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Can't open file 'demo.py': [Errno 2] No such file or directory when running through a bash script

I have a folder named test-folder with two files.
# file name: demo.py
print('hello world')
# file name: script.sh
python3.7 demo.py
The test-folder is present inside /home/username/Documents/
I have exported the above path to the .bashrc.
Now when I try to execute the script.sh using the following command,
bash script.sh
I get the following error
python3.7: can't open file 'demo.py': [Errno 2] No such file or directory
How can I solve this? I'm on Ubuntu 18.04

Move to the folder containing the file :
$ cd /home/username/Documents/test-folder
after making sure you are in the folder ccontaining the file try:
$ python file_name.py
if it doesn't work try the full path to python and the full path to the file like this:
$ /usr/lib/python3.7 /home/username/Documents/test-folder/file_name.py

If you want to use script.sh from anywhere, your approach won't work.
When executing $ bash script.sh, the file script.sh is expected to be located in the working directory.
You can use absolute paths to solve this:
# filename: script.sh
python3.7 /home/username/Documents/test-folder/demo.py
Try executing this with bash /home/username/Documents/test-folder/script.py.
You can achieve your goal in an arguably simpler way by adding /home/username/Documents/test-folder to your PATH, adding a shebang to demo.py and making demo.sh executable.
$ export PATH=/home/username/Documents/test-folder:$PATH
$ chmod +x /home/username/Documents/test-folder/demo.py
demo.sh will look like this with a shebang:
#!/usr/lib/python3.7
print('hello world')
and can be executed from anywhere with $ demo.sh.

It works for me. Is there anything about your setup that is different than mine?
$ cat demo.py
print("Hello World")
$ cat script.sh
python3.7 demo.py
$ tree
.
└── test-folder
├── demo.py
└── script.sh
1 directory, 2 files
$ cd test-folder/
$ bash script.sh
Hello World
Edit: Try the following.
$ cat script.sh
python3.7 "`dirname "$0"`/demo.py"

If your script.sh file is only there to run the Python script, it’s not needed. Instead make the Python script itself executable.
(1) add this as the first line in your Python file — this tells Unix which interpreter to use when running this file:
#!/usr/bin/env python37
(2) make your Python file executable so it can be run directly:
chmod +x demo.py
(3) put that file on the path, e.g. a path you set via a Bash config file, or one that’s already on the path like /usr/local/bin.
Now any time you give the command demo.py your script will run. Note the .py extension isn’t required — you can name the file anything you like.

Execute a python script as a command

I'm writing a python script which will be placed in a location. I want to execute it just like a command. for ex.
$ find_branch test
where find_branch is a script placed in anywhere in the system.
I would like to know how to achieve this. I can run it on the place where the script is present by chmod u+x on the script and removing the .py from the script

sudo nano /usr/bin/testpyscript
Then inside the script:
#!/usr/bin/python
print("I'm a python script")
Give it x permission:
sudo chmod +x /usr/bin/testpyscript
Now you can use it as a regular command:
bash-4.2$ testpyscript
I'm a python script
It doesn't have to be exactly at /usr/bin, any location that is inside your $PATH will do. Let's say you want it to be located at some folder inside your home directory, you could do something like this:
pwd
/home/brunorb
mkdir somedir
sudo mv /usr/bin/testpyscript somedir/
export PATH=$PATH:/home/brunorb/somedir/
testpyscript # from any folder in the system
I'm a python script

Make sure python has been added to your path and #!/usr/bin/python is located at the top of your script.
Note You could just try adding your script to your /usr/local/bin/ directory and give it the proper permissions.
sudo cp <your script> /usr/local/bin/

You have a number of options on how to achieve this.
Add the location where you put the script to your PATH environment variable, for example in your ~/.bashrc script:
export PATH="${PATH}:/folder/where/you/put/the/script"
Install the script to a location that is already on your path. It does not have to be a system folder like /usr/bin. Many default Bash setups will include ~/bin in your PATH.
Give the full path to your script on the command line:
/folder/where/you/put/the/script/find_branch test
Run the script through Python. This is very similar to option #2:
python /folder/where/you/put/the/script/find_branch test
Create an alias for the script in your environment. In bash you would do something like the following in your ~/.bashrc:
alias find_branch='/folder/where/you/put/the/script/find_branch'
OR
alias find_branch='python /folder/where/you/put/the/script/find_branch'
For options #1, #2, #3 and #5a to work properly, you should have a shebang with the version of python as the first line of the script. Any of the following will do, depending on how you have/want your environment set up:
#!/usr/bin/python
#!/usr/bin/python2
#!/usr/bin/python3
#!/usr/bin/env python
#!/usr/bin/env python2
#!/usr/bin/env python3
Finally, you do not have to remove the .py extension from the script if you do not want to. Many bash scripts have a .sh extension, for example, which does not prevent them from running as-is. You just have to include the extension in the name of the script when you run it.

How to modify the command to call Python

On my Mac, python will run Python 2 and python3 will run Python 3 in the terminal. How can I modify the command so I can call Python 3 with just py?

Add a new file in your home directory called .profile. In there, add a line:
alias py=python3
You'll have to start a new terminal session for this to have any effect.

Place you in the directory where the python3 executable is (with cd command).
Then, enter :
ln -s python3 py
This creates a symbolic link.

where to save a file in cygwin

noobiest question ever:
I'm trying to work with python via cygwin on my pc laptop - I have a file (foo.py) but python can't find it. The error it's giving me is:
$ chmod +x foo.py
chmod: cannot access `foo.py': No such file or directory
Is there a special location within the Cygwin folder that I need to save my foo.py?
Thanks!
AP

It's not python that can't find your file, it's the chmod command. C drive is mapped to /cygdrive/c in Cygwin, and D drive is mapped to /cygdrive/d and so on and so forth.
Are you in the same directory as the file when you are running chmod?
If your file is at C:\mycode\python\foo.py then you should either change to that directory first -
$ cd c:
$ cd mycode/python/
or as #Ahmed mentioned above, you could also run the command as
$ chmod +x /cygdrive/c/mycode/python/foo.py
But you only need chmod if your python script starts with
#!/bin/python
To execute such a file, you'd say
$ /cygdrive/c/mycode/python/foo.py
Or if you are in the same directory
./foo.py
If the first line of the python script isn't "#!/bin/python" then you can skip the chmod and just type
python /cygdrive/c/mycode/python/foo.py

You go right click on foo.py figure out its full path then do this:
chmod +x foos-full-directory/foo.py
And this should work for you and btw its not Python problem it's your pwd other than the foo.py working directory and you even didn't use python word in your command.

How do I install a script to run anywhere from the command line?

If I have a basic Python script, with it's hashbang and what-not in place, so that from the terminal on Linux I can run
/path/to/file/MyScript [args]
without executing through the interpreter or any file extensions, and it will execute the program.
So would I install this script so that I can type simply
MyScript [args]
anywhere in the system and it will run? Can this be implemented for all users on the system, or must it be redone for each one? Do I simply place the script in a specific directory, or are other things necessary?

The best place to put things like this is /usr/local/bin.
This is the normal place to put custom installed binaries, and should be early in your PATH.
Simply copy the script there (probably using sudo), and it should work for any user.

Walkthrough of making a python script available anywhere:
Make a python script:
cd /home/el/bin
touch stuff.py
chmod +x stuff.py
Find out where your python is:
which python
/usr/bin/python
Put this code in there:
#!/usr/bin/python
print "hi"
Run in it the same directory:
python stuff.py
Go up a directory and it's not available:
cd ..
stuff.py
-bash: stuff.py: command not found
Not found! It's as we expect, add the file path of the python file to the $PATH
vi ~/.bashrc
Add the file:
export PATH=$PATH:/home/el/bin
Save it out, re apply the .bashrc, and retry
source ~/.bashrc
Try again:
cd /home/el
stuff.py
Prints:
hi
The trick is that the bash shell knows the language of the file via the shebang.

you can also use setuptools (https://pypi.org/project/setuptools/)
your script will be:
def hi():
print("hi")
(suppose the file name is hello.py)
also add __init__.py file next to your script (with nothing in it).
add setup.py script, with the content:
#!/usr/bin/env python3
import setuptools
install_requires = [
'WHATEVER PACKAGES YOU NEED GOES HERE'
]
setuptools.setup(
name="some_utils",
version="1.1",
packages=setuptools.find_packages(),
install_requires=install_requires,
entry_points={
'console_scripts': [
'cool_script = hello:hi',
],
},
include_package_data=True,
)
you can now run python setup.py develop in this folder
then from anywhere, run cool_script and your script will run.

Just create ~/bin and put export PATH=$PATH:$HOME/bin in your bashrc/profile. Don't mess with the system, it will bite you back, trust me.
Few more things (relevant to the question but not part of the answer):
The other way export PATH=$HOME/bin:$PATH is NOT safe, for bash will will look into your ~/bin folder for executables, and if their name matches with other executables in your original $PATH you will be surprised by unexpected/non working command execution.
Don't forget to chmod+x when you save your script in ~/bin.
Be aware of what you are putting in your ~/bin folder, if you are just testing something or working on unfinished script, its always better to use ./$SCRIPT_NAME from your CWD to execute the script than putting it under ~/bin.

The quick answer is to symlink your script to any directory included in your system $PATH.
The long answer is described below with a walk through example, (this is what I normally do):
a) Create the script e.g. $HOME/Desktop/myscript.py:
#!/usr/bin/python
print("Hello Pythonista!")
b) Change the permission of the script file to make it executable:
$ chmod +x myscript.py
c) Add a customized directory to the $PATH (see why in the notes below) to use it for the user's scripts:
$ export PATH="$PATH:$HOME/bin"
d) Create a symbolic link to the script as follows:
$ ln -s $HOME/Desktop/myscript.py $HOME/bin/hello
Notice that hello (can be anything) is the name of the command that you will use to invoke your script.
Note:
i) The reason to use $HOME/bin instead of the /usr/local/bin is to separate the local scripts from those of other users (if you wish to) and other installed stuff.
ii) To create a symlink you should use the complete correct path, i.e.
$HOME/bin GOOD ~/bin NO GOOD!
Here is a complete example:
$ pwd
~/Desktop
$ cat > myscript.py << EOF
> #!/usr/bin/python
> print("Hello Pythonista!")
> EOF
$ export PATH="$PATH:$HOME/bin"
$ ln -s $HOME/Desktop/myscript.py $HOME/bin/hello
$ chmod +x myscript.py
$ hello
Hello Pythonista!

Just create symbolic link to your script in /usr/local/bin/:
sudo ln -s /path/to/your/script.py /usr/local/bin/script

Putting the script somewhere in the PATH (like /usr/local/bin) is a good solution, but this forces all the users of your system to use/see your script.
Adding an alias in /etc/profile could be a way to do what you want allowing the users of your system to undo this using the unalias command. The line to be added would be:
alias MyScript=/path/to/file/MyScript

i find a simple alias in my ~/.bash_profile or ~/.zshrc is the easiest:
alias myscript="python path/to/my/script.py"

Type echo $PATH in a shell. Those are the directories searched when you type command, so put it in one of those.
Edit: Apparently don't use /usr/bin, use /usr/local/bin

Acording to FHS, the /usr/local/bin/ is the good place for custom scripts.
I prefer to make them 755 root:root, after copying them there.