python in bash: os.environ behaviour

Question

According to voithos's answer, os.environ can set environment variables and subprocess automatically inherit from parent process.

However, compare below to cases

First case, in python interaction mode

>>>import os
>>>os.environ['a']='1'
>>>os.system('echo $a')
1
0

The result is fine.

Second case, in bash script

#!/bin/bash
python3 - <<EOF
import os
os.environ['a']='1'
os.system('echo $a')
EOF

save the above as test.bash and run bash test.bash we got nothing!

Why in the second case, os.system doesn't inherit variable?

summary

Any dollar sign $ in bash here document will be expanded by default, no matter it is inside single quotes or not.

One way is to escape $ with backslash \ like \$

There is another way to avoid this expand, that is to single quote the first here doc delimiter, compare following

a=0

python3 - <<here
import os
os.environ['a']='1'
os.system('echo $a')
here

python3 - <<'here'
import os
os.environ['a']='1'
os.system('echo $a')
here

Slightly related question: Is your child-process (the python-process) able to set persistent environment variables? I know that NodeJS doesn't allow you to set environment variables that persist after the program ends. — geostocker
– geostocker, Commented Apr 24, 2017 at 8:43
it inherits variable, but $a must be evaluated before because of interactive stuff. I'm using windows and echo %a% yields 1 even ran from bash. — Jean-François Fabre
– Jean-François Fabre ♦, Commented Apr 24, 2017 at 8:47
@ChristosPapoulas I got confused. I thought $a only expand inside double quotes. It should not expand inside single quotes, right? — user15964
– user15964, Commented Apr 24, 2017 at 9:19
it's expanded before the command is run, while in the multiline redirect. — Jean-François Fabre
– Jean-François Fabre ♦, Commented Apr 24, 2017 at 9:20

Alex L · Accepted Answer · 2017-04-24 09:03:00Z

1

What @ChristosPapoulas says is right. The $a is getting evaluated by the shell when you're typing it in. The $a never makes it to your python interpreter. This can be seen in the following:

$ cat >/tmp/foo <<EOF
> import os
> os.environ['a'] = '1'
> os.system('echo $a')
> EOF
$ cat /tmp/foo
import os
os.environ['a'] = '1'
os.system('echo ')
$

answered Apr 24, 2017 at 9:03

Alex L

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3 Comments

Jean-François Fabre Over a year ago

yes, that we saw. But what's the fix? trying echo \$a seems to fix it but I cannot test, being on windows.

Alex L Over a year ago

Oh. If you write actual python code with a text editor, and put it in a file, then none of this is a problem, because you can put the $ in the file just fine. Is that an acceptable solution?

user15964 Over a year ago

@Jean-FrançoisFabre \$a works. I am on windows too. But win 10 has built-in sub linux and bash on windows, so I can confirm

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