How to get into python environment and run some python commands and return to normal terminal using shell script

Question

Sorry about the title it may not be clear. Here is the complete explanation of my doubt. I am writing the below shell script and expecting the mentioned output.

#!/bin/bash python3 print("Hello World") exit() echo "The execution is completed" 

The output what i am expecting is, it should enter the python3 interpreter and execute the print and exit() commands and after executing the exit() command as the interpreter exits if we do it manually and then execute the echo command.But it is not working that way, after executing python3 it is entering the python3 interpreter but not executing the print and exit().

>>> 

It is entering the python3 correctly and then stops there till i manually exit the python interpreter.

What changes should i make in order to get my expected output.

1. The follow up question: Is it possible to get the output Hello world or any other output that is generated in the python interpreter to bash environment and use it in the bash script. 2.And method which you specify to run the python commands will that work for other tools as well?

Answer 1

#!/bin/bash python3 print("Hello World") exit() echo "The execution is completed" 

Scripts work differently from typing commands directly in a terminal, ie. what you want to do won't work this easily.

The #!/bin/bash line tells the kernel that the program it's trying to start is a script that need to be run with /bin/bash. So when you run ./scriptname the kernel instead runs /bin/bash ./scriptname and bash will read commands from scriptname instead of standard input (the terminal).

When bash gets to the python line it will start the Python interpreter and kindly wait for it exit before continuing to the next line. When Python starts it does the same thing it would when you type python in the terminal, it will start the interactive interpreter.

Python is unaware that you want to run commands from a script file, and you also can't tell it to skip a few lines.
When Python exits, bash will continue from the next line quit() which is a syntax error, you also can't tell Bash to skip a few lines after python.

There are three ways to get Python to run commands:

1. Have bash write commands to the input of python via a heredoc (*1). Python can't read user input from the terminal if you do this.
2. Use the -c option to supply a short list of commands. This is only suitable for extremely short programs that also don't need to use any quotes (for strings). Multiple commands can be separated with semicolons.
3. Have a separate Python script, this requires a second file. If you understand the security risks associated with temporary files (*2), you can make bash create the file for you if you wish. Just don't create the files in /tmp and you should be fine.

The follow up question: Is it possible to get the output Hello world or any other output that is generated in the python interpreter to bash environment and use it in the bash script.

You can use command substitution using $( and ), please use these inside double quoted strings. You can even nest double quoted strings on the inside:

test="$(python -c "import sys; print(sys.version)")" echo "Python has finished" echo "test = $test" 

And method which you specify to run the python commands will that work for other tools as well?

All three methods listed above will work for many other scripting languages too.

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*1 Heredoc example

read -p "How many numbers: " n python <<END for i in range($n): print(i) print("Literal dollar sign \$") END 

The <<END syntax makes the shell (bash) read all lines until it reads the word END. Note that END must be at the beginning of the line.

This way you don't need to escape quotes. The shell will still interpret dollar signs, so you can use variables. If you want an actual dollar sign you need to escape it like this \$.

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*2 Example of the dangers of temp files. DO NOT USE THIS CODE

cat >/tmp/python-script <<END for i in range(10): print(i) END python /tmp/python-script 

If someone else creates /tmp/python-script first, this shell script won't even stop if it fails to overwrite the file. A malicious user on the system could make a harmful Python script which will run instead of the intended script.

This is not the only thing that can go wrong.

There are ways to do this safely, but the simplest way would be to create the files in the current working directory or in a dedicated directory in the home directory.

cat >dopythonstuff <<END ... END python dopythonstuff rm dopythonstuff 

Answer 2

Use Python to launch bash script and pass variable to bash script:

piping python variable value to bash script (inside python script)

There are 62 answers on this Stack Overflow question of how to call a shell command from a Python script:

https://stackoverflow.com/questions/89228/how-to-call-an-external-command

Python cannot exit and then call a shell command. You must call the shell command before you exit the Python script.

https://www.geeksforgeeks.org/python-exit-commands-quit-exit-sys-exit-and-os-_exit/

This article under the link above covers how to exit Python using various commands:

The functions quit(), exit(), sys.exit() and os._exit() have almost same functionality as they raise the SystemExit exception by which the Python interpreter exits and no stack traceback is printed. We can catch the exception to intercept early exits and perform cleanup activities; if uncaught, the interpreter exits as usual.

The article concludes:

Among above four exit functions, sys.exit() is preferred mostly, because the exit() and quit() functions cannot be used in production code while os._exit() is for special cases only when immediate exit is required.

You can use sys.exit("Exit message here") to exit the Python script and write the exit message to the console which would then return to the shell prompt.