Tuesday, January 31, 2012

Command Redirection

Redirection is a function common to most command-line interpreters, that can redirect standard streams to user-specified locations. The input or output stream location is referred to as a handle.
Definition: Redirection is the switching of a standard stream of data so that it comes from a source other than its default source or or that it goes to some destination other than its default destination.
standard streams for input, output, and error
Redirection operatorDescription
>Writes the command output to a file or a device, such as a printer, instead of the Command Prompt window.
<Reads the command input from a file, instead of reading input from the keyboard.
>>Appends the command output to the end of a file without deleting the information that is already in the file.
>&Writes the output from one handle to the input of another handle.
<&Reads the input from one handle and writes it to the output of another handle.
|Reads the output from one command and writes it to the input of another command. Also known as a pipe.
The following table lists the available handles:
HandleNumeric equivalentDescription
STDIN0Keyboard input
STDOUT1Output to the Command Prompt window
STDERR2Error output to the Command Prompt window
UNDEFINED3-9These handles are defined individually by the application and are specific to each tool.
The numbers zero through nine (that is, 0-9) represent the first 10 handles. You can use Cmd.exe to run a program and redirect any of the first 10 handles for the program. To specify which handle you want to use, type the number of the handle before the redirection operator. If you do not define a handle, the default < redirection input operator is zero (0) and the default > redirection output operator is one (1). After you type the < or > operator, you must specify where you want to read or write the data. You can specify a file name or another existing handle.
To specify redirection to existing handles, use the ampersand (&) character followed by the handle number that you want to redirect (that is, &handle#). For example, the following command redirects handle 2 (that is, STDERR) into handle 1 (that is, STDOUT): 1<&2
Duplicating handles:
command 2> filenameRedirect any error message into a file
command 2>> filenameAppend any error message into a file
(command) 2> filenameRedirect any CMD.exe error into a file
command > file 2>&1Redirect errors and output to one file
command > file 2<&1Redirect output and errors to one file
command > fileA 2> fileBRedirect output and errors to separate files
command 2>&1 > filenameThis will fail!
Redirect to NUL (hide errors):
command 2> nulRedirect error messages to NUL
command >nul 2>&1Redirect error and output to NUL
command >filename 2> nulRedirect output to file but suppress error
(command) >filename 2> nulRedirect output to file but suppress CMD.exe errors

Note, any long filenames must be surrounded in "double quotes". A CMD error is an error raised by the command processor itself rather than the program/command.

Redirection with > or 2> will overwrite any existing file.

You can also redirect to a printer with > PRN or >LPT1

To prevent the > and < characters from causing redirection, escape with a caret: ^> or ^<

Using the pipe operator (|)
The pipe operator (|) takes the output (by default, STDOUT) of one command and directs it into the input (by default, STDIN) of another command. For example, the following command sorts a directory:
dir | sort
In this example, both commands start simultaneously, but then the sort command pauses until it receives the dir command's output. The sort command uses the dir command's output as its input, and then sends its output to handle 1 (that is, STDOUT).

Examples of redirection:
DIR >MyFileListing.txt
   DIR /o:n >"Another list of Files.txt"

   ECHO y| DEL *.txt

   ECHO Some text ^<html tag^> more text
   MEM /C >>MemLog.txt

   Date /T >>MemLog.txt

   SORT < MyTextFile.txt

   SET _output=%_missing% 2>nul

   DIR C:\ >List_of_C.txt 2>errorlog.txt
   FIND /i "Jones" < names.txt >logfile.txt

   DIR C:\ >List_of_C.txt & DIR D:\ >List_of_D.txt

   ECHO DIR C:\ ^> c:\logfile.txt >NewScript.cmd

   (TYPE logfile.txt >> newfile.txt) 2>nul

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.