Chapter 19: Filters and Utilities

Overview

One of the most valuable features of the UNIX System is the rich set of commands it gives you. This chapter surveys a particularly useful set of commands that are often referred to as tools or utilities. They are small, modular commands, each of which performs a specific function, such as sorting a list or searching for a word in a file. You can use them singly and in combination to carry out many common tasks.

Most of the tools described in this chapter are what are often referred to as filters. Filters are programs that read standard input, operate on it, and produce the result as standard output. They are not interactive-they do not prompt you or wait for input. Filters are often used with other commands in a command pipeline. By allowing you to combine filters in pipelines, the UNIX System makes it easy to accomplish tasks that would be overly difficult and time-consuming in other operating systems.

Most of the filters are designed to work with text or with text files. In general, filters do not modify the original file, so you can experiment without much risk of overwriting data. (Exceptions to this rule are carefully noted.) Also, most of the tools in this chapter have other command-line options that are not included here. To get more details about the options that are available, check the man pages or the references at the end of this chapter.

A number of the tools described in this chapter have features that are especially useful in dealing with files containing structured lists. Such files are often used as simple databases. Typically, each line in the file is a separate record containing information about a particular item. The information is often structured in fields. For example, each line in a personnel file may contain a record consisting of information about one employee, with fields for name, address, phone number, and so forth. The UNIX System includes tools to search, edit, and reformat this type of file.

This chapter also describes a number of miscellaneous tools, including commands for compressing files, performing numerical calculations, and monitoring input and output. For other utilities, see Chapter 3 (which includes the commands for working with files and directories) and Chapter 5 (which explains the main tools for editing text). The chapter after this one, which shows you how to write shell scripts, includes many uses of the tools presented here. And Chapter 21 explains how to use awk and sed, a very powerful pair of tools for working with files and pattern matching.

Most of the tools described here can be found in any standard UNIX or Linux system. A few, such as patch and tac, come with Linux but are not part of the standard UNIX command set. You can download free versions of many of these tools through the GNU project, at http://www.gnu.org/. Versions of most of the tools mentioned in this chapter are also available for Microsoft Windows through the MKS toolkit (http://www.mkssoftware.com/).