file command

 

Command: file

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🔍 Purpose

The file command is used to identify the type of a file.
It examines the file’s contents, not its extension, and tells you what kind of data it holds.

This helps you distinguish between:

• Text files

• Executable files

• Object files

• Scripts

• Images, PDFs, etc.

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🧩 Basic Syntax

file [options] filename

Example:

file a.out

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⚙️ How It Works

The file command uses a three-step test to determine a file’s type:

1. Filesystem test – Checks the type from filesystem information (e.g., directory, symbolic link, block device).

2. Magic test – Reads the first few bytes of the file (called the magic number) and compares them with entries in /usr/share/misc/magic.mgc.

3. Language test – If it’s a text file, file checks whether it contains source code (like C, Python, shell script, etc.).

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Example 1: Identifying Common Files

Let’s try on different files.

file /bin/ls

Output:

/bin/ls: ELF 64-bit LSB executable, x86-64, dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2

🧩 Explanation:

• ELF → Executable and Linkable Format (used for binaries in Linux)

• 64-bit → Architecture type

• Dynamically linked → Uses shared libraries

• Interpreter → Loader that runs the binary

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file hello.c

Output:

hello.c: C source, ASCII text

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file test.txt

Output:

test.txt: ASCII text

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file image.png

Output:

image.png: PNG image data, 800 x 600, 8-bit/color RGBA

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file script.sh

Output:

script.sh: Bourne-Again shell script, ASCII text executable

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Example 2: Using Wildcards

You can test multiple files at once:

file *

Output:

a.out:       ELF 64-bit LSB executable
hello.c:     C source, ASCII text
notes.txt:   UTF-8 Unicode text

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Example 3: Checking a Directory or Link

file /etc

Output:

/etc: directory

file /bin/sh

Output:

/bin/sh: symbolic link to bash

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Example 4: Check Character Encoding

file notes.txt

Output:

notes.txt: UTF-8 Unicode text

This helps detect encoding issues (ASCII, UTF-8, ISO-8859, etc.).

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Example 5: Checking Object or Library Files

file sum.o

Output:

sum.o: ELF 64-bit LSB relocatable, x86-64, with debug_info

🧩 Explanation:

• Relocatable → Object file (not yet linked)

• with debug_info → Contains debugging symbols (compiled with -g)

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Example 6: Checking Compressed Files

file archive.tar.gz

Output:

archive.tar.gz: gzip compressed data, from Unix

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Example 7: Detect Script Type

file myscript.py

Output:

myscript.py: Python script, ASCII text executable

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📚 Commonly Used Options

Option	Description
-b	Brief output (omit filename)
-i	Print MIME type instead of description
-z	Examine compressed files
-L	Follow symbolic links
-s	Read block/character special files
--mime-type	Show MIME type (e.g., text/plain)
--mime-encoding	Show encoding (e.g., utf-8)

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Examples

file -b a.out

Output:

ELF 64-bit LSB executable

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file --mime-type hello.c

Output:

hello.c: text/x-c

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file --mime-encoding notes.txt

Output:

notes.txt: utf-8

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Lab Exercise

Aim:

To study the use of the file command to identify the type and properties of files in Linux.

Procedure:

1. Create different types of files:

   echo "Hello OS Lab" > text.txt
   cat > script.sh
   #!/bin/bash
   echo "Test Script"
   (Ctrl+D)
   gcc -o hello hello.c
   

2. Execute the following commands:

   file text.txt
   file script.sh
   file hello.c
   file hello
   file /bin/ls
   

3. Record and compare outputs.

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Sample Observation Table

Filename	Command Used	Output
text.txt	file text.txt	ASCII text
script.sh	file script.sh	Bash script
hello.c	file hello.c	C source text
hello	file hello	ELF executable
/bin/ls	file /bin/ls	ELF 64-bit binary

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Learning Outcomes

Students will:

• Understand how Linux identifies files based on content, not extensions.

• Learn about ELF executables, text encoding, and symbolic links.

• Develop a foundational understanding of how the OS classifies and manages files.

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Summary

Command	Purpose
file filename	Identify file type
file -b filename	Show type only
file -i filename	Show MIME type
file *	Identify all files in directory
file --mime-type file	Display MIME type
file --mime-encoding file	Display text encoding