Issues with handling special characters when reading environment variables from python script-CodePudding

I have setup environment variables for to access MongoDB database. All works except for the MongoDB connection string which is pretty long.

Mongodb string can be provided in different format,

mongodb://myDBReader:D1fficultP@[email protected]:27017/?authSource=admin

Other examples are here - https://www.fosslinux.com/50317/connection-string-in-mongodb-with-examples.htm

In my case the string was provided by the database admin, so I am using it as it is provided. All, the environmental variables are shown at it is in .bashrc file except the MONGODB connection string. If I use the string from within my python script it works well, but when I call it through environment variables (.bashrc file) something changes the string.

Setup on .bashrc

export MGDB_CON_STRING="mongodb://myMGDB:someuname....................server/dbname?authSource=$external&authMechanism=PLAIN....."

When this variable is called by python script as

mgdb_con_str = os.environ["MGDB_CON_STRING"]

something is eating up the text $external& to be exact) in this string and returning it as

"mongodb://myMGDB:someuname....................server/dbname?authSource=authMechanism=PLAIN....."

However, if I override this variable again by using it within python script it works

mgdb_con_str = r"mongodb://myMGDB:someuname....................server/dbname?authSource=$external&authMechanism=PLAIN....." - so this works

Something is eating that $external& within the string and I cannot find what exactly is causing this. I also cannot find any question related to this problem elsewhere on stack or general google search. But, string value of all other environment variables do not change. And, similar problem arises if I read the mongodb string through config file.

CodePudding user response：

One other way to handle this situation is to use single quotes when preparing the environment variables and it work bash and csh scripts.

Instead of this

export MGDB_CON_STRING="mongodb://myMGDB:someuname....................server/dbname?authSource=$external&authMechanism=PLAIN....."

Do this

export MGDB_CON_STRING='mongodb://myMGDB:someuname....................server/dbname?authSource=$external&authMechanism=PLAIN.....'

When this variable is called by python script as

mgdb_con_str = os.environ["MGDB_CON_STRING"]
print(mgdb_con_str)
# returns the raw string
'mongodb://myMGDB:someuname....................server/dbname?authSource=$external&authMechanism=PLAIN.....'

CodePudding user response：

This has nothing to do with Python, just with how linux handles env variables.

When you are doing

export A="...authSource=$external&something_else"

$external is interpreted as the environment variable $external. Since it does not exist, it is replaced with an empty string:

bash-4.2$ export A="...authSource=$external&something_else"
bash-4.2$ echo $A
...authSource=&something_else

You need an actual $ in your env variable value, so you need to escape it with \:

bash-4.2$ export A="...authSource=\$external&something_else"
bash-4.2$ echo $A
...authSource=$external&something_else

Of course assigning mgdb_con_str = r"mongodb://myMGDB:someuname....................server/dbname?authSource=$external&authMechanism=PLAIN....." directly in the Python script will work, since it does not try to evaluate env variables and $external is just the literal string $external.

Actual example:

bash-4.2$ export NO_ESCAPED="aa$aa&g"
bash-4.2$ export ESCAPED="aa\$aa&g"
bash-4.2$ echo $NO_ESCAPED
aa&g
bash-4.2$ echo $ESCAPED
aa$aa&g
bash-4.2$ python
>>> import os
>>> os.getenv('NO_ESCAPED')
'aa&g'
>>> os.getenv('ESCAPED')
'aa$aa&g'