file Command Cheat Sheet

The file command determines the file type. Unlike Windows, which relies on extensions (.exe, .txt), Linux uses the valid content of the file (magic numbers) to decide what it is.

Synopsis

file [OPTION...] [FILE...]

Basic Usage

Check File Type

file document.pdf
# Output: document.pdf: PDF document, version 1.4

Check Multiple Files

file *

Interpreting Scripts

It identifies shebangs to tell you the script type.

file script.py
# Output: script.py: Python script, ASCII text executable

Detailed Analysis

Show MIME Type (-i)

Useful for web servers and scripts that need the exact MIME string.

file -i image.png
# Output: image.png: image/png; charset=binary

Keep Going (-k)

Don't stop at the first match. Some polyglot files might be valid as multiple types (e.g., a ZIP file that is also a valid shell script).

file -k setup.sh

Look Inside Compressed Files (-z)

Normally, file just says "gzip compressed data". Use -z to look inside the archive.

file -z archive.tar.gz
# Output: archive.tar.gz: POSIX tar archive (gzip compressed data)

Output Options

Brief Mode (-b)

Don't print the filename, just the result. Great for scripting.

file -b myvideo.mp4
# Output: ISO Media, MP4 Base Media v1 [ISO 14496-12:2003]

Separator (-F)

Change the separator (default is :).

file -F " -> " photo.jpg
# Output: photo.jpg -> JPEG image data...

Practical Examples

Find All Executables

Combine with find to verify actual executables (ignoring permission bits).

find /bin -type f | xargs file | grep "ELF"

Verify Download Integrity

Did you download a fake image?

file suspicious.jpg
# If Output: "Bourne-Again shell script", don't open it!

Fix Broken Extensions

Renaming files based on their actual content.

for name in *; do
    type=$(file -b --mime-type "$name")
    case $type in
        image/jpeg) mv "$name" "$name.jpg" ;;
        image/png)  mv "$name" "$name.png" ;;
    esac
done

Magic Database

file relies on a database of "magic numbers" (signatures) located at /usr/share/file/magic (or similar).

• Compile Magic File: file -C
• Use Custom Magic File: file -m mymagic

Exit Status

Notes

• Charset Detection: file -i is one of the most reliable ways to check if a text file is utf-8, us-ascii, or iso-8859-1.
• System Dependent: The detail level of descriptions depends on the file version (standard is typically 5.x) and the installed magic database.