How to call python with script from StdIn from BASH with dynamic input-parameters?

Question

As part of a BASH-script I need to launch some python-code with an input-parameter. I also want to avoid creating an temporary *.py-files.

This is what I have so far:

input=123
output=$(
python - <<'END_SCRIPT'
print("$input")
END_SCRIPT
)
echo $output

This snippet writes the output of the python-call into an output-variable. But instead of returning "123" it returns "$input", because the variable with not be interpreted during the piping into python. Is there any way to forward the input variable as a parameter to python while reading from StdIn?

tripleee · Accepted Answer · 2020-12-10 15:29:32Z

7

I guess you are looking for

input=123
output=$(python - "$input" <<'END_SCRIPT'
import sys
print(sys.argv[1])
END_SCRIPT
)
echo "$output"

(Notice also When to wrap quotes around a shell variable.)

answered Dec 10, 2020 at 15:29

tripleee

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6 Comments

chepner Over a year ago

As a stylistic choice, you can also put the here document outside the command substitution: output=$(python - "$input") << 'END_SCRIPT', as Python will inherit its standard input from the assignment.

Carsten Over a year ago

@chepner I dont get that comment. How to make this into a solution or to improve the given solution?

chepner Over a year ago

@Peter Literally, you just move the ) to just before the <<. Like I said, it's a style choice, not an improvement, as I'm not aware of any significant difference between putting the quoted here-document inside the command substitution versus leaving it outside.

Carsten Over a year ago

@chepner I tried moving the bracket, but that does not run at all on MacOS with BASH.

tripleee Over a year ago

I can confirm - contrary to my expectation - that Bash 3.2 on MacOS Catalina starts an interactive Python interpreter if you move the redirection outside the command substitution.

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prithajnath · Accepted Answer · 2020-12-10 15:39:47Z

0

Just get rid of the single quotes around END_SCRIPT

input=123
output=$(
python - << END_SCRIPT
print("$input")
END_SCRIPT
)
echo $output

edited Dec 10, 2020 at 15:39

answered Dec 10, 2020 at 15:28

prithajnath

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3 Comments

tripleee Over a year ago

This is problematic if the real Python script needs to contain literal backticks or dollar signs anywhere in it.

prithajnath Over a year ago

@tripleee good call. I edited my answer. I assumed OP has some control over input in which case they could escape those characters before passing them to Python

chepner Over a year ago

Making the caller responsible for adapting the input is just asking for mistakes.

Carsten · Accepted Answer · 2020-12-13 15:53:16Z

0

At the end I decided to skip passing any input variable to python at all and better set an environment variable as needed. Here my final result:

input='<123!>'
output=$(python3 - <<'END_SCRIPT'
import os
print(os.environ['input'])
END_SCRIPT
)
echo "$output"

edited Dec 13, 2020 at 15:53

answered Dec 13, 2020 at 15:38

Carsten

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