Redirections

Redirections allow standard input and outputs to me redirected by the command interpretor.

Interpreter’s input and outputs

Traditionally, every console program is given three files that a referred to as standard output and inputs.

• Standard input (stdin) : The default input of console programs. Usually this input refers to text typed directly in the the terminal. As an example, when no file path is given to pBat, it reads commands directly from the standard input.

• Standard Output (stdout) : The default output for console programs. Usually, this output refers to the terminal. This output is usually the default output for programs. As an example, TYPE uses stdout to output files.

• Standard Error (stderr) : This is the default output used to print error message. It is usually used to separate error message from actual output in redirections.

Theses three output can be redirected using pBat. However, cmd.exe allows user to redirect any file descriptor through redirections. But this could lead to absurd results since knowing which descriptor refers to which file. Moreover, this behavior can lead to strange side effects. Thus, this feature is not supported by pBat.

Redirecting input and outputs

pBat uses < and > characters to set redirections. > character is used to redirect stdout and stderr, whereas the < is used to specify stdin redirections.

When pbat encounters either a > or a <, then pBat uses the very first parameter to the right of the same an use it as the name of a file to which the input or output will be redirected. The name of the file and the sign do not have to be separated by any blank character. Then pBat removes the file name from the command line. The > or < signs can be used anywhere in the command line, all of the following lines are valid.

echo >file redirection test
> file echo redirection test
echo redirection test > file

Any of the previous syntax are valid. However, some of them give the code more readability, as such, using the third syntax is highly recommended.

Although the is only one syntax using <, redirections using > are provided with a bunch of different syntaxes, which are the following.

• The default behaviour if you use only the > sign, is to redirect the standard output to the specified file.

    echo redirecting stdout > file
  

  The previous example will redirect stdout to file. That is, file will be emptied and will recieve “redirecting stdout” as its new content.

  File emptying can be inhibited by doubling the > sign to get the following code. In that case, the line “redirecting stdout” will be added at the end of file.

    echo redirecting stdout >> file
  

• Either of stdout or stderr can be chosen by preceding the > sign by either 1 or 2. As a result, the stream that gets redirected to file change depending on the number:

  • 1 : standard output.

  • 2 : standard error.

  As an example, stderr can be redirected to file using:

    echo redirecting stderr 2> file
  

• It possible to redirect either stderr to stdout or stdout to stderr using 2>&1 or 1>&2.

  Thus, stderr and stdout can be redirected to file using the following syntax:

    echo redirecting stderr and stdout at the same > file 2>&1
  

  Note that pBat allow the 2>&1 to be placed before the actual redirection, even though this would raise an error using cmd.exe

Compatibility

Partially compatible with cmd.exe.

Available since 0.48.

See Also

Command line, Conditionnal operators