CIS120 Book

CIS120 Linux Fundamentals by Scott Shaper

file Command

Think of the file command as a detective that examines files to tell you what they really are. Unlike Windows or macOS, Linux doesn't rely on file extensions (like .txt or .jpg) to know what type of file something is. Instead, it looks at the actual content of the file to determine its type.

Why Use the file Command?

You might need the file command when:

You download a file and don't know what it is
You find a file without an extension
You want to verify a file's type before opening it
You're working with files from different operating systems

Basic Usage

The simplest way to use the file command is to type:

file filename

For example, if you have a file called notes.txt:

file notes.txt

Output:

notes.txt: ASCII text

Common Options

Here are the most useful options you'll need as a beginner:

Option	What It Does	When to Use It
`-b`	Shows only the file type (no filename)	When you want cleaner output
`-i`	Shows the MIME type	When working with web files
`-z`	Looks inside compressed files	When checking zip or tar files

Practical Examples

Checking Different File Types

Let's check some common file types you might encounter:

file document.pdf image.jpg script.sh

Output:

document.pdf: PDF document, version 1.4
image.jpg: JPEG image data, JFIF standard 1.01
script.sh: Bourne-Again shell script, ASCII text executable

Using Brief Mode

If you just want to know the file type without the filename:

file -b notes.txt

Output:

ASCII text

Checking Web Files

For web development, you might want to see the MIME type:

file -i index.html style.css

Output:

index.html: text/html; charset=utf-8
style.css: text/css; charset=utf-8

Tips for Success

Start Simple: Begin with basic file checks before using options
Check Unknown Files: Always use file on files you're not sure about
Use Brief Mode: The -b option gives cleaner output
Practice: Try checking different types of files to learn the output formats

Common Mistakes to Avoid

Forgetting that Linux doesn't rely on file extensions
Not checking file types before opening unknown files
Getting confused between similar file types
Not using the -b option when you want cleaner output

Advanced Usage (For Later)

When you're more comfortable with Linux, you might want to try these advanced features:

Using -f to read filenames from a file
Using -L to follow symbolic links
Using -s to read special files