Shell Scripting: How to Read, Display, and Append Files

Hello, scripters! Today, we're moving on to one of the most common and essential tasks in any scripting language: file manipulation. Almost everything you do on a Linux system involves reading from or writing to a file.

In this tutorial, we'll write two separate scripts to cover two fundamental operations:

1. Reading a file specified by the user and displaying its contents to the screen.
2. Appending (adding) the contents of one file to the end of another file.

These scripts will teach you about the cat command, file-checking, and the all-important "redirect" operators.

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Part 1: Script to Read and Display a File

Our first script will ask the user for a filename, check if that file actually exists, and if it does, print its contents.

The Script: `read_file.sh`

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

How It Works: Breakdown

• read -p "..." filename: Prompts the user and stores their input in the filename variable.
• if [ -f "$filename" ]; then: This is a crucial test.
  • -f checks if the path stored in $filename "exists and is a regular file."
  • We put "$filename" in quotes to protect against errors if the user enters a filename with spaces.
• cat "$filename": The cat (concatenate) command is the simplest way to read a file and print its contents to standard output (your terminal).
• else ... fi: If the if test fails (the file doesn't exist), this block runs, printing our custom error message.

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Part 2: Script to Append Content to Another File

This second script will ask for two files: a "source" file and a "destination" file. It will then add the *entire* contents of the source file to the *very end* of the destination file, without deleting anything.

The Script: `append_file.sh`

#!/bin/bash

# This script appends the content of one file to another.

# 1. Ask for the source file
read -p "Enter the source file (to copy FROM): " source_file

# 2. Ask for the destination file
read -p "Enter the destination file (to append TO): " dest_file

# 3. Check if the SOURCE file exists
if [ -f "$source_file" ]; then
    # 4. If it exists, append its content to the destination file
    cat "$source_file" >> "$dest_file"
    
    echo "-----------------------------------------"
    echo "Success: Content of '$source_file' has been appended to '$dest_file'."
else
    # 5. If the source file doesn't exist, show an error
    echo "Error: Source file '$source_file' not found. No action taken."
fi

How It Works: Breakdown

• read -p ...: We get two filenames from the user and store them.
• if [ -f "$source_file" ]; then: We only need to check if the source file exists. If it doesn't, we have nothing to copy, so we show an error.
• cat "$source_file" >> "$dest_file": This is the core of the script.
  • cat "$source_file": This reads the source file.
  • >>: This is the append redirect operator. It takes the output from the command on its left (cat) and adds it to the very end of the file on its right ($dest_file).
  • Note: If $dest_file doesn't exist, this command will automatically create it!
  • Warning: Be careful not to use a single >. A single > is the overwrite operator and will delete everything in the destination file before writing.

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How to Run These Scripts

You can run these scripts using the same standard process.

Step 1: Create the file
Open your terminal and use nano (or any editor) to create your file. Let's use the first script as an example:

nano read_file.sh

Step 2: Paste and Save
Copy the code for the first script and paste it into nano. Press Ctrl+O to save and Ctrl+X to exit.

Step 3: Make it executable
Give the script permission to run:

chmod +x read_file.sh

Step 4: Run it!
Before you run it, you'll need a file to read. Let's make one:

echo "Hello, this is a test file." > my_file.txt

Now, run your script:

$ ./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.
-----------------------------------------

Try running it again with a file that doesn't exist to see your error message in action!

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

You can follow the exact same steps to test your append_file.sh script.

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Challenge Yourself

You've now learned how to check for files, read them, and append to them. Here's a challenge to combine your skills:

Can you modify the append_file.sh script to ask the user, "Are you sure you want to append? (y/n)" and only proceed if they type 'y'?

Happy scripting!