bin/bash
 
echo "Reading the file hello.txt"
 
cat hello.txt | while read r
do
        echo $r
done

Using “IFS” and pipe command.

If the file has special characters like file indentation or a space or a tab or a newline, those values cannot be ready by the above methods. Hence at that place we use “IFS” keyword. IFS stands for Internal Field Separator.

#!/bin/bash
 
echo "Reading the file /etc/resolv.conf"
 
while IFS= read -r line
do
        echo $line
done < /etc/resolv.conf

Output:

sh 2_file.sh
Reading the file /etc/resolv.conf
# Generated by dhcpcd
# /etc/resolv.conf.head can replace this line
# /etc/resolv.conf.tail can replace this line