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

After running the course setup, try it on a file in ~/playground/chapter2. For example:

cd ~/playground/chapter2
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

The course setup creates several file types in ~/playground/chapter2. Try checking a few at once:

cd ~/playground/chapter2
file notes.txt sample.sh index.html style.css readme empty_file sample.pdf sample.jpg

You should see output like:

notes.txt:    ASCII text
sample.sh:    Bourne-Again shell script, ASCII text executable
index.html:   HTML document, ASCII text
style.css:    ASCII text
readme:       ASCII text
empty_file:   empty
sample.pdf:   PDF document, version 1.4
sample.jpg:   JPEG image data, JFIF standard 1.01

Notice that readme has no extension, but file still identifies it as text by looking at its content. The empty_file is reported as "empty" because it has no content. The sample.pdf and sample.jpg files are minimal examples (they contain only the magic bytes that identify the format); they are not real documents or images, but file correctly reports their type.

Using Brief Mode

If you just want the file type without the filename, use -b:

cd ~/playground/chapter2
file -b notes.txt
file -b sample.sh

Output:

ASCII text
Bourne-Again shell script, ASCII text executable

Checking Web Files (MIME Type)

For web files, use -i to see the MIME type. The setup provides index.html and style.css in ~/playground/chapter2:

cd ~/playground/chapter2
file -i index.html style.css

Output:

index.html: text/html; charset=us-ascii
style.css: text/css; charset=us-ascii

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