CODEWITHSUNDEEP
pythonlinuxbash
user15964
user15964

Reputation: 2639

python in bash: os.environ behaviour

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

Upvotes: 1

Views: 4763

Answers (1)

Alex L
Alex L

Reputation: 1114

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 ')
$

Upvotes: 1

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