file

file

The file command in Unix and Linux is used to determine the type of a file by examining its contents, rather than relying on the filename extension alone. It's a handy utility for identifying the format of a file, especially when dealing with unknown or ambiguous file types.

Basic Usage

The basic syntax for the file command is:

file [options] [file(s)]

• options: Optional command-line options to modify the behavior of file.

• file(s): The name(s) of the file(s) to examine.

Examples

Determine File Type

To determine the type of a file:

file filename.txt

This command examines filename.txt and prints out a description of its contents, such as ASCII text, HTML document, JPEG image, etc.

Handling Multiple Files

To check the types of multiple files:

file file1.txt file2.jpg file3.pdf

This command checks the types of file1.txt, file2.jpg, and file3.pdf and displays their respective file types.

Options

Verbose Output

• -v: Verbose mode, providing additional details about the file.

Force Check

• -f: Force file check, even if the file does not exist.

Mime Type

• --mime-type: Display MIME type (Multipurpose Internet Mail Extensions) of the file.

Practical Use Cases

Scripting and Automation

In scripts, the file command can be used to automate decisions based on file types. For example:

#!/bin/bash

for f in *.dat; do
    file "$f" | grep -q "ASCII text" && echo "$f is a text file"
done

This script checks all .dat files in the current directory and prints a message if they are identified as ASCII text files.

File System Management

When managing files with unknown extensions or when dealing with diverse file types, the file command helps in quickly identifying the nature of each file.

Summary

The file command is a valuable tool in Unix and Linux for identifying file types based on their contents. It's useful in scripting, file system management, and when handling a variety of files with ambiguous extensions. Understanding its usage and options can streamline file handling tasks and improve efficiency.

help

Usage: file [OPTION...] [FILE...]
Determine type of FILEs.

      --help                 display this help and exit
  -v, --version              output version information and exit
  -m, --magic-file LIST      use LIST as a colon-separated list of magic
                               number files
  -z, --uncompress           try to look inside compressed files
  -Z, --uncompress-noreport  only print the contents of compressed files
  -b, --brief                do not prepend filenames to output lines
  -c, --checking-printout    print the parsed form of the magic file, use in
                               conjunction with -m to debug a new magic file
                               before installing it
  -e, --exclude TEST         exclude TEST from the list of test to be
                               performed for file. Valid tests are:
                               apptype, ascii, cdf, compress, csv, elf,
                               encoding, soft, tar, json, text,
                               tokens
      --exclude-quiet TEST         like exclude, but ignore unknown tests
  -f, --files-from FILE      read the filenames to be examined from FILE
  -F, --separator STRING     use string as separator instead of `:'
  -i, --mime                 output MIME type strings (--mime-type and
                               --mime-encoding)
      --apple                output the Apple CREATOR/TYPE
      --extension            output a slash-separated list of extensions
      --mime-type            output the MIME type
      --mime-encoding        output the MIME encoding
  -k, --keep-going           don't stop at the first match
  -l, --list                 list magic strength
  -L, --dereference          follow symlinks (default if POSIXLY_CORRECT is set)
  -h, --no-dereference       don't follow symlinks (default if POSIXLY_CORRECT is not set) (default)
  -n, --no-buffer            do not buffer output
  -N, --no-pad               do not pad output
  -0, --print0               terminate filenames with ASCII NUL
  -p, --preserve-date        preserve access times on files
  -P, --parameter            set file engine parameter limits
                                   bytes 1048576 max bytes to look inside file
                               elf_notes     256 max ELF notes processed
                               elf_phnum    2048 max ELF prog sections processed
                               elf_shnum   32768 max ELF sections processed
                                encoding   65536 max bytes to scan for encoding
                                   indir      50 recursion limit for indirection
                                    name      50 use limit for name/use magic
                                   regex    8192 length limit for REGEX searches
  -r, --raw                  don't translate unprintable chars to \ooo
  -s, --special-files        treat special (block/char devices) files as
                             ordinary ones
  -S, --no-sandbox           disable system call sandboxing
  -C, --compile              compile file specified by -m
  -d, --debug                print debugging messages