Demystifying the `whatis` Command in Linux

When you're getting started with Linux or even if you're an experienced user, there will be times when you need to quickly understand what a particular command does. The whatis command in Linux serves this purpose by providing a brief description of command-line programs, system calls, and library functions. In this article, we will delve into the whatis command, discussing its syntax, options, and examples of how to use it.

Syntax

The basic syntax of the whatis command is quite simple:

whatis [options] keyword

In this format, [options] refers to optional flags you can use to alter the behavior of whatis, and keyword is the name of the command, system call, or function you wish to know more about.

Options Table

Option	Shorthand	Description
--regex	-r	Interpret keyword as a regular expression
--wildcard	-w	Interpret keyword as a wildcard expression
--long	-l	Do not trim output to fit terminal width
--apropos	-a	Also search for approximate matches
--case-insensitive	-i	Case-insensitive search

Examples with Outcomes

Basic Usage

To quickly get an understanding of the ls command, you can run:

Command:

whatis ls

Sample Output:

ls (1)               - list directory contents

This output tells you that ls is used to list directory contents and that it falls under section 1 (general commands) of the manual.

Using --regex Option

To find commands that contain the pattern ls, you can use the --regex option:

Command:

whatis -r "ls"

Sample Output:

ls (1)               - list directory contents
vls (1)              - ls-like utility for VIPTV streams

Using --apropos Option

For example, let's say you want to find out about commands that have something to do with listing (list). Using the --apropos option will find approximate matches related to the keyword.

Command:

whatis --apropos list

Sample Output:

list (n)             - Element of a list. Lists may be nested.
ls (1)               - list directory contents
lscpu (1)            - display information about the CPU architecture
lsattr (1)           - list file attributes on a Linux second extended filesystem
...

In this example, --apropos expanded the search to include commands and terms related to 'list', including ls, lscpu, lsattr, and even a term like ' list' from the Tcl programming language (as indicated by section 'n' for Tcl/Tk commands).

This option makes whatis a more flexible tool when you're exploring commands but are not exactly sure what you're looking for.

Using --wildcard Option

If you'd like to look for commands that start with 'ls', you can use a wildcard search:

Command:

whatis -w 'ls*'

Sample Output:

ls (1)               - list directory contents
lsattr (1)           - list file attributes on a Linux second extended filesystem
lscpu (1)            - display information about the CPU architecture
...

Combining Multiple Options

You can also combine multiple options for a more nuanced search:

Command:

whatis -wi 'LS'

Sample Output:

ls (1)               - list directory contents

Here we used the case-insensitive (-i) option along with the wildcard (-w) option to find commands that start with 'ls' regardless of the case.

Conclusion

The whatis command is an incredibly useful tool for quickly finding out what a particular Linux command, system call, or library function does. Whether you're a beginner learning the ropes or an advanced user looking for quick reminders, whatis can help you navigate Linux's vast array of commands more efficiently. This guide aimed to provide a comprehensive look at whatis, covering its syntax, options, and practical examples, to get you started on mastering this helpful command.

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