[SOLVED] Checking the existence of env variable

mahmoodn · 06-12-2010, 04:35 AM

I have two questions:
1- How can I check a directory path is in $PATH?
2- I wrote this script to check an environment variable:

Code:

echo -n "checking for \$SOMETHING... "
if [ -n "$SOMETHING" ] 
then 
   echo "yes"
else
   echo "export SOMETHING=$HOME/sim" >> $HOME/.bashrc
fi;

but here is the result I get:

Code:

mahmood@hocalhost:myfoler$ echo $SOMETHING

mahmood@hocalhost:myfoler$ ./install.sh 
checking for $SOMETHING... yes

Why it doesn't work?
Thanks,

carltm · 06-12-2010, 05:45 AM

Question 1:
if echo :"$PATH": | grep :"$SomeDirectory": > /dev/null
then echo "$SomeDirectory" is in the current PATH
else echo "$SomeDirectory" is not in the current PATH
fi

Question 2:
How do you know it isn't working?

From your results, we can't tell whether or not $SOMETHING
is empty. For example it may have a space character.

It looks to me like it is working. The logic says if
$SOMETHING is empty then echo yes, and it did.

mahmoodn · 06-12-2010, 06:26 AM

Consider this script:

Code:

echo -n "checking for \$SOMETHING... "
if [ -n "$SOMETHING" ] 
then 
	echo "yes"
else
	export SOMETHING=$HOME/sim
	echo "export SOMETHING=$HOME/sim" >> $HOME/.bashrc
	echo "no -> added"
fi;

Now I run this script for the first time:

Code:

mahmood@localhost:~$ ./install.sh 
checking for $SOMETHING... no -> added

without closing the current terminal, I checked:

Code:

mahmood@localhost:~$ grep "SOMETHING" .bashrc
export SOMETHING=/home/mahmood/sim
mahmood@localhost:~$ echo $SOMETHING

mahmood@localhost:~$

I ran the script again and got this output:

Code:

mahmood@localhost:~$ ./install.sh 
checking for $SOMETHING... no -> added
mahmood@localhost:~$ grep "SOMETHING" .bashrc
export SOMETHING=/home/mahmood/sim
export SOMETHING=/home/mahmood/sim

If I close the terminal and reopen it again, every thing is fine:

Code:

mahmood@localhost:~$ echo $SOMETHING
/home/mahmood/sim
mahmood@localhost:~$ ./install.sh 
checking for $SOMETHING... yes

I don't know what is wrong with that. I know that the script logic is true.

unSpawn · 06-12-2010, 06:52 AM

AFAIK there is nothing wrong. The first time SOMETHING is empty (and exporting SOMETHING would then affect the environment of the current shell and children) but since you dumped the export in your .bashrc instead (which isn't exactly behaviour you would want to propagate beyond your own system) it gets sourced when starting a new shell session. See the "invocation" part in 'man bash'?

mahmoodn · 06-12-2010, 07:23 AM

In this part:

Code:

else
	export SOMETHING=$HOME/sim
	echo "export SOMETHING=$HOME/sim" >> $HOME/.bashrc
	echo "no -> added"

First I am exporting the variable in the current session and then add that entry to the .bashrc file for future uses.
Now:

Code:

mahmood@localhost:~$ grep "SOMETHING" .bashrc
export SOMETHING=/home/mahmood/sim
mahmood@localhost:~$ echo $SOMETHING

mahmood@localhost:~$

variable is added to the .bashrc file and *seems* that the first command (exporting to current shell) was not added because echo command return nothing.

colucix · 06-12-2010, 07:41 AM

Please re-read the post from unSpawn, since it has the answer to your question:

Code:

.bashrc ... gets sourced when starting a new shell session

this means that it's not enough to write the export statement into $HOME/.bashrc... it has to be sourced to apply the changes (otherwise it is simply something written in a text file and nothing more). If you want to apply the changes to the current shell just do:

Code:

. $HOME/.bashrc

Anyway, what is the aim of this? If you want to provide an installation script, I think it's better to not modify the user's .bashrc without he/she is aware of the change. I personally would prefer the installation script warns me about the changes I must apply (that is it makes notice about a necessary environment variable) then I will apply it at my pleasure, eventually following the advice to write the export statement in my .bashrc. Just my

Finally, following again the answer from unSpawn, you should definitively have a look at the bash man page or at the Bash reference manual to see how environment variables work and how they are assigned by the login shell when invoked.

mahmoodn · 06-12-2010, 08:58 AM

believe me that sourcing .bashrc in a bash file does not work.
This is my script (I remove all other actions)

Code:

#! /bin/bash
echo "export SOMETHING=$HOME/sim" >> $HOME/.bashrc
source .bashrc

And this is the run:

Code:

mahmood@localhost:~$ chmod +x var-test.sh
mahmood@localhost:~$ ./var-test.sh 
mahmood@localhost:~$ echo $SOMETHING

mahmood@localhost:~$ grep "SOMETHING" .bashrc
export SOMETHING=/home/mahmood/sim

but sourcing from the terminal works:

Code:

mahmood@localhost:~$ source .bashrc
mahmood@localhost:~$ echo $SOMETHING
/home/mahmood/sim

I will be very thankful if you run this script in your own system and let me know the result.

colucix · 06-12-2010, 09:14 AM

Quote:

Originally Posted by mahmoodn

believe me that sourcing .bashrc in a bash file does not work.

Believe me, I believe you! The shell cannot retrieve environment variable from its childs. Every variable assigned into the script (even with a source command) is lost when the script terminates. The parent shell will not be aware of the changes.

Indeed it would not be useful at all trying to set environment variables from a script. This is the reason why configuration files like .bashrc, .bash_profile or the system-wide /etc/bashrc and so on, exist and are sourced every time you invoke a new shell.

In your example, you modify the $HOME/.bashrc from a script, but you have to source it in the parent shell (after the script has finished its job) to see the changes applied to the current shell (without closing it).

mahmoodn · 06-12-2010, 09:51 AM

So I think I have to leave it at this time until finding a better solution. I will write in my readme file that the users should enter the variables manually and then run the install script.

Using this solution will then satisfy the privacy of users because they are aware of changing their env variables.

Thank you all