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file Command in Linux
The file command in Linux determines the file type. It performs a sequence of tests for each argument. These tests include the filesystem test, magic test, and language test. The first successful test determines the file type.
The filesystem test uses a stat system call to identify the attributes of a file, such as size and other parameters. The magic test identifies a file's magic number and determines the file type. The magic number is the byte sequence found at the beginning of the file, indicating the file format.
The language test identifies file types based on specific patterns and signatures. It searches for particular strings that can appear in the first few blocks of a file to identify the file type.
Table of Contents
Here is a comprehensive guide to the options available with the file command −
Syntax of file Command
The syntax of the Linux file command is as follows −
file [options] [filename]
The [options] field is used to specify the options to modify the command’s behavior. The [filename] option is used to specify the filename whose type is needed to be determined.
file Command Options
The options used with the file command are listed below −
| Flags | Options | Description |
|---|---|---|
| -b | --brief | It is used to skip the filename from the output |
| -C | --compile | It is used to compile file specified by -m |
| -c | --checking-printout | It is used to print the parsed form of a magic file, used with -m to debug a magic file before installing |
| -d | --debug | It is used to print the debugging messages |
| -e testname | --exclude testname | It is used to exclude a specific test to be performed; valid tests are: apptype, ascii, cdf, compress, csv, elf, encoding, soft, tar, json, simh, text, tokens |
| -F string | --separator string | It is used to specify a string to be used as a separator instead of a colon (:) |
| -f file | --files-from file | It is used to read the files from the specified file to determine the file type |
| -h | --no-dereference | It is used to skip symlinks (default) |
| --help | It is used to display help related to command | |
| -i | --mime | It is used to display mime type and mime-encoding instead of human-readable form |
| --apple | It is used to display output in Apple CREATOR/TYPE format | |
| --extension | It is used to display the slash-separated list of extensions | |
| --mime-type | It is used to display the mime-type | |
| --mime-encoding | It is used to display mime encoding | |
| -k | --keep-going | It is used to tell the command not to stop at the first match |
| -L | --dereference | It is used to follow the symlinks |
| -m files | --magic-file files | It is used to specify alternative magic files and directories |
| -N | --no-pad | It is used to display output without padding |
| -n | --no-buffer | It is used to prevent the output buffer |
| -p | --preserve-date | It is used to preserve access times on files |
| -P | --parameter | It is used to set the engine parameter limits |
| -r | --raw | It is used to avoid unprintable characters to \ooo translation |
| -s | --special-files | It is used to treat the special files as ordinary |
| -S | --no-sandbox | It is used to disable a system call sandboxing |
| -v | --version | It is used to print the command version |
| -z | --uncompress | It is used to look inside the compressed files |
| -Z | --uncompress-noreport | It is used to print the contents of compressed files |
| -0 | --print0 | It is used to terminate the filenames with ASCII null |
Examples of file Command in Linux
This section demonstrates the usage of the Linux file command through various examples −
- Displaying File Type
- Displaying File Type without Filename
- Displaying the Debugging Message
- Displaying Additional Information on a Block Device
- Displaying the MIME Type and MIME Encoding
- Excluding a Test Type
Displaying File Type
To display the file type of file, use the file command without any option −
file test
It is a useful tool to determine the file type when the extension is not mentioned.
In the example, the file has no extension, but the file command identifies it as an executable, as shown in the output image.
Similarly, other file types can be determined.
Displaying File Type without Filename
By default, the filename comes with the file type in the file command standard output. To skip the filename, use the -b or --brief option −
file -b test
Displaying the Debugging Message
To display the debugging messages with the output, use the -d or --debug option −
file -d test
Displaying Additional Information on a Block Device
By default, determining the file type of a block device file will output a simple file type. To treat the block device file as an ordinary file, the -s or --special-file option is used −
sudo file -s /dev/vda
As an ordinary file, the block device file will be analyzed for further information.
Displaying the MIME Type and MIME Encoding
To display the file type in MIME format, use the -i or --mime option −
file -i test
The charset in the output indicates the MIME encoding.
MIME stands for Multipurpose Internet Mail Extensions and is used to specify the nature of a file or data. For example, a text will be displayed as text/plain, and an image as image/png.
Note that the MIME type and MIME encoding can be displayed separately using the --mime-type and --mime-encoding options:
file --mime-type test file --mime-encoding test
Excluding a Test Type
It should be "To exclude a specific test from determining the file type, use the -e or --exclude option. For example, to exclude the text test, use −
file -e text test
Other test types that can be excluded are apptype, ascii, cdf, compress, csv, elf, encoding, soft, tar, json, simh, tokens.
Conclusion
The file command in Linux is used to determine the file type by performing a sequence of tests. These tests include filesystem, magic, and language tests. It is a handy tool to determine file types without relying on the extensions.
In this tutorial, we explained the file command, its syntax, options, and usage in Linux through various examples.