The environment dictionary, env

A simple but integral aspect of Fabric is what is known as the “environment”: a Python dictionary subclass which is used as a combination settings registry and shared inter-task data namespace.

The environment dict is currently implemented as a global singleton, fabric.state.env, and is included in fabric.api for convenience. Keys in env are sometimes referred to as “env variables”.

Environment as configuration

Most of Fabric’s behavior is controllable by modifying env variables, such as env.hosts (as seen in the tutorial). Other commonly-modified env vars include:

  • user: Fabric defaults to your local username when making SSH connections, but you can use env.user to override this if necessary. The Execution model documentation also has info on how to specify usernames on a per-host basis.
  • password: Used to explicitly set your default connection or sudo password if desired. Fabric will prompt you when necessary if this isn’t set or doesn’t appear to be valid.
  • warn_only: a Boolean setting determining whether Fabric exits when detecting errors on the remote end. See Execution model for more on this behavior.

There are a number of other env variables; for the full list, see Full list of env vars at the bottom of this document.

The settings context manager

In many situations, it’s useful to only temporarily modify env vars so that a given settings change only applies to a block of code. Fabric provides a settings context manager, which takes any numbr of key/value pairs and will use them to modify env within its wrapped block.

For example, there are many situations where setting warn_only (see below) is useful. To apply it to a few lines of code, use settings(warn_only=True), as seen in this simplified version of the contrib exists function:

from fabric.api import settings, run

def exists(path):
    with settings(warn_only=True):
        return run('test -e %s' % path)

See the Context Managers API documentation for details on settings and other, similar tools.

Environment as shared state

As mentioned, the env object is simply a dictionary subclass, so your own fabfile code may store information in it as well. This is sometimes useful for keeping state between multiple tasks within a single execution run.

Note

This aspect of env is largely historical: in the past, fabfiles were not pure Python and thus the environment was the only way to communicate between tasks. Nowadays, you may call other tasks or subroutines directly, and even keep module-level shared state if you wish.

In future versions, Fabric will become threadsafe, at which point env may be the only easy/safe way to keep global state.

Other considerations

While it subclasses dict, Fabric’s env has been modified so that its values may be read/written by way of attribute access, as seen in some of the above material. In other words, env.host_string and env['host_string'] are functionally identical. We feel that attribute access can often save a bit of typing and makes the code more readable, so it’s the recommended way to interact with env.

The fact that it’s a dictionary can be useful in other ways, such as with Python’s dict-based string interpolation, which is especially handy if you need to insert multiple env vars into a single string. Using “normal” string interpolation might look like this:

print("Executing on %s as %s" % (env.host, env.user))

Using dict-style interpolation is more readable and slightly shorter:

print("Executing on %(host)s as %(user)s" % env)

Full list of env vars

Below is a list of all predefined (or defined by Fabric itself during execution) environment variables. While any of them may be manipulated directly, it’s often best to use context_managers, either generally via settings or via specific context managers such as cd.

Note that many of these may be set via fab‘s command-line switches – see fab options and arguments for details. Cross-links will be provided where appropriate.

abort_on_prompts

Default: False

When True, Fabric will run in a non-interactive mode, calling abort anytime it would normally prompt the user for input (such as password prompts, “What host to connect to?” prompts, fabfile invocation of prompt, and so forth.) This allows users to ensure a Fabric session will always terminate cleanly instead of blocking on user input forever when unforeseen circumstances arise.

New in version 1.1.

all_hosts

Default: None

Set by fab to the full host list for the currently executing command. For informational purposes only.

See also

Execution model

always_use_pty

Default: True

When set to False, causes run/sudo to act as if they have been called with pty=False.

The command-line flag --no-pty, if given, will set this env var to False.

New in version 1.0.

combine_stderr

Default: True

Causes the SSH layer to merge a remote program’s stdout and stderr streams to avoid becoming meshed together when printed. See Combining stdout and stderr for details on why this is needed and what its effects are.

New in version 1.0.

command

Default: None

Set by fab to the currently executing command name (e.g. when executed as $ fab task1 task2, env.command will be set to "task1" while task1 is executing, and then to "task2".) For informational purposes only.

See also

Execution model

command_prefixes

Default: []

Modified by prefix, and prepended to commands executed by run/sudo.

New in version 1.0.

connection_attempts

Default: 1

Number of times Fabric will attempt to connect when connecting to a new server. For backwards compatibility reasons, it defaults to only one connection attempt.

New in version 1.4.

cwd

Default: ''

Current working directory. Used to keep state for the cd context manager.

disable_known_hosts

Default: False

If True, the SSH layer will skip loading the user’s known-hosts file. Useful for avoiding exceptions in situations where a “known host” changing its host key is actually valid (e.g. cloud servers such as EC2.)

See also

SSH behavior

exclude_hosts

Default: []

Specifies a list of host strings to be skipped over during fab execution. Typically set via --exclude-hosts/-x.

New in version 1.1.

fabfile

Default: fabfile.py

Filename pattern which fab searches for when loading fabfiles. To indicate a specific file, use the full path to the file. Obviously, it doesn’t make sense to set this in a fabfile, but it may be specified in a .fabricrc file or on the command line.

host_string

Default: None

Defines the current user/host/port which Fabric will connect to when executing run, put and so forth. This is set by fab when iterating over a previously set host list, and may also be manually set when using Fabric as a library.

See also

Execution model

forward_agent

Default: False

If True, enables forwarding of your local SSH agent to the remote end.

New in version 1.4.

See also

-A

host

Default: None

Set to the hostname part of env.host_string by fab. For informational purposes only.

hosts

Default: []

The global host list used when composing per-task host lists.

See also

Execution model

keepalive

Default: 0 (i.e. no keepalive)

An integer specifying an SSH keepalive interval to use; basically maps to the SSH config option ClientAliveInterval. Useful if you find connections are timing out due to meddlesome network hardware or what have you.

See also

--keepalive

New in version 1.1.

key_filename

Default: None

May be a string or list of strings, referencing file paths to SSH key files to try when connecting. Passed through directly to the SSH layer. May be set/appended to with -i.

linewise

Default: False

Forces buffering by line instead of by character/byte, typically when running in parallel mode. May be activated via --linewise. This option is implied by env.parallel – even if linewise is False, if parallel is True then linewise behavior will occur.

New in version 1.3.

local_user

A read-only value containing the local system username. This is the same value as user‘s initial value, but whereas user may be altered by CLI arguments, Python code or specific host strings, local_user will always contain the same value.

no_agent

Default: False

If True, will tell the SSH layer not to seek out running SSH agents when using key-based authentication.

New in version 0.9.1.

no_keys

Default: False

If True, will tell the SSH layer not to load any private key files from one’s $HOME/.ssh/ folder. (Key files explicitly loaded via fab -i will still be used, of course.)

New in version 0.9.1.

parallel

Default: False

When True, forces all tasks to run in parallel. Implies env.linewise.

New in version 1.3.

password

Default: None

The default password used by the SSH layer when connecting to remote hosts, and/or when answering sudo prompts.

See also

passwords

passwords

Default: {}

This dictionary is largely for internal use, and is filled automatically as a per-host-string password cache. Keys are full host strings and values are passwords (strings).

New in version 1.0.

path

Default: ''

Used to set the remote $PATH when executing commands in run/sudo. It is recommended to use the path context manager for managing this value instead of setting it directly.

New in version 1.0.

pool_size

Default: 0

Sets the number of concurrent processes to use when executing tasks in parallel.

New in version 1.3.

port

Default: None

Set to the port part of env.host_string by fab when iterating over a host list. May also be used to specify a default port.

real_fabfile

Default: None

Set by fab with the path to the fabfile it has loaded up, if it got that far. For informational purposes only.

rcfile

Default: $HOME/.fabricrc

Path used when loading Fabric’s local settings file.

reject_unknown_hosts

Default: False

If True, the SSH layer will raise an exception when connecting to hosts not listed in the user’s known-hosts file.

See also

SSH behavior

roledefs

Default: {}

Dictionary defining role name to host list mappings.

See also

Execution model

roles

Default: []

The global role list used when composing per-task host lists.

See also

Execution model

shell

Default: /bin/bash -l -c

Value used as shell wrapper when executing commands with e.g. run. Must be able to exist in the form <env.shell> "<command goes here>" – e.g. the default uses Bash’s -c option which takes a command string as its value.

skip_bad_hosts

Default: False

If True, causes fab (or non-fab use of execute) to skip over hosts it can’t connect to.

New in version 1.4.

ssh_config_path

Default: $HOME/.ssh/config

Allows specification of an alternate SSH configuration file path.

New in version 1.4.

sudo_prompt

Default: sudo password:

Passed to the sudo program on remote systems so that Fabric may correctly identify its password prompt. This may be modified by fabfiles but there’s no real reason to.

See also

The sudo operation

timeout

Default: 10

Network connection timeout, in seconds.

New in version 1.4.

use_shell

Default: True

Global setting which acts like the use_shell argument to run/sudo: if it is set to False, operations will not wrap executed commands in env.shell.

use_ssh_config

Default: False

Set to True to cause Fabric to load your local SSH config file.

New in version 1.4.

user

Default: User’s local username

The username used by the SSH layer when connecting to remote hosts. May be set globally, and will be used when not otherwise explicitly set in host strings. However, when explicitly given in such a manner, this variable will be temporarily overwritten with the current value – i.e. it will always display the user currently being connected as.

To illustrate this, a fabfile:

from fabric.api import env, run

env.user = 'implicit_user'
env.hosts = ['host1', 'explicit_user@host2', 'host3']

def print_user():
    with hide('running'):
        run('echo "%(user)s"' % env)

and its use:

$ fab print_user

[host1] out: implicit_user
[explicit_user@host2] out: explicit_user
[host3] out: implicit_user

Done.
Disconnecting from host1... done.
Disconnecting from host2... done.
Disconnecting from host3... done.

As you can see, during execution on host2, env.user was set to "explicit_user", but was restored to its previous value ("implicit_user") afterwards.

Note

env.user is currently somewhat confusing (it’s used for configuration and informational purposes) so expect this to change in the future – the informational aspect will likely be broken out into a separate env variable.

See also

Execution model

version

Default: current Fabric version string

Mostly for informational purposes. Modification is not recommended, but probably won’t break anything either.

warn_only

Default: False

Specifies whether or not to warn, instead of abort, when run/sudo/local encounter error conditions.

See also

Execution model