Redirecting Command Output and Input
Windows 7 is always directing things here and there. This generally falls into two categories:
A device called CON (console)
normally handles standard input and standard output, which is your
keyboard and monitor. Windows 7 assumes that all command input comes
from the keyboard and that all command output (such as a DIR listing or a system message) goes to the screen. Redirection is just a way of specifying different input and output devices.
Redirecting Command Output
To send command output to somewhere other than the screen, you use the output redirection operator (>).
One of the most common uses for output redirection is to capture the
results of a command in a text file. For example, you might want to use
the report produced by the SYSTEMINFO command as part of a word processing document. You could use the following command to first capture the report as the file systeminfo.csv:
systeminfo /fo csv > %userprofile%\systeminfo.csv
When you run this command, the usual SYSTEMINFO data doesn’t appear onscreen. That’s because you directed it away from the screen and into the systeminfo.csv file.
You can use this technique to capture DIR listings, CHKDSK
reports, and more. One caveat: If the file you specify as the output
destination already exists, Windows 7 overwrites it without warning. To
avoid this, you can use the double output redirection symbol (>>).
This tells Windows 7 to append the output to the end of the file if the
file exists. For example, suppose you used the following command to
output the results of the CHKDSK C: command to chkdsk.txt:
chkdsk c: > %userprofile%\chkdsk.txt
If you then want to append the results of the CHKDSK D: command to chkdsk.txt, you enter the following command:
chkdsk d: >> %userprofile%\chkdsk.txt
You can also redirect output to different devices. Table 4 lists the various devices that Windows 7 installs each time you start your system.
Table 4. Devices Installed by Windows 7 When You Start Your System
| Device Name | Device |
|---|
| AUX | Auxiliary device (usually COM1) |
| CLOCK$ | Real-time clock |
| COMn | Serial port (COM1, COM2, COM3, or COM4) |
| CON | Console (keyboard and screen) |
| LPTn | Parallel port (LPT1, LPT2, or LPT3) |
| NUL | NUL device (nothing) |
| PRN | Printer (usually LPT1) |
For example, you can send a DIR
listing to the printer with the following command. (Of course, you need
to be sure that your printer is on before doing this. Also note that
this only works for a printer attached to a parallel port; it doesn’t
work for USB printers.)
The NUL device usually throws people for a loop when they first see it. This device (affectionately known as the bit bucket)
is, literally, nothing. Batch files normally use it to suppress the
usual messages Windows 7 displays when it completes a command. For
example, Windows 7 normally says 1 file(s) copied when you copy a file. However, the following command sends that message to NUL, so you wouldn’t see it onscreen:
copy somefile.doc \\server\users\paul\ > nul
Tip
Unfortunately,
Windows 7 gives you no way to redirect output to a USB port. However,
there’s a workaround you can use if you’re trying to redirect output to
a USB printer. Assuming the printer is shared and that no other device
is using the port LPT2, run the following command:
NET USE LPT2 \\server\printer
Here, replace server with the name of your Windows 7 computer and printer with the share name of the USB printer. Now, when you redirect output to LPT2, Windows 7 sends the output to the USB printer.
Redirecting Input
The input redirection operator (<)
handles getting input to a Windows 7 command from somewhere other than
the keyboard. Input redirection is almost always used to send the
contents of a text file to a Windows 7 command. The most common example
is the MORE command, which displays one screen of information at a
time. If you have a large text file that scrolls off the screen when
you use TYPE, the following command, which sends the contents of BIGFILE.TXT to the MORE command, solves the problem:
When you run this command, the first screen of text appears, and the following line shows up at the bottom of the screen:
Just press any key, and MORE displays the next screen. (Whatever you do, don’t mix up < and > when using MORE. The command more > bigfile.txt erases BIGFILE.TXT!) MORE is an example of a filter command. Filters process whatever text is sent through them. The other Windows 7 filters are SORT and FIND, which I discuss in a moment.
Another handy use for input redirection is to send keystrokes to Windows 7 commands. For example, create a text file called enter.txt that consists of a single press of the Enter key, and then try this command:
Windows 7 displays the current date, and instead of waiting for you to either type in a new date or press Enter, it just reads enter.txt
and uses its single carriage return as input. (For an even easier way
to input the Enter key to a command, check out the next section.)
One common recipient of redirected input is the SORT command. SORT,
as you might guess from its name, sorts the data sent to it and
displays the results onscreen. So, for example, here’s how you would
sort a file called JUMBLED.TXT:
Instead of merely displaying the results of the sort onscreen, you can use > to redirect them to another file.
Tip
SORT normally starts with the first column and works across. To start with any other column, use the /+n switch, where n
is the number of the column you want to use. To sort a file in reverse
order (that is, a descending sort—Z to A, then 9 to 0—instead of an
ascending sort—0 to 9, then A to Z), use the /R switch.
Piping Commands
Piping
is a technique that combines both input and output redirection. Using
the pipe operator (|), the output of one command is captured and sent
as input to another command. For example, the SYSTEMINFO
command displays about five screens of data, so you usually need to
scroll back to see the data you’re looking for. However, you can pause
the output by piping it to the MORE command:
The pipe operator captures the SYSTEMINFO output and sends it as input to MORE, which then displays the SYSTEMINFO results one screen at a time.
Note
Piping
works by first redirecting the output of a command to a temporary file.
It then takes this temporary file and redirects it as input to the
second command. A command such as SYSTEMINFO | MORE is approximately equivalent to the following two commands:
SYSTEMINFO > tempfile
MORE tempfile
I
showed you in the preceding section how to use input redirection to
send keystrokes to a Windows 7 command. But if you have to send only a
single key, piping offers a much nicer solution. The secret is to use
the ECHO command to echo the character you need and then pipe it to the Windows 7 command.
For example, if you use the command DEL *.*,
Windows 7 always asks whether you’re sure that you want to delete all
the files in the current directory. This is a sensible precaution, but
you can override it if you do things this way:
Here, the y that would normally be echoed to the screen is sent to DEL
instead, which interprets it as a response to its prompt. This is a
handy technique for batch files in which you want to reduce or even
eliminate user interaction.
Tip
You can even use this technique to send an Enter keypress to a command. The command ECHO. (that’s ECHO
followed by a period) is equivalent to pressing Enter. So, for example,
you could use the following command in a batch file to display the time
without user input:
