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Output Redirection and Pipes

As in POSIX shells, output of a command can be redirected and/or piped. This feature is fully cross-platform and works identically on Windows, macOS, and Linux.

Output Redirection

Redirect to a file

Redirecting the output of a cmd2 command to a file works just like in POSIX shells:

  • send to a file with >, as in mycommand args > filename.txt
  • append to a file with >>, as in mycommand args >> filename.txt

If you need to include any of these redirection characters in your command, you can enclose them in quotation marks, mycommand 'with > in the argument'.

Redirect to the clipboard

cmd2 output redirection supports an additional feature not found in most shells - if the file name following the > or >> is left blank, then the output is redirected to the operating system clipboard so that it can then be pasted into another program.

  • overwrite the clipboard with mycommand args >
  • append to the clipboard with mycommand args >>

Pipes

Piping the output of a cmd2 command to a shell command works just like in POSIX shells:

  • pipe as input to a shell command with |, as in mycommand args | wc

Multiple Pipes and Redirection

Multiple pipes, optionally followed by a redirect, are supported. Thus, it is possible to do something like the following:

(Cmd) help | grep py | wc > output.txt

The above runs the help command, pipes its output to grep searching for any lines containing py, then pipes the output of grep to the wc "word count" command, and finally redirects the output of that to a file called output.txt.

Disabling Redirection

Note

If you wish to disable cmd2's output redirection and pipes features, you can do so by setting the allow_redirection attribute of your cmd2.Cmd class instance to False. This would be useful, for example, if you want to restrict the ability for an end user to write to disk or interact with shell commands for security reasons:

from cmd2 import Cmd
class App(Cmd):
    def __init__(self):
        super().__init__(allow_redirection=False)

cmd2's parser will still treat the >, >>, and | symbols as output redirection and pipe symbols and will strip arguments after them from the command line arguments accordingly. But output from a command will not be redirected to a file or piped to a shell command.

Limitations of Redirection

Warning

Some limitations apply to redirection and piping within cmd2 applications:

  • Can only pipe to shell commands, not other cmd2 application commands
  • stdout gets redirected/piped, stderr does not