Unexpected behavior when nesting cd calls using Fa

Question

I am experiencing some troubles with Fabric (version 1.7.0 on Ubuntu 13.04).

Consider this function:

def does_not_work():
  with cd('/absolute/folder/one/'):
    with prefix('change_path_command'):
      with cd('/absolute/folder/two/'):
        run('some_random_command')

I expect it to execute the same command as:

def works():
  run('cd /absolute/folder/one/ && change_path_command && cd /absolute/folder/two/ && some_random_command')

However, here is the Fabric output of fab does_not_work:

Requested: some_random_command
Executed: /bin/bash -l -c "cd /absolute/folder/two/ && change_path_command && some_random_command"

It seems that nesting cds is causing me troubles.

Is there a good explanation?

Answer 1

The cd context manager and the prefix context manager don't actually run commands when you invoke them, they just modify some local environment settings that affect any subsequent invocations of run() and/or sudo().

So when your run('some_random_command') gets executed, it gets executed, it runs in the context of (cd=/folder/one, prefix=change_path_command, cd=/folder/two), and since the inner cd takes precedence over the outer cd, the end result is a single command executed with cd /folder/two && change_path_command && some_random_command.

Take a look at the source code for cd and prefix to a get a better idea of how that works -- all they ultimately do is modify the dictionary fabric.state.env when they enter and exit. These later get applied in the call to _prefix_commands(), which gets called from run() and sudo() via the _run_command() function.