Bash script

1) #!/bin/bash -p
What is -p means ?

2) killall dhcpcd > dev/null 2 > & 1
What is 2 > & 1 means ?

3) if [ $? -ne 0 ]
then
ifconfig eth0 up;
exit 1;
fi
What is "$?" ? Does it refer to the last variable reference ?

4) suid putdata update &&
What is && means ?

I did look at the web site that gives instruction on BASH programming, but some of the things mentioned above was not covered.

I checked the man page on that and did not find that option. I did find a --posix option that might be equivalent.

You should do a web search on bash and -p to see what you can find. If the archaic K&R syntax in your other post is any indication, this -p option might also be archaic.


It means to redirect stderr to the same place you redirected stdout. That's a real handy notation that I wish worked in DOS.


That's a predefined special environment variable, which in this case is the exit code of the previous command/process. So this statement says that if the previous command failed (0 means it ran successfully), then bring the ethernet interface back up and exit with a non-zero exit code to indicate failure. BTW, that is why your C programs should always return an int: zero if successful (using return 0; ) and non-zero if an error was encountered (using something like exit(1) ).

There's an entire set of these special variables; eg:
$$ == the current process id
$? == the exit code of the previous process
$# == the number of parameters passed to this process; just like argc
$* == the argument list -- there are two different variables for this that handle quoting differently, so you'd better research this one.

perl uses many of the same special variables, plus many more.


This one I'm not at all sure. You'd better do a web search on suid and "&&". Are you sure that it's "&&" and not "$$"?

&& is a logical AND and is often used to decouple, or serialise commands at the bash promt, as in

# ./configure && make

ie if the first process (left hand side of &&) fails, then the system won't evaulate the right hand side of the expression.

This all implies that there could be a missing command on the end of this last example...

just a thought


christo