Shell Scripting : How to Read and Display a File's Content

Hello, scripters! Welcome to another foundational tutorial in our shell scripting series. One of the most common tasks you'll ever perform in a script is reading data from a file. This could be for checking logs, processing data, or just displaying information.

Today, we're going to write a simple, safe, and useful script that does just that: it will ask the user for a filename, check if that file actually exists, and if it does, print all of its contents to the screen.

Let's get started!

The Shell Script

Here is the complete, commented script. We're using the cat command, which is the simplest and most direct way to display a file's contents.

#!/bin/bash

# This script reads and displays the content of a file.

# 1. Ask the user for the filename
read -p "Enter the name of the file you want to display: " filename

# 2. Check if the file exists and is a regular file
#    This is an important safety check!
if [ -f "$filename" ]; then

    # 3. If it exists, print a header and then the file's content
    echo "-----------------------------------------"
    echo "Displaying content of '$filename':"
    echo "-----------------------------------------"
    
    cat "$filename"
    
    echo "-----------------------------------------"
    echo "End of file."

else
    # 4. If it doesn't exist, show an error message
    echo "Error: File '$filename' not found or is not a regular file."
fi

---

How It Works: A Step-by-Step Breakdown

Let's break down the logic of this script.

• #!/bin/bash
  The "shebang." This must be the very first line. It tells your system to use the bash shell to run the script.


• read -p "..." filename
  This command prints a prompt (-p) with the text in quotes and then waits for the user to type something. Whatever the user types is stored in the filename variable.


• if [ -f "$filename" ]; then
  This is our crucial safety check.
  • if [ ... ]: This is a "test" command.
  • -f: This is a test operator that checks if the path stored in $filename "exists and is a regular file."
  • "$filename": We use quotes around the variable to prevent errors in case the user enters a filename with spaces (e.g., "my test file.txt").


• cat "$filename"
  This is the core command. cat (short for "concatenate") reads one or more files and prints their contents to the terminal. It's the simplest way to display a file.


• else
  This block of code runs only if the if test fails (meaning the file was not found).


• echo "Error: ..."
  Prints our user-friendly error message so the user knows what went wrong.


• fi
  This signals the end of the if/else block (it's "if" spelled backward).

---

How to Run Your Script

Let's test our new script. First, we need a file to read.

Step 1: Create a test file
In your terminal, run this command to create a simple text file:

echo "Hello, this is a test file." > my_file.txt
echo "It has two lines." >> my_file.txt

Step 2: Create the script file
Use nano (or any text editor) to create your file:

nano read_file.sh

Step 3: Paste and Save
Copy the script from the top of this post, paste it into nano, and save/exit (Ctrl+O, Enter, Ctrl+X).

Step 4: Make it executable
Give the script permission to run:

chmod +x read_file.sh

Step 5: Run it! (Two Tests)
First, let's test the "success" case by giving it the file we created:

$ ./read_file.sh
Enter the name of the file you want to display: my_file.txt
-----------------------------------------
Displaying content of 'my_file.txt':
-----------------------------------------
Hello, this is a test file.
It has two lines.
-----------------------------------------
End of file.

Now, let's test the "error" case by giving it a file that doesn't exist:

$ ./read_file.sh
Enter the name of the file you want to display: nonexistent.txt
Error: File 'nonexistent.txt' not found or is not a regular file.

---

Challenge Yourself

Great job! You've learned how to read user input, check for files, and display their contents.
Want a challenge? Try modifying this script to also count the number of lines in the file and display that count. (Hint: The command wc -l counts lines, and you can pipe cat to it: cat filename | wc -l).

Happy scripting!