5.2. Pipes and Filters
We've seen how to redirect input from a file and output to a file. You can also connect two programs together so that the output from one program becomes the input of the next program. Two or more programs connected in this way form a pipe. To make a pipe, put a vertical bar (|) on the command line between two commands. When a pipe is set up between two commands, the standard output of the command to the left of the pipe symbol becomes the standard input of the command to the right of the pipe symbol. Any two commands can form a pipe as long as the first program writes to standard output and the second program reads from standard input.
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. A common use of filters is to modify output. Just as a common filter culls unwanted items, Unix filters can restructure output.
Most Unix programs can be used to form pipes. Some programs that are commonly used as filters are described in the next sections. Note that these programs aren't used only as filters or parts of pipes. They're also useful on their own.
grep "pattern" file(s)
The name "grep" derives from the ed (a Unix line editor) command g/re/p, which means "globally search for a regular expression and print 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.
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. But let's start with an example reading from files: searching all files in the working directory for a word--say, Unix. We'll use the wildcard * to quickly give grep all filenames in the directory.
$ grep "Unix" * ch01:Unix is a flexible and powerful operating system ch01:When the Unix designers started work, little did ch05:What can we do with Unix? $
When grep searches multiple files, it shows the filename where it finds each matching line of text. Alternatively, if you don't give grep a filename to read, it reads its standard input; that's the way all filter programs work:
$ ls -l | grep "Aug" -rw-rw-rw- 1 john doc 11008 Aug 6 14:10 ch02 -rw-rw-rw- 1 john doc 8515 Aug 6 15:30 ch07 -rw-rw-r-- 1 john doc 2488 Aug 15 10:51 intro -rw-rw-r-- 1 carol doc 1605 Aug 23 07:35 macros $
First, the 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.
Table 5-1. Some grep 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:
$ ls -l | grep "carol.*Aug" -rw-rw-r-- 1 carol doc 1605 Aug 23 07:35 macros $
The sort program arranges lines of text alphabetically or numerically. The following example sorts the lines in the food file (from Section 4.5 in Chapter 4) alphabetically. sort doesn't modify the file itself; it reads the file and writes the sorted text to the standard output.
$ sort food Afghani Cuisine Bangkok Wok Big Apple Deli Isle of Java Mandalay Sushi and Sashimi Sweet Tooth Tio Pepe's Peppers
By default, sort arranges lines of text alphabetically. Many options control the sorting, and Table 5-2 lists some of them.
Table 5-2. Some sort options
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 uses the commands ls, grep, and sort:
$ ls -l | grep "Aug" | sort +4n -rw-rw-r-- 1 carol doc 1605 Aug 23 07:35 macros -rw-rw-r-- 1 john doc 2488 Aug 15 10:51 intro -rw-rw-rw- 1 john doc 8515 Aug 6 15:30 ch07 -rw-rw-rw- 1 john doc 11008 Aug 6 14:10 ch02 $
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, filtered by grep, is sorted by the file size (this is 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 program. Or you could print the listing by piping the sort output to your printer command (either lp or lpr).
5.2.3. Piping to a Pager
The less program, which you saw in Section 3.2 in Chapter 3, can also be used as a filter. A long output normally zips by you on the screen, but if you run text through less, 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 such as /bin or /usr/bin.) To make it easier to read the sorted listing, pipe the output through less:
$ ls -l | grep "Aug" | sort +4n | less -rw-rw-r-- 1 carol doc 1605 Aug 23 07:35 macros -rw-rw-r-- 1 john doc 2488 Aug 15 10:51 intro -rw-rw-rw- 1 john doc 8515 Aug 6 15:30 ch07 -rw-rw-r-- 1 john doc 14827 Aug 9 12:40 ch03 . . . -rw-rw-rw- 1 john doc 16867 Aug 6 15:56 ch05 :
less reads a screenful of text from the pipe (consisting of lines sorted by order of file size), then prints a colon (:) prompt. At the prompt, you can type a less command to move through the sorted text. less reads more text from the pipe and shows it to you, as well as saves a copy of what it has read, so you can go backwards to reread previous text if you want to. (The simpler pager programs more and pg generally can't back up while reading from a pipe.) When you're done seeing the sorted text, the q command quits less.
22.214.171.124. Exercise: redirecting input/output
In the following exercises you redirect output, create a simple pipe, and use filters to modify output.
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