How to set virtualenv for a crontab?

How to set virtualenv for a crontab? - python

I want to set up a crontab to run a Python script.
Say the script is something like:
#!/usr/bin/python
print "hello world"
Is there a way I could specify a virtualenv for that Python script to run in? In shell I'd just do:
~$ workon myenv
Is there something equivalent I could do in crontab to activate a virtualenv?

Is there something equivalent I could do in crontab to activate a virtualenv?
This works well for me...
## call virtualenv python from crontab
0 9 * * * /path/to/virtenv/bin/python /path/to/your_cron_script.py
I prefer using python directly from the virtualenv instead of hard-coding the virtualenv $PATH into the script's shebang... or sourcing the venv activate

If you're using "workon" you're actually using "virtualenv wrapper" which is another layer of abstraction that sits on top of virtualenv. virtualenv alone can be activated by cd'ing to your virtualenv root directory and running:
source bin/activate
workon is a command provided by virtualenv wrapper, not virtualenv, and it does some additional stuff that is not necessarily required for plain virtualenv. All you really need to do is source the bin/activate file in your virtualenv root directory to "activate" a virtualenv.
You can setup your crontab to invoke a bash script which does this:
#! /bin/bash
cd my/virtual/env/root/dir
source bin/activate
# virtualenv is now active, which means your PATH has been modified.
# Don't try to run python from /usr/bin/python, just run "python" and
# let the PATH figure out which version to run (based on what your
# virtualenv has configured).
python myScript.py

With bash, you can create a generic virtual env wrapper that you can use to invoke any command, much like how time can wrapper any command.
virt_env_wrapper.bash:
#!/bin/bash
source path/to/virtual/env/bin/activate
"$#"
Bash's magical incantation "$#" re-escapes all tokens on the original command line so that if you were to invoke:
virt_env_wrapper.bash python foo.py bar 'baz blap'
foo.py would see a sys.argv of ['bar', 'baz blap']

I'm not sure about workon, but it's pretty straightforward for venv. The only thing to remember is that crontab uses sh by default, not bash, so you need to use the . command instead of source.
Here are examples if you have a file ~/myproject/main.py:
* * * * * cd ~/myproject && . .venv/bin/activate && python main.py > /tmp/out1 2>&1
You could also directly call the specific path of the python in the venv directory, then you don't need to call activate.
* * * * * ~/myproject/.venv/bin/python ~/myproject/main.py > /tmp/out2 2>&1
The downside of that is you would need to specify the project path twice, which makes maintenance trickier. To avoid that, you could use a shell variable so you only specify the project path once:
* * * * * project_dir=~/myproject ; $project_dir/.venv/bin/python $project_dir/main.py > /tmp/out3 2>&1

Related

How to source an env file and then execute Python script via cronjob? [duplicate]

I am trying to run a Django management command from cron. I am using virtualenv to keep my project sandboxed.
I have seen examples here and elsewhere that show running management commands from within virtualenv's like:
0 3 * * * source /home/user/project/env/bin/activate && /home/user/project/manage.py command arg
However, even though syslog shows an entry when the task should have started, this task never actually runs (the log file for the script is empty). If I run the line manually from the shell, it works as expected.
The only way I can currently get the command to run via cron, is to break the commands up and put them in a dumb bash wrapper script:
#!/bin/sh
source /home/user/project/env/bin/activate
cd /home/user/project/
./manage.py command arg
EDIT:
ars came up with a working combination of commands:
0 3 * * * cd /home/user/project && /home/user/project/env/bin/python /home/user/project/manage.py command arg
At least in my case, invoking the activate script for the virtualenv did nothing. This works, so on with the show.

You should be able to do this by using the python in your virtual environment:
/home/my/virtual/bin/python /home/my/project/manage.py command arg
EDIT: If your django project isn't in the PYTHONPATH, then you'll need to switch to the right directory:
cd /home/my/project && /home/my/virtual/bin/python ...
You can also try to log the failure from cron:
cd /home/my/project && /home/my/virtual/bin/python /home/my/project/manage.py > /tmp/cronlog.txt 2>&1
Another thing to try is to make the same change in your manage.py script at the very top:
#!/home/my/virtual/bin/python

Running source from a cronfile won't work as cron uses /bin/sh as its default shell, which doesn't support source. You need to set the SHELL environment variable to be /bin/bash:
SHELL=/bin/bash
*/10 * * * * root source /path/to/virtualenv/bin/activate && /path/to/build/manage.py some_command > /dev/null
It's tricky to spot why this fails as /var/log/syslog doesn't log the error details. Best to alias yourself to root so you get emailed with any cron errors. Simply add yourself to /etc/aliases and run sendmail -bi.
More info here:
http://codeinthehole.com/archives/43-Running-django-cronjobs-within-a-virtualenv.html
the link above is changed to:
https://codeinthehole.com/tips/running-django-cronjobs-within-a-virtualenv/

Don't look any further:
0 3 * * * /usr/bin/env bash -c 'cd /home/user/project && source /home/user/project/env/bin/activate && ./manage.py command arg' > /dev/null 2>&1
Generic approach:
* * * * * /usr/bin/env bash -c 'YOUR_COMMAND_HERE' > /dev/null 2>&1
The beauty about this is you DO NOT need to change the SHELL variable for crontab from sh to bash

I am sorry for that nth answer but I checked the answers and there is really simpler and neater.
Long story short
Use the python binary of your venv in your cron :
0 3 * * * /home/user/project/env/bin/python /home/user/project/manage.py
Long story
We activate the virtual environment when we want to set the current shell with the python config of that specific virtual environment(that is binaries and modules of that).
It is relevant to work with the current shell : execute multiple python commands on the current shell without the need to reference the full python path of the venv.
In the frame of a cron or even a bash, which value to activate the environment ?
Besides I read in some answers some references to bash rather than sh or still to define a wrapper to call the Python code. But why the hell should we bother with these ?
I repeat, just do it :
0 3 * * * /home/user/project/env/bin/python /home/user/project/manage.py
The documentation confirms that :
You don’t specifically need to activate an environment; activation
just prepends the virtual environment’s binary directory to your path,
so that “python” invokes the virtual environment’s Python interpreter
and you can run installed scripts without having to use their full
path. However, all scripts installed in a virtual environment should
be runnable without activating it, and run with the virtual
environment’s Python automatically.

The only correct way to run python cron jobs when using a virtualenv is to activate the environment and then execute the environment's python to run your code.
One way to do this is use virtualenv's activate_this in your python script, see: http://virtualenv.readthedocs.org/en/latest/userguide.html#using-virtualenv-without-bin-python
Another solution is echoing the complete command including activating the environment and piping it into /bin/bash. Consider this for your /etc/crontab:
***** root echo 'source /env/bin/activate; python /your/script' | /bin/bash

Rather than mucking around with virtualenv-specific shebangs, just prepend PATH onto the crontab.
From an activated virtualenv, run these three commands and python scripts should just work:
$ echo "PATH=$PATH" > myserver.cron
$ crontab -l >> myserver.cron
$ crontab myserver.cron
The crontab's first line should now look like this:
PATH=/home/me/virtualenv/bin:/usr/bin:/bin: # [etc...]

This is a simple way that keeps the crontab command very similar to regular command (tested in Ubuntu 18.04). Some key notes to keep in mind:
You can use the . command instead of source. (crontab uses sh by default, not bash, so it doesn't have source.)
~ and $variables are expanded in crontab commands. (It's only crontab environment statements that don't do variable expansion.)
Here are examples if you have a file ~/myproject/main.py:
* * * * * cd ~/myproject && . .venv/bin/activate && python main.py > /tmp/out1 2>&1
You could also directly call the specific path of the python in the venv directory, then you don't need to call activate.
* * * * * ~/myproject/.venv/bin/python ~/myproject/main.py > /tmp/out2 2>&1
The downside of that is you would need to specify the project path twice, which makes maintenance trickier. To avoid that, you could use a shell variable so you only specify the project path once:
* * * * * project_dir=~/myproject ; $project_dir/.venv/bin/python $project_dir/main.py > /tmp/out3 2>&1

The best solution for me was to both
use the python binary in the venv bin/ directory
set the python path
to include the venv modules directory.
man python mentions modifying the path in shell at $PYTHONPATH or in python with sys.path
Other answers mention ideas for doing this using the shell. From python, adding the following lines to my script allows me to successfully run it directly from cron.
import sys
sys.path.insert(0,'/path/to/venv/lib/python3.3/site-packages');
Here's how it looks in an interactive session --
Python 3.3.2+ (default, Feb 28 2014, 00:52:16)
[GCC 4.8.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.path
['', '/usr/lib/python3.3', '/usr/lib/python3.3/plat-x86_64-linux-gnu', '/usr/lib/python3.3/lib-dynload']
>>> import requests
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ImportError: No module named 'requests'
>>> sys.path.insert(0,'/path/to/venv/modules/');
>>> import requests
>>>

I'd like to add this because I spent some time solving the issue and did not find an answer here for combination of variables usage in cron and virtualenv. So maybe it'll help someone.
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
DIR_SMTH="cd /smth"
VENV=". venv/bin/activate"
CMD="some_python_bin do_something"
# m h dom mon dow command
0 * * * * $DIR_SMTH && $VENV && $CMD -k2 some_target >> /tmp/crontest.log 2>&1
It did not work well when it was configured like
DIR_SMTH="cd /smth && . venv/bin/activate"
Thanks #davidwinterbottom, #reed-sandberg and #mkb for giving the right direction. The accepted answer actually works fine until your python need to run a script which have to run another python binary from venv/bin directory.

If you're on python and using a Conda Virtual Environment where your python script contains the shebang #!/usr/bin/env python the following works:
* * * * * cd /home/user/project && /home/user/anaconda3/envs/envname/bin/python script.py 2>&1
Additionally, if you want to capture any outputs in your script (e.g. print, errors, etc) you can use the following:
* * * * * cd /home/user/project && /home/user/anaconda3/envs/envname/bin/python script.py >> /home/user/folder/script_name.log 2>&1

python script
from datetime import datetime
import boto # check wheather its taking the virtualenv or not
import sys
param1=sys.argv[1] #Param
myFile = open('appendtxt.txt', 'a')
myFile.write('\nAccessed on ' + param1+str(datetime.now()))
Cron command
*/1 * * * * cd /Workspace/testcron/ && /Workspace/testcron/venvcron/bin/python3 /Workspace/testcron/testcronwithparam.py param
In above command
*/1 * * * * - Execute every one minte
cd /Workspace/testcron/ - Path of the python script
/Workspace/testcron/venvcron/bin/python3 - Virtualenv path
Workspace/testcron/testcronwithparam.py - File path
param - parameter

I've added the following script as manage.sh inside my Django project, it sources the virtualenv and then runs the manage.py script with whatever arguments you pass to it. It makes it very easy in general to run commands inside the virtualenv (cron, systemd units, basically anywhere):
#! /bin/bash
# this is a convenience script that first sources the venv (assumed to be in
# ../venv) and then executes manage.py with whatever arguments you supply the
# script with. this is useful if you need to execute the manage.py from
# somewhere where the venv isn't sourced (e.g. system scripts)
# get the script's location
DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
# source venv <- UPDATE THE PATH HERE WITH YOUR VENV's PATH
source $DIR/../venv/bin/activate
# run manage.py script
$DIR/manage.py "$#"
Then in your cron entry you can just run:
0 3 * * * /home/user/project/manage.sh command arg
Just remember that you need to make the manage.sh script executable

Add the path of the Python installation in the venv but don't activate the environment.
* * * * * /HDD1/shritam_kumar/VENOM/venv/bin/python /HDD1/shritam_kumar/Projects/Voelkner-DE/schedule_product_BA.py
It's that simple.

I had the same issue and spent a lot of time solving that.
None of the solutions here helped me, so I'm sharing what worked for me:
Open a new file "pick_name.sh" open it inside of your project directory.
Inside the "pick_name.sh" file, write and save the following lines:
#!/bin/bash
source /YOUR_VIRTUAL_ENV_PATH/bin/activate
export PYTHONPATH="${PYTHONPATH}:/PATH_TO_CUSTOM_MODULE_YOU_CREATED**OPTIONAL**"
export PYTHONPATH="${PYTHONPATH}:/PATH_TO_ANOTHER_CUSTOM_MODULE_YOU_CREATED**OPTIONAL**"
cd /PATH_TO_DIR_STORING_FILE_NAME.PY
python file_name.py
Go to /var/spool/cron/crontabs (or to where your cron management file sits) and open the 'root' file.
Add these lines to the root file which's inside the crontab folder:
# m h dom mon dow command
* * * * * /PATH_TO_DIR_WHERE_PICK_NAME.SH_SITS/pick_name.sh >> /YOUR_PROJECT_DIR/cron_output.txt 2>&1
Notes:
This command (section 4.) will run the "pick_name.sh" file.
In this example it runs every minute, so make sure you change it according to your needs.
It writes all logs to a log file called "cron_ouput".
No need to create the file before, it will be created automatically.
Make sure to replace all paths (I wrote them in capital letters) to your paths.
You can change file names, if so, make sure to change it in all appearances in my instructions to avoid errors.
If you want to add another py file to run by cron, you need to add it to the "pick_nam.sh" file* not to the cron. Simply duplicate section 2. lines in the "pick_nam.sh" but without the "#!/bin/bash" part.
Then, every time the cron will run "pick_name.sh" it will run all the files you specified inside of it.
Make sure to restart cron after changes, it could have saved me a lot of debugging time, use this command:
systemctl restart cron

Since a cron executes in its own minimal sh environment, here's what I do to run Python scripts in a virtual environment:
* * * * * . ~/.bash_profile; . ~/path/to/venv/bin/activate; python ~/path/to/script.py
(Note: if . ~/.bash_profile doesn't work for you, then try . ~/.bashrc or . ~/.profile depending on how your server is set up.)
This loads your bash shell environment, then activates your Python virtual environment, essentially leaving you with the same setup you tested your scripts in.
No need to define environment variables in crontab and no need to modify your existing scripts.

If you are a MacOS user like me, you can check the crontab error message at /var/mail/{username} file. like this
tail /var/mail/{username}
If there is an "operation not permitted" error, maybe you have to add cron to the Full Disk Access apps (Security & Privacy > Privacy > Full Disk Access apps/execs).
And click + button, go to /usr/sbin, double click the cron file.
then it will fix the "not permitted" error. detailed steps
And this is my code：
0 19 * * * cd /Users/user/Desktop/Project && source /Users/user/Desktop/Project/venv/bin/activate && python command arg

This is a solution that has worked well for me.
source /root/miniconda3/etc/profile.d/conda.sh && \
conda activate <your_env> && \
python <your_application> &
I am using miniconda with Conda version 4.7.12 on a Ubuntu 18.04.3 LTS.
I am able to place the above inside a script and run it via crontab as well without any trouble.

This will also work on crontab -e
* */5 * * * cd /home/project && sudo /home/project/venv/bin/python scripte.py

I had this same issue:
I had written a custom django command to check for geodjango position coordinates inside of geodjango polygons and had trouble automating the task to run, however using this command with crontab worked for me:
* * * * * ./home/project/locations/locations.sh >> /var/log/locations.log 2>&1

Run python script on cron using conda

Trying to send some output to Slack using cron on an instance of GCP Compute Engine running Linux Ubuntu 18.04.2 LTS.
Output is generated by python script.
Python script is usually run using conda activate my_env and python my_script.py
I have made the bash file executable by doing chmod +x my_script.bash
Here is content of bash file:
#!/bin/bash
source /home/user/miniconda3/bin/activate
conda activate my_env
python /home/user/folder/cron/reports.py -r check_stocks
I would expect adding the following line to crontab -e:
00 21 * * * cd /home/user/folder/cron/ && /bin/bash my_script.bash would give me the same results.
I run cd /home/user/folder/cron/ && /bin/bash my_script.bash in my shell and the script runs fine.

Make your .py file exacutable too (chmod +x file.py) - otherwise it won't work.
You can find similar issue resolved here.

I had a similar issue and I gave up activating the conda environment and instead called directly the python bin in the miniconda environment folder, like this:
00 21 * * * /home/myuser/miniconda3/envs/my_env/bin/python /home/user/folder/cron/reports.py
I don't think this is the recommended solution, especially if you have a complex project with dependencies and you are importing a lot of modules, but for my simple script it solved the problem.

Writing Shell Commands to VirtualEnv?

Is there a way I can write commands to a virtual environment after it's been activated? For example lets say I have a Python or Bash script which does some, stuff i.e.
Make a virtualenv
Activates it.
Executes the commands to the shell of the newly created virtual environment?
For example I am doing something like this:
activate_this = subprocess.call("/bin/bash --rcfile " + "/home/" + os.getlogin() + "/mission-control/venv/bin/activate", shell=True)
process = execfile(activate_this, dict(__file__=activate_this))
process.communicate(subprocess.call(virtualenv.create_bootstrap_script(textwrap.dedent
("""
import subprocess
subprocess.call("pip install -r " + os.environ['VIRTUAL_ENV'] + "/requirements.txt", shell=True)
"""
))))
I would like to install the requirements.txt file after I activate the environment however I can't get the subprocess module to communicate with the shell after the virtual environment is created. I think it might have to do with me creating a new virtual environment via execfile, which therefore is creating a new process.
Also I know shell=True is bad practice but as of right now I am not concerned with the possibility of unsanitized input.

. "$VIRTUAL_ENV/bin/activate"
pip install -r "$VIRTUAL_ENV/requirements.txt"

First of all, thanks to #Ryne Everett for the help. So I solved this by just ditching the Python solution and creating a Bash file which I call from subprocess in my Python script. The subprocess call executes the Bash file which handles creating and executing within the virtualenv. I am not sure how to solve this using just Python. I am sure there is a way but this seems like a simpler solution. The Bash script is the following:
#!/bin/bash
MISSION_CONTROL="$PWD"
if [ ! -d "$MISSION_CONTROL/venv" ]; then
virtualenv $MISSION_CONTROL/venv --no-site-packages
echo "Welcome to Mission Control..."
/bin/bash --rcfile $MISSION_CONTROL/venv/bin/activate
fi
if [ -d "$MISSION_CONTROL/venv" ]; then
pip install -r $MISSION_CONTROL/requirements.txt
fi
EDIT: This may also be useful for people who are trying to do something similiar: How to source virtualenv activate in a Bash script

Shell : workon not found [duplicate]

So, once again, I make a nice python program which makes my life ever the more easier and saves a lot of time. Ofcourse, this involves a virtualenv, made with the mkvirtualenv function of virtualenvwrapper. The project has a requirements.txt file with a few required libraries (requests too :D) and the program won't run without these libraries.
I am trying to add a bin/run-app executable shell script which would be in my path (symlink actually). Now, inside this script, I need to switch to the virtualenv before I can run this program. So I put this in
#!/bin/bash
# cd into the project directory
workon "$(cat .venv)"
python main.py
A file .venv contains the virtualenv name. But when I run this script, I get workon: command not found error.
Of course, I have the virtualenvwrapper.sh sourced in my bashrc but it doesn't seem to be available in this shell script.
So, how can I access those virtualenvwrapper functions here? Or am I doing this the wrong way? How do you launch your python tools, each of which has its own virtualenv!?

Just source the virtualenvwrapper.sh script in your script to import the virtualenvwrapper's functions. You should then be able to use the workon function in your script.
And maybe better, you could create a shell script (you could name it venv-run.sh for example) to run any Python script into a given virtualenv, and place it in /usr/bin, /usr/local/bin, or any directory which is in your PATH.
Such a script could look like this:
#!/bin/sh
# if virtualenvwrapper.sh is in your PATH (i.e. installed with pip)
source `which virtualenvwrapper.sh`
#source /path/to/virtualenvwrapper.sh # if it's not in your PATH
workon $1
python $2
deactivate
And could be used simply like venv-run.sh my_virtualenv /path/to/script.py

I can't find the way to trigger the commands of virtualenvwrapper in shell. But this trick can help: assume your env. name is myenv, then put following lines at the beginning of scripts:
ENV=myenv
source $WORKON_HOME/$ENV/bin/activate

This is a super old thread and I had a similar issue. I started digging for a simpler solution out of curiousity.
gnome-terminal --working-directory='/home/exact/path/here' --tab --title="API" -- bash -ci "workon aaapi && python manage.py runserver 8001; exec bash;"
The --workingdirectory forces the tab to open there by default under the hood and the -ci forces it to work like an interactive interface, which gets around the issues with the venvwrapper not functioning as expected.
You can run as many of these in sequence. It will open tabs, give them an alias, and run the script you want.
Personally I dropped an alias into my bashrc to just do this when I type startdev in my terminal.
I like this because its easy, simple to replicate, flexible, and doesn't require any fiddling with variables and whatnot.

It's a known issue. As a workaround, you can make the content of the script a function and place it in either ~/.bashrc or ~/.profile
function run-app() {
workon "$(cat .venv)"
python main.py
}

If your Python script requires a particular virtualenv then put/install it in virtualenv's bin directory. If you need access to that script outside of the environment then you could make a symlink.
main.py from virtualenv's bin:
#!/path/to/virtualenv/bin/python
import yourmodule
if __name__=="__main__":
yourmodule.main()
Symlink in your PATH:
pymain -> /path/to/virtualenv/bin/main.py
In bin/run-app:
#!/bin/sh
# cd into the project directory
pymain arg1 arg2 ...

Apparently, I was doing this the wrong way. Instead of saving the virtualenv's name in the .venv file, I should be putting the virtualenv's directory path.
(cdvirtualenv && pwd) > .venv
and in the bin/run-app, I put
source "$(cat .venv)/bin/activate"
python main.py
And yay!

add these lines to your .bashrc or .bash_profile
export WORKON_HOME=~/Envs
source /usr/local/bin/virtualenvwrapper.sh
and reopen your terminal and try

You can also call the virtualenv's python executable directly. First find the path to the executable:
$ workon myenv
$ which python
/path/to/virtualenv/myenv/bin/python
Then call from your shell script:
#!/bin/bash
/path/to/virtualenv/myenv/bin/python myscript.py

Cron and virtualenv

I am trying to run a Django management command from cron. I am using virtualenv to keep my project sandboxed.
I have seen examples here and elsewhere that show running management commands from within virtualenv's like:
0 3 * * * source /home/user/project/env/bin/activate && /home/user/project/manage.py command arg
However, even though syslog shows an entry when the task should have started, this task never actually runs (the log file for the script is empty). If I run the line manually from the shell, it works as expected.
The only way I can currently get the command to run via cron, is to break the commands up and put them in a dumb bash wrapper script:
#!/bin/sh
source /home/user/project/env/bin/activate
cd /home/user/project/
./manage.py command arg
EDIT:
ars came up with a working combination of commands:
0 3 * * * cd /home/user/project && /home/user/project/env/bin/python /home/user/project/manage.py command arg
At least in my case, invoking the activate script for the virtualenv did nothing. This works, so on with the show.

You should be able to do this by using the python in your virtual environment:
/home/my/virtual/bin/python /home/my/project/manage.py command arg
EDIT: If your django project isn't in the PYTHONPATH, then you'll need to switch to the right directory:
cd /home/my/project && /home/my/virtual/bin/python ...
You can also try to log the failure from cron:
cd /home/my/project && /home/my/virtual/bin/python /home/my/project/manage.py > /tmp/cronlog.txt 2>&1
Another thing to try is to make the same change in your manage.py script at the very top:
#!/home/my/virtual/bin/python

Running source from a cronfile won't work as cron uses /bin/sh as its default shell, which doesn't support source. You need to set the SHELL environment variable to be /bin/bash:
SHELL=/bin/bash
*/10 * * * * root source /path/to/virtualenv/bin/activate && /path/to/build/manage.py some_command > /dev/null
It's tricky to spot why this fails as /var/log/syslog doesn't log the error details. Best to alias yourself to root so you get emailed with any cron errors. Simply add yourself to /etc/aliases and run sendmail -bi.
More info here:
http://codeinthehole.com/archives/43-Running-django-cronjobs-within-a-virtualenv.html
the link above is changed to:
https://codeinthehole.com/tips/running-django-cronjobs-within-a-virtualenv/

Don't look any further:
0 3 * * * /usr/bin/env bash -c 'cd /home/user/project && source /home/user/project/env/bin/activate && ./manage.py command arg' > /dev/null 2>&1
Generic approach:
* * * * * /usr/bin/env bash -c 'YOUR_COMMAND_HERE' > /dev/null 2>&1
The beauty about this is you DO NOT need to change the SHELL variable for crontab from sh to bash

I am sorry for that nth answer but I checked the answers and there is really simpler and neater.
Long story short
Use the python binary of your venv in your cron :
0 3 * * * /home/user/project/env/bin/python /home/user/project/manage.py
Long story
We activate the virtual environment when we want to set the current shell with the python config of that specific virtual environment(that is binaries and modules of that).
It is relevant to work with the current shell : execute multiple python commands on the current shell without the need to reference the full python path of the venv.
In the frame of a cron or even a bash, which value to activate the environment ?
Besides I read in some answers some references to bash rather than sh or still to define a wrapper to call the Python code. But why the hell should we bother with these ?
I repeat, just do it :
0 3 * * * /home/user/project/env/bin/python /home/user/project/manage.py
The documentation confirms that :
You don’t specifically need to activate an environment; activation
just prepends the virtual environment’s binary directory to your path,
so that “python” invokes the virtual environment’s Python interpreter
and you can run installed scripts without having to use their full
path. However, all scripts installed in a virtual environment should
be runnable without activating it, and run with the virtual
environment’s Python automatically.

The only correct way to run python cron jobs when using a virtualenv is to activate the environment and then execute the environment's python to run your code.
One way to do this is use virtualenv's activate_this in your python script, see: http://virtualenv.readthedocs.org/en/latest/userguide.html#using-virtualenv-without-bin-python
Another solution is echoing the complete command including activating the environment and piping it into /bin/bash. Consider this for your /etc/crontab:
***** root echo 'source /env/bin/activate; python /your/script' | /bin/bash

Rather than mucking around with virtualenv-specific shebangs, just prepend PATH onto the crontab.
From an activated virtualenv, run these three commands and python scripts should just work:
$ echo "PATH=$PATH" > myserver.cron
$ crontab -l >> myserver.cron
$ crontab myserver.cron
The crontab's first line should now look like this:
PATH=/home/me/virtualenv/bin:/usr/bin:/bin: # [etc...]

This is a simple way that keeps the crontab command very similar to regular command (tested in Ubuntu 18.04). Some key notes to keep in mind:
You can use the . command instead of source. (crontab uses sh by default, not bash, so it doesn't have source.)
~ and $variables are expanded in crontab commands. (It's only crontab environment statements that don't do variable expansion.)
Here are examples if you have a file ~/myproject/main.py:
* * * * * cd ~/myproject && . .venv/bin/activate && python main.py > /tmp/out1 2>&1
You could also directly call the specific path of the python in the venv directory, then you don't need to call activate.
* * * * * ~/myproject/.venv/bin/python ~/myproject/main.py > /tmp/out2 2>&1
The downside of that is you would need to specify the project path twice, which makes maintenance trickier. To avoid that, you could use a shell variable so you only specify the project path once:
* * * * * project_dir=~/myproject ; $project_dir/.venv/bin/python $project_dir/main.py > /tmp/out3 2>&1

The best solution for me was to both
use the python binary in the venv bin/ directory
set the python path
to include the venv modules directory.
man python mentions modifying the path in shell at $PYTHONPATH or in python with sys.path
Other answers mention ideas for doing this using the shell. From python, adding the following lines to my script allows me to successfully run it directly from cron.
import sys
sys.path.insert(0,'/path/to/venv/lib/python3.3/site-packages');
Here's how it looks in an interactive session --
Python 3.3.2+ (default, Feb 28 2014, 00:52:16)
[GCC 4.8.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.path
['', '/usr/lib/python3.3', '/usr/lib/python3.3/plat-x86_64-linux-gnu', '/usr/lib/python3.3/lib-dynload']
>>> import requests
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ImportError: No module named 'requests'
>>> sys.path.insert(0,'/path/to/venv/modules/');
>>> import requests
>>>

I'd like to add this because I spent some time solving the issue and did not find an answer here for combination of variables usage in cron and virtualenv. So maybe it'll help someone.
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
DIR_SMTH="cd /smth"
VENV=". venv/bin/activate"
CMD="some_python_bin do_something"
# m h dom mon dow command
0 * * * * $DIR_SMTH && $VENV && $CMD -k2 some_target >> /tmp/crontest.log 2>&1
It did not work well when it was configured like
DIR_SMTH="cd /smth && . venv/bin/activate"
Thanks #davidwinterbottom, #reed-sandberg and #mkb for giving the right direction. The accepted answer actually works fine until your python need to run a script which have to run another python binary from venv/bin directory.

If you're on python and using a Conda Virtual Environment where your python script contains the shebang #!/usr/bin/env python the following works:
* * * * * cd /home/user/project && /home/user/anaconda3/envs/envname/bin/python script.py 2>&1
Additionally, if you want to capture any outputs in your script (e.g. print, errors, etc) you can use the following:
* * * * * cd /home/user/project && /home/user/anaconda3/envs/envname/bin/python script.py >> /home/user/folder/script_name.log 2>&1

python script
from datetime import datetime
import boto # check wheather its taking the virtualenv or not
import sys
param1=sys.argv[1] #Param
myFile = open('appendtxt.txt', 'a')
myFile.write('\nAccessed on ' + param1+str(datetime.now()))
Cron command
*/1 * * * * cd /Workspace/testcron/ && /Workspace/testcron/venvcron/bin/python3 /Workspace/testcron/testcronwithparam.py param
In above command
*/1 * * * * - Execute every one minte
cd /Workspace/testcron/ - Path of the python script
/Workspace/testcron/venvcron/bin/python3 - Virtualenv path
Workspace/testcron/testcronwithparam.py - File path
param - parameter

I've added the following script as manage.sh inside my Django project, it sources the virtualenv and then runs the manage.py script with whatever arguments you pass to it. It makes it very easy in general to run commands inside the virtualenv (cron, systemd units, basically anywhere):
#! /bin/bash
# this is a convenience script that first sources the venv (assumed to be in
# ../venv) and then executes manage.py with whatever arguments you supply the
# script with. this is useful if you need to execute the manage.py from
# somewhere where the venv isn't sourced (e.g. system scripts)
# get the script's location
DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
# source venv <- UPDATE THE PATH HERE WITH YOUR VENV's PATH
source $DIR/../venv/bin/activate
# run manage.py script
$DIR/manage.py "$#"
Then in your cron entry you can just run:
0 3 * * * /home/user/project/manage.sh command arg
Just remember that you need to make the manage.sh script executable

Add the path of the Python installation in the venv but don't activate the environment.
* * * * * /HDD1/shritam_kumar/VENOM/venv/bin/python /HDD1/shritam_kumar/Projects/Voelkner-DE/schedule_product_BA.py
It's that simple.

Since a cron executes in its own minimal sh environment, here's what I do to run Python scripts in a virtual environment:
* * * * * . ~/.bash_profile; . ~/path/to/venv/bin/activate; python ~/path/to/script.py
(Note: if . ~/.bash_profile doesn't work for you, then try . ~/.bashrc or . ~/.profile depending on how your server is set up.)
This loads your bash shell environment, then activates your Python virtual environment, essentially leaving you with the same setup you tested your scripts in.
No need to define environment variables in crontab and no need to modify your existing scripts.

If you are a MacOS user like me, you can check the crontab error message at /var/mail/{username} file. like this
tail /var/mail/{username}
If there is an "operation not permitted" error, maybe you have to add cron to the Full Disk Access apps (Security & Privacy > Privacy > Full Disk Access apps/execs).
And click + button, go to /usr/sbin, double click the cron file.
then it will fix the "not permitted" error. detailed steps
And this is my code：
0 19 * * * cd /Users/user/Desktop/Project && source /Users/user/Desktop/Project/venv/bin/activate && python command arg

This is a solution that has worked well for me.
source /root/miniconda3/etc/profile.d/conda.sh && \
conda activate <your_env> && \
python <your_application> &
I am using miniconda with Conda version 4.7.12 on a Ubuntu 18.04.3 LTS.
I am able to place the above inside a script and run it via crontab as well without any trouble.

This will also work on crontab -e
* */5 * * * cd /home/project && sudo /home/project/venv/bin/python scripte.py

I had this same issue:
I had written a custom django command to check for geodjango position coordinates inside of geodjango polygons and had trouble automating the task to run, however using this command with crontab worked for me:
* * * * * ./home/project/locations/locations.sh >> /var/log/locations.log 2>&1

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