I'm implement a simple shell using python's cmd module.
Now, I'd like to implement a unix pipe in this shell, that is when I type:
ls | grep "a"
it will pass the result of do_ls to the input of do_grep,
what's the simplest way to do this?
Sorry CryptoJones, I forgot to say that my platform is Windows.
Here is a simple example which could help you:
from cmd import Cmd
class PipelineExample(Cmd):
def do_greet(self, person):
if person:
greeting = "hello, " + person
else:
greeting = 'hello'
self.output = greeting
def do_echo(self, text):
self.output = text
def do_pipe(self, args):
buffer = None
for arg in args:
s = arg
if buffer:
# This command just adds the output of a previous command as the last argument
s += ' ' + buffer
self.onecmd(s)
buffer = self.output
def postcmd(self, stop, line):
if hasattr(self, 'output') and self.output:
print self.output
self.output = None
return stop
def parseline(self, line):
if '|' in line:
return 'pipe', line.split('|'), line
return Cmd.parseline(self, line)
def do_EOF(self, line):
return True
if __name__ == '__main__':
PipelineExample().cmdloop()
Here is an example session:
(Cmd) greet wong
hello, wong
(Cmd) echo wong | greet
hello, wong
(Cmd) echo wong | greet | greet
hello, hello, wong
The simplest way is probably to store the output of your do_ls in a buffer and feed it to do_grep. You probably want to do it line by line or by groups of lines, rather than in a single go, especially if you want to implement a more command.
A more complete approach would be to run all of your commands in sub-processes and rely on an existing standard library module for pipe support, e.g. subprocess.
You may want to use the cmd2 module. It is a replacement for cmd with additional features.
See the Output redirection section of its documentation.
Use the built in pipe function, not cmd.
