1.7. Input and Output

You may have used some of these before and noticed that they take names of input files as arguments and produce output on standard output. You may not know, however, that all of them (and most other UNIX utilities) accept input from standard input if you omit the argument. ^[13]

^[13] If a particular UNIX utility doesn't accept standard input when you leave out the filename argument, try using a dash ( - ) as the argument. Some UNIX systems provide standard input as a file, so you could try providing the file /dev/stdin as the input file argument.

For example, the most basic utility is cat , which simply copies its input to its output. If you type cat with a filename argument, it will print out the contents of that file on your screen. But if you invoke it with no arguments, it will expect standard input and copy it to standard output. Try it: cat will wait for you to type a line of text; when you type RETURN, cat will repeat the text back to you. To stop the process, hit CTRL-D at the beginning of a line. You will see ^D when you type CTRL-D. Here's what this should look like:

$

cat


Here is a line of text.

Here is a line of text.

This is another line of text.

This is another line of text.

^D

$

1.7.2. I/O Redirection

cat is short for "catenate," i.e., link together. It accepts multiple filename arguments and copies them to the standard output. But let's pretend , for now, that cat and other utilities don't accept filename arguments and accept only standard input. As we said above, the shell lets you redirect standard input so that it comes from a file. The notation command < filename does this; it sets things up so that command takes standard input from a file instead of from a terminal.

For example, if you have a file called cheshire that contains some text, then cat < cheshire will print cheshire 's contents out onto your terminal. sort < cheshire will sort the lines in the cheshire file and print the result on your terminal (remember: we're pretending that these utilities don't take filename arguments).

Similarly, command > filename causes the command 's standard output to be redirected to the named file. The classic "canonical" example of this is date > now : the date command prints the current date and time on the standard output; the previous command saves it in a file called now .

Input and output redirectors can be combined. For example: the cp command is normally used to copy files; if for some reason it didn't exist or was broken, you could use cat in this way:

$

cat <


file1


>


file2

This would be similar to cp file1 file2 .

1.7.3. Pipelines

It is also possible to redirect the output of a command into the standard input of another command instead of a file. The construct that does this is called the pipe, notated as . A command line that includes two or more commands connected with pipes is called a pipeline.

Pipes are very often used with the more command, which works just like cat except that it prints its output screen by screen, pausing for the user to type SPACE (next screen), RETURN ( next line), or other commands. If you're in a directory with a large number of files and you want to see details about them, ls -l more will give you a detailed listing a screen at a time.

Pipelines can get very complex, and they can also be combined with other I/O directors. To see a sorted listing of the file cheshire a screen at a time, type sort < cheshire more . To print it instead of viewing it on your terminal, type sort < cheshire lp .

Here's a more complicated example. The file /etc/passwd stores information about users' accounts on a UNIX system. Each line in the file contains a user's login name, user ID number, encrypted password, home directory, login shell, and other information. The first field of each line is the login name ; fields are separated by colons (:). A sample line might look like this:

cam:LM1c7GhNesD4GhF3iEHrH4FeCKB/:501:100:Cameron Newham:/home/cam:/bin/bash

To get a sorted listing of all users on the system, type:

$

cut -d: -f1 < /etc/passwd  sort

(Actually, you can omit the < , since cut accepts input filename arguments.) The cut command extracts the first field ( -f1 ), where fields are separated by colons ( -d :), from the input. The entire pipeline will print a list that looks like this:

adm

bin

cam

daemon

davidqc

ftp

games

gonzo

...

If you want to send the list directly to the printer (instead of your screen), you can extend the pipeline like this:

$

cut -d: -f1 < /etc/passwd  sort  lp

Now you should see how I/O directors and pipelines support the UNIX building block philosophy. The notation is extremely terse and powerful. Just as important, the pipe concept eliminates the need for messy temporary files to store command output before it is fed into other commands.

For example, to do the same sort of thing as the above command line on other operating systems ( assuming that equivalent utilities are available...), you need three commands. On DEC's VAX/VMS system, they might look like this:

$

cut [etc]passwd /d=":" /f=1 /out=temp1

$

sort temp1 /out=temp2

$

print temp2

$

delete temp1 temp2

After sufficient practice, you will find yourself routinely typing in powerful command pipelines that do in one line what it would take several commands (and temporary files) in other operating systems to accomplish.