Frequently Asked Questions (FAQ)¶
These are some of the most commonly encountered problems or frequently asked questions which we receive from users. They aren’t intended as a substitute for reading the rest of the documentation, especially the usage docs, so please make sure you check those out if your question is not answered here.
Fabric installs but doesn’t run!¶
On systems with old versions of
setuptools (notably OS X Mavericks [10.9]
as well as older Linux distribution versions) users frequently have problems
running Fabric’s binary scripts; this is because these
setuptools are too
old to deal with the modern distribution formats Fabric and some of its
dependencies may use.
One method we’ve used to recreate this error:
OS X 10.9 using system Python
Pip obtained via e.g.
sudo easy_install pipor
sudo python get-pip.py
pip install fabric
fab [args]then results in the following traceback:
Traceback (most recent call last): File "/usr/local/bin/fab", line 5, in <module> from pkg_resources import load_entry_point File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 2603, in <module> working_set.require(__requires__) File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 666, in require needed = self.resolve(parse_requirements(requirements)) File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 565, in resolve raise DistributionNotFound(req) # XXX put more info here pkg_resources.DistributionNotFound: paramiko>=1.10
The best solution is to obtain a newer
setuptools (which fixes this bug
among many others) like so:
$ sudo pip install -U setuptools
Uninstalling, then reinstalling Fabric after doing so should fix the issue.
Another approach is to tell
pip not to use the
wheel format (make sure
you’ve already uninstalled Fabric and Paramiko beforehand):
$ sudo pip install fabric --no-use-wheel
How can I run something after my task is done on all hosts?¶
Init scripts don’t work!¶
Init-style start/stop/restart scripts (e.g.
sometimes don’t like Fabric’s allocation of a pseudo-tty, which is active by
default. In almost all cases, explicitly calling the command in question with
pty=False works correctly:
sudo("/etc/init.d/apache2 restart", pty=False)
If you have no need for interactive behavior and run into this problem
frequently, you may want to deactivate pty allocation globally by setting
export/etc) calls don’t seem to work!¶
While Fabric can be used for many shell-script-like tasks, there’s a slightly
unintuitive catch: each
call has its own distinct shell session. This is required in order for Fabric
to reliably figure out, after your command has run, what its standard out/error
and return codes were.
Unfortunately, it means that code like the following doesn’t behave as you might assume:
def deploy(): run("cd /path/to/application") run("./update.sh")
If that were a shell script, the second
run call would
have executed with a current working directory of
but because both commands are run in their own distinct session over SSH, it
actually tries to execute
$HOME/update.sh instead (since your remote home
directory is the default working directory).
A simple workaround is to make use of shell logic operations such as
which link multiple expressions together (provided the left hand side executed
without error) like so:
def deploy(): run("cd /path/to/application && ./update.sh")
You might also get away with an absolute path and skip directory changing altogether:
def deploy(): run("/path/to/application/update.sh")
However, this requires that the command in question makes no assumptions about your current working directory!
How do I use
su to run commands as another user?¶
This is a special case of My (cd/workon/export/etc) calls don’t seem to work!. As that FAQ explains,
su which are ‘stateful’ do not work well in Fabric, so
workarounds must be used.
In the case of running commands as a user distinct from the login user, you have two options:
with settings(sudo_user="otheruser"): sudo("command 1") sudo("command 2") ...
If your target system cannot use
sudofor some reason, you can still use
su, but you need to invoke it in a non-interactive fashion by telling it to run a specific command instead of opening a shell. Typically this is the
su otheruser -c "command".
To run multiple commands in the same
su -c“wrapper”, you could e.g. write a wrapper function around
def run_su(command, user="otheruser"): return run('su %s -c "%s"' % (user, command))
Why do I sometimes see
err: stdin: is not a tty?¶
This message is typically generated by programs such as
lurking within your remote user’s
.bashrc files (or any
other such files, including system-wide ones.) Fabric’s default mode of
operation involves executing the Bash shell in “login mode”, which causes these
files to be executed.
Because Fabric also doesn’t bother asking the remote end for a tty by default (as it’s not usually necessary) programs fired within your startup files, which expect a tty to be present, will complain – and thus, stderr output about “stdin is not a tty” or similar.
There are multiple ways to deal with this problem:
- Find and remove or comment out the offending program call. If the program was not added by you on purpose and is simply a legacy of the operating system, this may be safe to do, and is the simplest approach.
env.shellto remove the
-lflag. This should tell Bash not to load your startup files. If you don’t depend on the contents of your startup files (such as aliases or whatnot) this may be a good solution.
sudo, which will force allocation of a pseudo-tty on the remote end, and hopefully cause the offending program to be less cranky.
Why can’t I run programs in the background with
&? It makes Fabric hang.¶
Because Fabric executes a shell on the remote end for each invocation of
sudo (see also), backgrounding a
process via the shell will not work as expected. Backgrounded processes may
still prevent the calling shell from exiting until they stop running, and this
in turn prevents Fabric from continuing on with its own execution.
The key to fixing this is to ensure that your process’ standard pipes are all disassociated from the calling shell, which may be done in a number of ways (listed in order of robustness):
Use a pre-existing daemonization technique if one exists for the program at hand – for example, calling an init script instead of directly invoking a server binary.
- Or leverage a process manager such as
systemd- such tools let you define what it means to “run” one of your background processes, then issue init-script-like start/stop/restart/status commands. They offer many advantages over classic init scripts as well.
- Or leverage a process manager such as
dtachto fully detach the process from the running shell; these tools have the benefit of allowing you to reattach to the process later on if needed (though they are more ad-hoc than
You may be able to the program under
nohupor similar “in-shell” tools - however we strongly recommend the prior approaches because
nohuphas only worked well for a minority of our users.
My remote system doesn’t have
bash installed by default, do I need to install
While Fabric is written with
bash in mind, it’s not an absolute
requirement. Simply change env.shell to call your desired shell, and
include an argument similar to
-c argument, which allows us to
build shell commands of the form:
/bin/bash -l -c "<command string here>"
/bin/bash -l -c is the default value of env.shell.
-l argument specifies a login shell and is not absolutely
required, merely convenient in many situations. Some shells lack the option
entirely and it may be safely omitted in such cases.
A relatively safe baseline is to call
/bin/sh, which may call the original
sh binary, or (on some systems)
csh, and give it the
argument, like so:
from fabric.api import env env.shell = "/bin/sh -c"
This has been shown to work on FreeBSD and may work on other systems as well.
I’m sometimes incorrectly asked for a passphrase instead of a password.¶
Due to a bug of sorts in our SSH layer, it’s not currently possible for Fabric to always accurately detect the type of authentication needed. We have to try and guess whether we’re being asked for a private key passphrase or a remote server password, and in some cases our guess ends up being wrong.
The most common such situation is where you, the local user, appear to have an SSH keychain agent running, but the remote server is not able to honor your SSH key, e.g. you haven’t yet transferred the public key over or are using an incorrect username. In this situation, Fabric will prompt you with “Please enter passphrase for private key”, but the text you enter is actually being sent to the remote end’s password authentication.
We hope to address this in future releases by modifying a fork of the aforementioned SSH library.
Is Fabric thread-safe?¶
Currently, no, it’s not – the present version of Fabric relies heavily on shared state in order to keep the codebase simple. However, there are definite plans to update its internals so that Fabric may be either threaded or otherwise parallelized so your tasks can run on multiple servers concurrently.