CIS120Linux Fundementals

file Command

The file command in Linux is used to determine the type of a file. This command helps users identify files without relying solely on file extensions. The file command examines the content of a file and outputs a human-readable description of the file type.

Common Options for the file Command

The file command comes with various options to refine its behavior. Below is a table of some of the most common options and their descriptions:

Option	Description
-b	Brief mode: do not prepend filenames to output lines
-i	Output MIME type strings rather than the traditional human-readable ones
-f <name>	Read the filenames to be examined from name instead of from the command line
-z	Try to look inside compressed files
-L	Follow symbolic links
-s	Read block or character special files
-r	Raw mode: do not translate unprintable characters to \ooo notation

Examples

Here are some examples of how to use the file command with different options, along with the expected output:

Determining the File Type

This command checks the type of the file example.txt.

file example.txt

Output

example.txt: ASCII text

Using the -b Option (Brief Mode)

This command outputs only the file type without the filename.

file -b example.txt

Output

ASCII text

Using the -i Option (MIME Type)

This command outputs the MIME type of the file example.txt.

file -i example.txt

Output

example.txt: text/plain; charset=us-ascii

Checking Multiple Files

This command checks the types of multiple files.

file file1.txt file2.jpg file3.zip

Output

file1.txt: ASCII text
file2.jpg: JPEG image data, JFIF standard 1.01
file3.zip: Zip archive data, at least v2.0 to extract

Reading File Names from a File

Each line in filenames.txt should contain the name of a file that actually exists in the directory where you run the command. For example:

file -f filenames.txt

Below is the contents of filenames.txt

document.pdf
image.png
script.sh
binaryfile
textfile.txt
directory/

Output

document.pdf: PDF document, version 1.4
image.png: PNG image data, 800 x 600, 8-bit/color RGB, non-interlaced
script.sh: Bourne-Again shell script, ASCII text executable
binaryfile: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux)
textfile.txt: ASCII text
directory/: directory

Explanation

• file -f filenames.txt reads each filename from the file filenames.txt and runs the file command on it.
• The output describes the type of each file, just as if you had run file filename on each one individually.
• If a filename in filenames.txt does not exist, an error message like filename: cannot open 'filename' (No such file or directory) will appear.

Looking Inside Compressed Files

This command attempts to look inside the compressed file compressed.tar.gz and identify its contents.

file -z compressed.tar.gz

Output

compressed.tar.gz: gzip compressed data, was "compressed.tar", last modified: ...

Following Symbolic Links

This command follows the symbolic link symlink and checks the type of the file it points to. The output depends on the file type of the target. NOTE: We will be looking at symbolic links in Chapter 2

file -L symlink

Output

symlink: ASCII text

Conclusion

The file command is a powerful tool for identifying the types of files in Linux. By understanding and utilizing its various options, you can efficiently determine file types and gain insights into the nature of the files you are working with. Practice using these options to become comfortable with the file command and enhance your file management skills in Linux.