Redirection
Redirection can be used to reassign the standard input, standard output,
and standard error devices from their default settings (the keyboard and
screen) to another device like the printer or serial port, to a file, or
to the clipboard. You must use some discretion when you use redirection
with a device; there is no way to get input from the printer, for example.
Redirection always applies to a specific command, and lasts only for the
duration of that command. When the command is finished, the assignments
for standard input, standard output, and standard error revert to whatever
they were before the command.
In the descriptions below, filename means either the name of a file or of
an appropriate device (PRN, LPT1, LPT2, or LPT3 for printers; COM1 to COM4
for serial ports; CON for the keyboard and screen; CLIP: for the
clipboard; NUL for the "null" device, etc.).
Here are the standard redirection options supported by CMD.EXE (see below
for additional redirection options using numeric file handles):
< filename To get input from a file or device instead of
from the keyboard
> filename Redirect standard output to a file or device
>& filename Redirect standard output and standard error to
a file or device
>&> filename Redirect standard error only to a file or
device
If you want to append output to the end of an existing file, rather than
creating a new file, replace the first ">" in the last three commands
above with ">>" (i.e., use >>, >>&, and >>&>).
To use redirection, place the redirection symbol and filename at the end
of the command line, after the command name and any parameters. For
example, to redirect the output of the DIR command to a file called
DIRLIST, you could use a command line like this:
[c:\] dir /b *.dat > dirlist
You can use both input and output redirection for the same command, if
both are appropriate: For example, this command sends input to SORT from
the file DIRLIST, and sends output from SORT to the file DIRLIST.SRT:
[c:\] sort < dirlist > dirlist.srt
You can redirect text to or from the OS/2 clipboard by using the
pseudo-device name CLIP: (the colon is required).
If you redirect the output of a single internal command like DIR, the
redirection ends automatically when that command is done. If you start a
batch file with redirection, all of the batch file's output is
redirected, and redirection ends when the batch file is done. Similarly,
if you use redirection at the end of a command group, all of the output
from the command group is redirected, and redirection ends when the
command group is done.
When output is directed to a file with >, >&, or >&>, if the file already
exists, it will be overwritten. You can protect existing files by using
the SETDOS /N1 command, the "Protect redirected output files" setting
available on the Options 1 page of the OPTION dialogs, or the NoClobber
directive in the .INI file.
When output is appended to a file with >>, >>&, or >>&>, the file will be
created if it doesn't already exist. However, if NoClobber is set as
described in the above, append redirection will not create a new file;
instead, if the output file does not exist a "File not found" or similar
error will be displayed.
You can temporarily override the current setting of NoClobber by using an
exclamation mark [!] after the redirection symbol. For example, to
redirect the output of DIR to the file DIROUT, and allow overwriting of
any existing file despite the NoClobber setting:
[c:\] dir >! dirout
Redirection is fully nestable. For example, you can invoke a batch file
and redirect all of its output to a file or device. Output redirection
on a command within the batch file will take effect for that command
only; when the command is completed, output will revert to the redirected
output file or device in use for the batch file as a whole.
You can use redirection if you need to create a zero-byte file. To do so,
enter >filename as a command, with no actual command before the >
character.
In addition to the standard redirection options above, CMD.EXE also
supports the OS/2 CMD.EXE syntax:
n>file Redirect handle n to the named file
n>&m Redirect handle n to the same place as handle m
[n] and [m] are one-digit file handles between 0 and 9. You may not put
any spaces between the n and the >, or between the >, &, and m in the
second form. OS/2 interprets "0" as standard input, "1" as standard
output, and "2" as standard error. Handles 3 to 9 will probably not be
useful unless you have an application which uses those handles for a
specific, documented purpose, or you have opened a file with the
%@FILEOPEN variable function and the file handle is between 3 and 9.
The n>file syntax redirects output from handle n to a file. You can use
this form to redirect two handles to different places. For example:
[c:\] dir > outfile 2> errfil
sends normal output to a file called OUTFILE and any error messages to a
file called ERRFILE.
The n>&m syntax redirects handle n to the same location as the previously
assigned handle m. For example, to send standard error to the same file
as standard output, you could use this command:
[c:\] .dir > outfile 2>&1
Notice that you can perform the same operations by using standard CMD.EXE
redirection features. The two examples above could be written as
[c:\] dir > outfile >&> errfile
and
[c:\] dir >&outfile
Created using Inf-PHP v.2 (c) 2003 Yuri Prokushev
Created using Inf-HTML v.0.9b (c) 1995 Peter Childs