Develop Reference

Piping to a python script from cmd shell

I have a python script (not created by me), let's call it myscript, which I call with several parameters.
So I run the script like this in Windows cmd:
Code:
/wherever/myscript --username=whoever /some/other/path/parameter
And then a prompt appears and I can pass arguments to the python script:
Process started successfully, blabla
Python 2.7.2 blabla
(LoggingConsole)
>>>
And I write my stuff, then quit to be back into cmd:
>>> command1()
>>> command2()
>>> quit()
I suspect some errors occurring in this part, but only once for a hundred trials. So I want to do it by a script.
I want to pipe to this script the internal command1 command2, so that I can test this function thousand times and see when it breaks. I have the following piece of code:
echo 'command1()' | py -i /wherever/myscript --username=whoever /some/other/path/parameter
This unfortunately doesn't generate the same behaviour, as if it would be manually entered.
Can I simulate this behaviour with pipes/redirecting output? Why doesn't it work? I expect that the 'command1()' text will be entered when the script waits for the commands, but it seems I'm wrong.
Thanks!
EDIT 16/02/2021 3:33PM :
I was looking for the cmd shell way to solve this, no python stuff
The piece of script
echo 'command1()' | py -i /wherever/myscript --username=whoever /some/other/path/parameter
is almost correct, just remove the '' :
echo command1() | py -i /wherever/myscript --username=whoever /some/other/path/parameter
my issues were coming from myscript. Once I fixed the weird things on this side, this part was all ok. You can even put all commands together:
echo command1();command2();quit(); | py -i /wherever/myscript --username=whoever /some/other/path/parameter
This question is adapted from a question of gplayersv the 23/08/2012 on unix.com, but the original purpose made the question not answered.

Easy to have pipes.
If you want to get the standard input :
import sys
imput = sys.stdin.read()
print(f'the standard imput was\n{imput}')
sys.stderr.write('This is an error message that will be ignored by piping')
If you want to use the standard input as argument:
echo param | xargs myprogram.py

Python's built-in fileinput module makes this simple and concise:
#!/usr/bin/env python3
import fileinput
with fileinput.input() as f:
for line in f:
print(line, end='')
Than you can accept input in whatever mechanism is easier for you:
$ ls | ./filein.py
$ ./filein.py /etc/passwd
$ ./filein.py < $(uname -r)

Executing string of python code within bash script

I've come across a situation where it would be convenient to use python within a bash script I'm writing. I call some executables within my script, then want to do a bit of light data processing with python, then carry on. It doesn't seem worth it to me to write a dedicated script for the processing.
So what I want to do is something like the following:
# do some stuff in bash script
# write some data into datafile.d
python_fragment= << EOF
f = open("datafile.d")
// do some stuff with opened file
print(result)
EOF
result=$(execute_python_fragment $python_fragment) # <- what I want to do
# do some stuff with result
Basically all I want to do is execute a string containing python code. I could of course just make another file containing the python code and execute that, but I'd prefer not to do so. I could do something like echo $python_fragment > temp_code_file, then execute temp_code_file, but that seems inelegant. I just want to execute the string directly, if that's possible.
What I want to do seems simple enough, but haven't figured it out or found the solution online.
Thanks!

You can run a python command direct from the command line with -c option
python -c 'from foo import hello; print (hello())'
Then with bash you could do something like
result=$(python -c '$python_fragment')

You only have to redirect that here-string/document to python
python <<< "print('Hello')"
or
python <<EOF
print('Hello')
EOF
and encapsulate that in a function
execute_python_fragment() {
python <<< "$1"
}
and now you can do your
result=$(execute_python_fragment "${python_fragment}")
You should also add some kind of error control, input sanitizing... it's up to you the level of security you need in this function.

If the string contains the exact python code, then this simple eval() function works.
Here's a really basic example:
>>> eval("print(2)")
2
Hope that helps.

maybe something like
result=$(echo $python_fragment | python3)
only problem is the heredoc assignment in the question doesn't work either. But https://stackoverflow.com/a/1167849 suggests a way to do it if that is what you want to do:
python_fragment=$(cat <<EOF
print('test message')
EOF
) ;
result=$(echo $python_fragment | python3)
echo result was $result

Pass variable from Python to Bash

I am writing a bash script in which a small python script is embedded. I want to pass a variable from python to bash. After a few search I only found method based on os.environ.
I just cannot make it work. Here is my simple test.
#!/bin/bash
export myvar='first'
python - <<EOF
import os
os.environ["myvar"] = "second"
EOF
echo $myvar
I expected it to output second, however it still outputs first. What is wrong with my script? Also is there any way to pass variable without export?
summary
Thanks for all answers. Here is my summary.
A python script embedded inside bash will run as child process which by definition is not able to affect parent bash environment.
The solution is to pass assignment strings out from python and eval it subsequently in bash.
An example is
#!/bin/bash
a=0
b=0
assignment_string=$(python -<<EOF
var1=1
var2=2
print('a={};b={}'.format(var1,var2))
EOF
)
eval $assignment_string
echo $a
echo $b

Unless Python is used to do some kind of operation on the original data, there's no need to import anything. The answer could be as lame as:
myvar=$(python - <<< "print 'second'") ; echo "$myvar"
Suppose for some reason Python is needed to spit out a bunch of bash variables and assignments, or (cautiously) compose code on-the-fly. An eval method:
myvar=first
eval "$(python - <<< "print('myvar=second')" )"
echo "$myvar"

Complementing the useful Cyrus's comment in question, you just can't do it. Here is why,
Setting an environment variable sets it only for the current process and any child processes it launches. os.environ will set it only for the shell that is running to execute the command you provided. When that command finishes, the shell goes away, and so does the environment variable.
You can pretty much do that with a shell script itself and just source it to reflect it on the current shell.

There are a few "dirty" ways of getting something like this done. Here is an example:
#!/bin/bash
myvar=$(python - <<EOF
print "second"
EOF
)
echo "$myvar"
The output of the python process is stored in a bash variable. It gets a bit messy if you want to return more complex stuff, though.

You can make python return val and pass it to bash:
pfile.py
print(100)
bfile.sh
var=$(python pfile.py)
echo "$var"
output: 100

Well, this may not be what you want but one option could be running the other batch commands in python using subprocess
import subprocess
x =400
subprocess.call(["echo", str(x)])
But this is more of a temporary work around. The other solutions are more along what you are looking for.
Hope I was able to help!

Changing BASH script to Python

I am trying to convert this script to Python but am having problems with the BASH syntax ${variable}. This does something I do not understand as I have very little experience with BASH scripting. My question is basically,how can I do the dollar brackets variable thing in Python ${variable}. In my case the variable is a number from zero to two-hundred and fifty-five. Please see the BASH script below. I am trying to do the exact same thing in Python.
do
for code in {0..255};
do echo -e "\e[38;05;${code}m $code: Test";
done;
done
Here is my attempt to convert the code to a Python script. Unfortunately, I am still having to call BASH via the os.system() method or function. Please check out my script below. The script does not function the same way though, with changing the text in the BASH shell. The Python script below simply prints out the crazy text below and increments the numbers... :/
#! /usr/bin/env python
import os
def run():
for code in range(0, 255):
os.system('echo -e "\e[38;05;%dm %s Test"' % (code,str(code)))
run()

You can use print command and string formatting to evaluate your variable during the print.
BTW, you can use xrange rather than range in order not to generate all the numbers in your memory, but to yield it one-by-one (for large range)
You can use this:
import os
def run():
for code in range(0, 256):
print "\x1b[38;05;{code}m {code} Test".format(code=code)
run()

You’ll need to use string formatting.
Strangely enough, you already do this in your code (but using the old % syntax instead of the new .format method). I’m not even sure why you think you have to call echo since the string you pass to it is already the string that you are trying to get (if I understand your question right). Just use the print function to output the string.

try this:
#! /usr/bin/env python
import os
def run():
for code in range(0, 255):
os.system('/bin/echo -e "\e[38;05;%dm %s Test"' % (code,str(code)))
run()
the echo is usually both extrnal program and internal command of shell, which can make difference sometime

Call Python script from bash with argument

I know that I can run a python script from my bash script using the following:
python python_script.py
But what about if I wanted to pass a variable / argument to my python script from my bash script. How can I do that?
Basically bash will work out a filename and then python will upload it, but I need to send the filename from bash to python when I call it.

To execute a python script in a bash script you need to call the same command that you would within a terminal. For instance
> python python_script.py var1 var2
To access these variables within python you will need
import sys
print(sys.argv[0]) # prints python_script.py
print(sys.argv[1]) # prints var1
print(sys.argv[2]) # prints var2

Beside sys.argv, also take a look at the argparse module, which helps define options and arguments for scripts.
The argparse module makes it easy to write user-friendly command-line interfaces.

Use
python python_script.py filename
and in your Python script
import sys
print sys.argv[1]

Embedded option:
Wrap python code in a bash function.
#!/bin/bash
function current_datetime {
python - <<END
import datetime
print datetime.datetime.now()
END
}
# Call it
current_datetime
# Call it and capture the output
DT=$(current_datetime)
echo Current date and time: $DT
Use environment variables, to pass data into to your embedded python script.
#!/bin/bash
function line {
PYTHON_ARG="$1" python - <<END
import os
line_len = int(os.environ['PYTHON_ARG'])
print '-' * line_len
END
}
# Do it one way
line 80
# Do it another way
echo $(line 80)
http://bhfsteve.blogspot.se/2014/07/embedding-python-in-bash-scripts.html

use in the script:
echo $(python python_script.py arg1 arg2) > /dev/null
or
python python_script.py "string arg" > /dev/null
The script will be executed without output.

I have a bash script that calls a small python routine to display a message window. As I need to use killall to stop the python script I can't use the above method as it would then mean running killall python which could take out other python programmes so I use
pythonprog.py "$argument" & # The & returns control straight to the bash script so must be outside the backticks. The preview of this message is showing it without "`" either side of the command for some reason.
As long as the python script will run from the cli by name rather than python pythonprog.py this works within the script. If you need more than one argument just use a space between each one within the quotes.

and take a look at the getopt module.
It works quite good for me!

Print all args without the filename:
for i in range(1, len(sys.argv)):
print(sys.argv[i])