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Finding out the active Unix shell

 
 
Mark McIntyre
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      06-07-2007
On Thu, 07 Jun 2007 13:42:50 +0100, in comp.lang.c , Mark McIntyre
<(E-Mail Removed)> wrote:

>On Thu, 07 Jun 2007 09:07:54 -0000, in comp.lang.c , Tom Gur
><(E-Mail Removed)> wrote:
>
>>Maybe I wasn't clear.
>>I'm writing a C program (a server) that runs a Java program (an agent)
>>with some arguments.
>>The agent's shell is unknown, so I need to check, within my C code
>>what kind of shell runs on the account that runs the agent.

>
>The answer is unfortunatly still the same. You need to ask the
>specialists in your os and /or compiler, since C itself has no
>facilities for doing this. Most probably there's a function called
>getenv() or something similar provided by your platform.


er... getenv() is of course Standard, though whether it will return
anything useful on your platform is what you'd need to ask the
specialists in your flavour of unix.

--
Mark McIntyre

"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan
 
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Walter Roberson
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      06-07-2007
In article <(E-Mail Removed). com>,
Tom Gur <(E-Mail Removed)> wrote:
>Maybe I wasn't clear.
>I'm writing a C program (a server) that runs a Java program (an agent)
>with some arguments.
>The agent's shell is unknown, so I need to check, within my C code
>what kind of shell runs on the account that runs the agent.


Are the agent and server even running on the same system? If so then
[OT]
you may be able to find out the client's username and use system-
specific functions to request information about the login shell
registered for the user in the user's password control entry;
unfortunately, the shell the user logs in to is not necessarily
the shell the user is running at the time they invoke the agent
[/OT]

But what does the server care what shell the agent is running under?
If the *server* executes system(), that is going to be in the context
of the *server's* process. In order to get the agent to execute system()
you would have to send something information back to the agent
that the agent knows to interpret to invoke the appropriate system
call. But you've already said that the agent is running Java, no C,
and if you want to find out about the Java equivilent of system(),
you will need to look in Java documentation or in a Java newsgroup.


>I was wondering whether you know any kind of function that can get
>environment variables - so I'll be able to check the value of $SHELL
>in the agent's machine.


[OT]
There is no TCP/IP mechanism that would allow a server to
inquire about the environment variables of the process at the far
end of a network socket, so if this is a client-server architecture,
the only thing you can do is have the server send a message to the
client (agent) that the agent knows to turn in to a request to send
back the environment information; but you've said the agent is
running in Java, not C, so if you are programming the agent you would
need to find out what the Java equivilent of getenv() is.

If this is not TCP/IP client/server (e.g., if this is running through
pipes or unix domain sockets) with both server and agent on the same
machine, your options are system-dependant. I don't know what can
be done in Windows. In the System V Unix family, there is no (or at least
traditionally was no) mechanism to find out about the environment
of another process; I don't know about the historical BSD family of
Unices. In -some- Unix or Unix-like operating systems (SGI IRIX,
Linux I think, possibly others), a process which starts another
process has (clumsy) mechanisms available to get at the memory
and environment of the child process; these mechanisms may also
be available to processes running under high levels of authorization
(e.g., 'root') with respect to arbitrary other processes on the same
system. But if you are thinking about anything even close to
this, then you are solving completely the wrong problem!
[/OT]


Why not solve the problem a completely different way?

1) use tmpname() to generate a temporary file name (this is ANSI C).

2) strcat() that filename onto the end of the string "echo \\\\ > "
(this is ANSI C).

3) Use system(NULL) to find out whether there is a shell there at all.
(this is ANSI C).

4) If there is a shell, system() the string generated in step (2).
ANSI C promises you can submit the string; it doesn't make any promises
about how the shell will interpret the string. check the return value of
the system() command; if it didn't work, then this mechanism cannot be
used so break out of this sequence.

5) try to open and read the filename generated in (1). If you are able
to open it and able to read it, then check to see how many \ it contained.
If it contains a single \ then you know that whatever shell is being
used is translating \\ into \ ; if the file contains two \ then you
know that the shell is treating each \ literally.
Close and remove the temporary file afterwards.

This arrangement won't work on -every- machine, especially as not
every machine will have an 'echo' shell command, but it will work on
a wide variety of machines and will directly answer the question of
whether the shell is preserving all \ or not, instead of trying to
answer the round-about question of which shell it is and having an
internal (necessarily incomplete) table of shell names and expected
behaviours, together with the risk that a user might have configured
the shell to act differently than you expect.
--
Programming is what happens while you're busy making other plans.
 
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Kenny McCormack
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      06-07-2007
In article <f49di8$pc$(E-Mail Removed)>,
Walter Roberson <(E-Mail Removed)-cnrc.gc.ca> wrote:
>In article <(E-Mail Removed). com>,
>Tom Gur <(E-Mail Removed)> wrote:
>>Maybe I wasn't clear.
>>I'm writing a C program (a server) that runs a Java program (an agent)
>>with some arguments.
>>The agent's shell is unknown, so I need to check, within my C code
>>what kind of shell runs on the account that runs the agent.

>
>Are the agent and server even running on the same system? If so then
>[OT]


You guys need to work on your reading comprehension skills.

I believe the issue is that different shells parse user input
differently. I.e., for a given sequence of keystrokes (that the user
types in order to invoke the program), the results (i.e., what the
application sees in argv[]) can differ. The OP wants to eliminate this
problem - i.e., make it so that when the user types a certain sequence of
keystrokes to invoke his program, his program does the right thing,
regardless of which shell lies between him and the user. Effectively,
he wants to kludge around shell differences via application-level coding.

Whether this is a noble or a foolish envdeavor is up to you to decide.

 
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Keith Thompson
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      06-07-2007
Tom Gur <(E-Mail Removed)> writes:
> Maybe I wasn't clear.
> I'm writing a C program (a server) that runs a Java program (an agent)
> with some arguments.
> The agent's shell is unknown, so I need to check, within my C code
> what kind of shell runs on the account that runs the agent.
> I was wondering whether you know any kind of function that can get
> environment variables - so I'll be able to check the value of $SHELL
> in the agent's machine.


This was in response to a followup in which I advised you to ask in
comp.unix.shell. Please provide context when you post a followup; see
<http://cfaj.freeshell.org/google/>.

So you should ask in comp.unix.programmer rather than comp.unix.shell.

--
Keith Thompson (The_Other_Keith) http://www.velocityreviews.com/forums/(E-Mail Removed) <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
 
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Richard Bos
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      06-08-2007
CBFalconer <(E-Mail Removed)> wrote:

> Richard Bos wrote:
> > (E-Mail Removed) wrote:
> > > On 7 Jun, 10:07, Tom Gur <(E-Mail Removed)> wrote:
> >>
> >>> I was wondering whether you know any kind of function that can
> >>> get environment variables - so I'll be able to check the value
> >>> of $SHELL in the agent's machine.
> >>
> >> In the C language as such there won't be any such thing.

> >
> > Yesno.
> >
> >> In the Unix APIs there may be an appropriate function but that's
> >> not in the remit of this group.
> >>
> >> You'll get better answers in a group relating to Unix - most
> >> likely comp.unix.programmer.
> >>
> >> <OffTopic>

> >
> > Not yet.
> >
> >> Unix has getenv()

> >
> > Actually, that's an ISO C function, but...
> >
> >> but the $SHELL variable does not seem to be altered if a new
> >> shell is launched.

> >
> > ...C makes no guarantees about which variables are available, let
> > alone what they mean.
> >
> > So the OP could easily use a C function, but _how_ he should use
> > this function does depend on the OS he uses (and frustratingly,
> > could even depend on the shell he uses... so to find out which
> > shell he has, he first has to find out which shell he has. Nice.)

>
> Who says ANY shell even exists? Not the C standard.


getenv() exists. That's what the C Standard says. _If_ you happen to
know about an environment variable that contains the active shell,
getenv() can get it for you. As you indicate, you may have to cater for
the possibility of an empty return value.

Richard
 
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Flash Gordon
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      06-08-2007
Richard Bos wrote, On 08/06/07 07:25:
> CBFalconer <(E-Mail Removed)> wrote:
>
>> Richard Bos wrote:
>>> (E-Mail Removed) wrote:
>>>> On 7 Jun, 10:07, Tom Gur <(E-Mail Removed)> wrote:
>>>>
>>>>> I was wondering whether you know any kind of function that can
>>>>> get environment variables - so I'll be able to check the value
>>>>> of $SHELL in the agent's machine.
>>>> In the C language as such there won't be any such thing.
>>> Yesno.
>>>
>>>> In the Unix APIs there may be an appropriate function but that's
>>>> not in the remit of this group.
>>>>
>>>> You'll get better answers in a group relating to Unix - most
>>>> likely comp.unix.programmer.
>>>>
>>>> <OffTopic>
>>> Not yet.
>>>
>>>> Unix has getenv()
>>> Actually, that's an ISO C function, but...
>>>
>>>> but the $SHELL variable does not seem to be altered if a new
>>>> shell is launched.
>>> ...C makes no guarantees about which variables are available, let
>>> alone what they mean.
>>>
>>> So the OP could easily use a C function, but _how_ he should use
>>> this function does depend on the OS he uses (and frustratingly,
>>> could even depend on the shell he uses... so to find out which
>>> shell he has, he first has to find out which shell he has. Nice.)

>> Who says ANY shell even exists? Not the C standard.

>
> getenv() exists. That's what the C Standard says. _If_ you happen to
> know about an environment variable that contains the active shell,
> getenv() can get it for you. As you indicate, you may have to cater for
> the possibility of an empty return value.


You might also want to allow for the user setting up a deliberate lie.
--
Flash Gordon
 
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Richard Tobin
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      06-08-2007
In article <(E-Mail Removed)>,
CBFalconer <(E-Mail Removed)> wrote:

>> So the OP could easily use a C function, but _how_ he should use
>> this function does depend on the OS he uses (and frustratingly,
>> could even depend on the shell he uses... so to find out which
>> shell he has, he first has to find out which shell he has. Nice.)


>Who says ANY shell even exists? Not the C standard.


The original poster stipulated it.

-- Richard
--
"Consideration shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.
 
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Kenny McCormack
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      06-08-2007
In article <(E-Mail Removed)>,
CBFalconer <(E-Mail Removed)> wrote:
>Richard Tobin wrote:
>> CBFalconer <(E-Mail Removed)> wrote:
>>
>>>> So the OP could easily use a C function, but _how_ he should use
>>>> this function does depend on the OS he uses (and frustratingly,
>>>> could even depend on the shell he uses... so to find out which
>>>> shell he has, he first has to find out which shell he has. Nice.)

>>
>>> Who says ANY shell even exists? Not the C standard.

>>
>> The original poster stipulated it.

>
>Well, I guess I could accept 'implied', but not stipulated.


Questioner: What model car did you just drive past my house in?
Chuck: Who says this is a car?

Yes, I think the original question implies the existence of a car.

 
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