A filter is a program that accepts input, transforms it, and outputs the transformed data. Filters are closely associated with several ideas basic to Unix: standard input, standard output, input/output redirection, and pipes.

Standard input and output

Standard input and output refer to the default places from which a program will take input and to which it will write output. The standard input (stdin) for a program running interactively at the command line is the keyboard; the standard output (stdout) is the terminal screen.

Redirection

With input/output redirection, a program can take input or send output someplace other than standard input or output -- to or from a file, for instance. Redirection of stdin is accomplished using the < symbol, and redirection of stdout by the > symbol. For example:

$ ls > list

redirects the output of the ls command, which would normally go into a file called list. Similarly,

$ cat < list

redirects the input for cat, which, in the absence of a filename, would be expected to come from the file list. So we print the contents of that file on screen.

Pipes

Pipes connect programs through I/O redirection and are denoted by the | symbol. For example:

$ ls | less

is a common way of comfortably viewing the output from a directory listing where there are more files than will fit on the screen.

grep

The principle of grep is very simple: search the input for a pattern, and then output that pattern. Here's an example:

$ grep 'Linus Torvalds' *

This searches all the files in the current directory for the name Linus Torvalds.

Various command-line switches can modify grep's behavior. For example, if we aren't sure about case, we can write:

$ grep -y 'linus torvalds'

The -y switch tells grep to match without considering case. If you use uppercase letters in the pattern, however, they'll still match only uppercase. (This is broken in GNU grep, which ignores case when given the -y switch -- that's what the -i switch is for).

Given even this much grep, it's easy to construct a practical application. Store name and address details in a file and you've got a searchable address book.

$ grep -y [search arg] ~/lib/phone-book

Put the command above in a text file called filename and make it executable:

$ chmod +x filename

But suppose we want to find all the occurrences of a text string that could be a reference to Linus Torvalds. Searching for Linus Torvalds won't find Linus or Torvalds individually. We need a way of saying, "This or this or this." Here's where egrep (extended grep) comes in. This handy program modifies standard grep to provide just such a syntax.

$ egrep 'Linus Torvalds|L\. Torvalds|L\. T\.|Mr\. Torvalds'

This will find most ways of naming the inventor of Linux. Note the backslashes to escape the full stop; because that's a special character in regular

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