What are Redirection Operators in Dos with example and how-to?

About

This page talks about shell redirection operator|operator in DOS.

The order of redirections is significant when redirecting a standard error stream to a standard output stream. See order of redirections

Handle

The standard predefined stream that can handle the operators are the below one and have a unique id

Operators

Redirecting Output

> (Greater than)

Greater than > redirect:

• a standard output stream (stdout, stderr, …)

to:

• a standard output stream (generally stderr to stdout)
• or to new file

Syntax:

[SourceHandleId]>[&]TargetWord

where:

• SourceHandleId is the unique id of an handle by default 1 (stdout). One is the default handle (ie > is equivalent to >1 (stdout))
• & indicates that the targetWord is a file descriptor and not a filename
• TargetWord is a word that is subject to expansion (ie you can use a variable, …) but normally you will set it to
  • TargetHandleId is the unique id of an handle if & is present
  • or filePath a file path that must be created or overwritten.

Example:

• Redirect stderr to a new file

command 2> stderr.log

• Redirect stdout to a new file

command 1> stdout.log
Rem or simply
command > stdout.log

• Redirect stderr to a new file as variable

set FILE_STDERR=err.log
command 2> %FILE_STDERR%

• Redirecting the stderr to nothing (discarding it)

command 2>nul

• Redirecting standard output and error output to File.txt. The redirections of the stderr to stdout must appears after, the redirection to the file. See order_of_precedence

dir >c:\file.txt 2>&1

» (Append)

>> has the same syntax than > but it will append to the file. It will not create it or truncate as > does.

Redirecting Input - < (Less than)

<&: Reads the input from one handle and writes it to the output of another handle.

[StandardStreamHandleId]<[&]SourceWord

where:

• HandleId is a handle of a standard stream (generally 2)
• & indicates that the targetWord is a file descriptor and not a filename
• SourceWord is a word that is subject to expansion (ie you can use a variable, …) but normally you will set it to
  • SourceHandleId is the unique id of an handle if & is present. Zero is the default handle (ie < is equivalent to <0 (stdin))
  • or filePath a file path that must be created or overwritten.

Example:

• Sort the command input from a file (File.txt) The < operator opens the specified file name with read-only access.

sort  <file.txt
REM or 
<file.txt  sort  

• Redirecting handle 2 (STDERR) into handle 1 (STDOUT):

2<&1

Redirecting Output to Input - | (Pipeline or Command Chaining)

The | pipeline (See also DOS - Filter Commands) operator redirect the stdout (not the stderr) of the source program to the stdin of the target program

Syntax:

command parameter | command parameter

mySourceCommand  2>&1 | myTargetCommand

mySourceCommand  2>&1 1>null | myTargetCommand

Order of precedence

The order of the standard streams redirection is important

The order of redirections is significant when you redirect the standard streams. The redirections of standard streams happens from left to right (and not from right to left)

dir >targetFile.txt 2>&1

dir 2>&1 >targetFile.txt

The redirections may appears before or after the command

• Example 1

command 2>&1 >targetFile.txt
Rem is equivalent to
2>&1 >targetFile.txt command

• Example 2: A command can have both its standard input and standard output redirected.

sort < file_to_be_sorted.txt > sorted_file_list.txt
Rem is equivalent to
< fileToBeSorted.txt > fileSorted.txt sort

How to

Create a file with content

DOS - File

set myFile=myFile.sql
> %myFile% echo.WHENEVER SQLERROR EXIT SQL.SQLCODE
>> %myFile% echo.select * from dual;
>> %myFile% echo.exit;
>> %myFile% echo.

Content:

WHENEVER SQLERROR EXIT SQL.SQLCODE
select * from dual;
exit;

Documentation / Reference

• Using command redirection operators