11

This answer discusses how to run a multi-line Python snippet from the command line in a terminal. I noticed that the answer works great within shell scripts, even with nested indentation, which is very nice, e.g.

#!/bin/bash
some_text="Hello world"
echo $some_text

cat <<EOF | python -
import sys;
for r in range(3):
  print r
  for a in range(2):
    print "hello"
EOF

prints:

0 
hello
hello
1
hello
hello
2
hello
hello

However, I am having a hard time sharing variables between the shell script and the Python snippet.

  1. How can I collect the output of the python subscript in the bash script? (e.g. in a variable such as $output).

  2. How can I pass a bash variable (e.g. $some_text) to the Python script?

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3
  • 1
    You can do python - <<EOF instead. Commented Apr 30, 2013 at 22:06
  • jftr imo this is bad style and you should try to avoid that Commented Apr 30, 2013 at 22:23
  • 1
    Why the /zsh tag? Commented May 1, 2013 at 20:34

4 Answers 4

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12

Getting a variable to Python

Since (when the EOF marker is not quoted) variable substitution occurs before text is passed from the heredoc to python's standard input, you can throw the variable right in the script.

python - <<EOF
some_text = "$some_text"
EOF

If some_text was test, python would see some_text = "test". Note however that it can be seen as a code injection vulnerability. If some_text was "; import os; os.system("evil-command"); x = ", for instance, python would see: 

some_text = ""; import os; os.system("evil-command"); x = ""

and run that evil command.

If you want to be able to pull your Python code right into a script without any modifications, you could export your variable.

export some_text

and use os.environ to retrieve it.

some_text = os.environ['some_text']

That's a much saner/safer approach.

Getting output from Python

You can use command substitution to collect the script's output.

output=$(
python - <<EOF
import sys;
for r in range(3):
  print r
  for a in range(2):
    print "hello"
EOF
)

(note that all trailing newline characters are removed)

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3
  • This looks great. When you said "You could also pass the variable to the script as an argument", how would you do that considering that the Python script is embedded within the shell script? Commented Apr 30, 2013 at 22:18
  • Sorry, I forgot about that for a second. I have removed that solution and added a better solution. Commented Apr 30, 2013 at 22:22
  • Be careful how you name the heredoc marker. When it's unquoted as in your example, shell expansion will occur within the heredoc string. For example, if your python code consisted of just print '$SHELL', the output would be /bin/bash or whatever your shell happens to be. If you change the first line to python - <<'END', the output would be $SHELL. If you are copying and pasting existing code to embed into a bash script, you almost certainly don't want shell substitutions -- you want the code to run the same way it does when it's not embedded in a bash script, right? Commented Aug 17, 2015 at 19:03
8

The problem with your approach is that the embedded python script no longer has access to the original stdin (since its stdin is... itself).

If that's an issue you can write:

python -c '
import sys;
for r in range(3):
  print r
  for a in range(2):
    print "hello"
'

Or if the python script may contain single quotes:

python -c "$(cat << 'EOF'
import sys;
for r in range(3):
  print r
  for a in range(2):
    print "hello"
EOF
)"

Or:

python <(cat << 'EOF'
import sys;
for r in range(3):
  print r
  for a in range(2):
    print "hello"
EOF
)
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6

Use a dash as the filename:

ruby - a b <<'END'
puts ARGV.join(",")
END

python - a b <<'END'
import sys
print ",".join(sys.argv[1:])
END

I don't know if sys.argv[1:] is the right way to do this in Python. For -e / -c you can specify end of arguments with --:

set -- -a -b -c
ruby -e 'puts ARGV.join(",")' -- "$@"
python -c 'import sys; print ",".join(sys.argv[2:])' -- "$@"

Capturing output and redirecting STDERR:

x=$(ruby <<'END' 2> /dev/null
puts "a"
abort "b"
END
)
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2

1) You could write variable assignments to a file in python, and then source that in your bash script.

2) Since your word (EOF) is not quoted, all the lines of the here-document are subjected to parameter expansion. You can use this to pass stuff to the python script.

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