In addition to redirecting input/output to a named file, you can connect two
commands together so that the output from one program becomes the input of
the next program.
Two or more commands connected in this way form a pipe
To make a pipe, put a vertical bar (
When a program takes its input from another program, performs some operation on that input, and writes the result to the standard output (which may be piped to yet another program), it is referred to as a filter . One of the most common uses of filters is to modify output. Just as a common filter culls unwanted items, the UNIX filters can be used to restructure output.
Almost all UNIX commands can be used to form pipes. Some programs that are commonly used as filters are described below. Note that these programs aren't used only as filters or parts of pipes. They're also useful on their own.
The name "grep" derives from the ed (a UNIX line editor) command g/re/p which means "g lobally search for a r egular e xpression and p rint all lines containing it." A regular expression is either some plain text (a word, for example) and/or special characters used for pattern matching. When you learn more about regular expressions, you can use them to specify complex patterns of text. See Mastering Regular Expressions , by Jeffrey Friedl (O'Reilly & Associates, 1997), and the references in Appendix A, Reading List .
The simplest use of grep is to look for a pattern consisting of a single word. It can be used in a pipe so that only those lines of the input files containing a given string are sent to the standard output. If you don't give grep a filename to read, it reads its standard input; that's the way all filter programs work:
First, our example runs ls -l to list your directory. The standard output of ls -l is piped to grep , which only outputs lines that contain the string "Aug" (that is, files that were last modified in August). Because the standard output of grep isn't redirected, those lines go to the terminal screen.
grep options let you modify the search. Table 5.1 lists some of the options.
Next, let's use a regular expression that tells grep to find lines with "carol", followed by zero or more other characters (abbreviated in a regular expression as ".*"), then followed by "Aug". For more about regular expressions, see the references in Appendix A .
The sort program arranges lines of text alphabetically or numerically. The example below sorts the lines in the food file (from Chapter 4, File Management ) alphabetically. sort doesn't modify the file itself; it reads the file and writes the sorted text to the standard output.
sort arranges lines of text alphabetically by default. There are many options that control the sorting. Some of these are given in Table 5.2 .
More than two commands may be linked up into a pipe. Taking a previous pipe example using grep , we can further sort the files modified in August by order of size. The following pipe consists of the commands ls , grep , and sort :
This pipe sorts all files in your directory modified in August by order of size, and prints them to the terminal screen. The sort option +4n skips four fields (fields are separated by blanks) then sorts the lines in numeric order. So, the output of ls (actually, the output of grep ) is sorted by the file size (the fifth column, starting with 1605). Both grep and sort are used here as filters to modify the output of the ls -l command. If you wanted to email this listing to someone, you could add a final pipe to the mail command. Or you could print the listing by piping the sort output to your printer command (like lp or lpr ).
The more and pg programs that you saw earlier can also be used as filters. A long output would normally zip by you on the screen, but if you run text through more or pg as a filter, the display stops after each screenful of text.
Let's assume that you have a long directory listing. (If you want to try this example and need a directory with lots of files, use cd first to change to a system directory like /bin or /usr/bin .) To make it easier to read the sorted listing, pipe the output through more :
The screen will fill up with one screenful of text consisting of lines sorted by order of file size. At the bottom of the screen is the more prompt where you can type a command to move through the sorted text. When you're done with this screen, you can use any of the commands listed in the discussion of the more program.
In the following exercises you'll redirect output, create a simple pipe, and use filters to modify output.