LinuxQuestions.org - Bash - how to write logical expressions?

I'm not that great at bash programming, and I have a class UNIX/Linux scripting. I've come across this a few times trying this assignment and the last ones, but now I want to figure it out.

I'm writing a script that provides a little wrapper around the 'ps aux' command. Basically there can be 0-3 arguments (-c, -u, -p):

-c - display all processes from the 'ps aux' output with non-zero CPU usage
-u - this requires an argument of a user to search (display processes owned by user entered)
-p - display a process by process ID entered

I'm getting all the cmd-line options from getopts - thats working fine. I'm kind of a bash newb, but I understand code logic pretty well for a first-year. I made 2 files, ps+ which controls startup flow and ps+fns.sc which has the main functions in it. In ps+ I do a while loop around my getopts like so (also note the variables I use):

Code:

declare USER_INPUT=""

declare PID_INPUT=""

declare NONZERO_FLAG=0



source "ps+fns.sc"



while getopts u:p:c arguments 2>/dev/null

do

  case $arguments in

    # I make sure that 2 instances of -u are passed to the program,

    # exiting if the user does. The function this case, and a few other

    # places calls is a simple error message and exit 1 command

    u) if [ "$USER_INPUT" = "" ]

        then

            USER_INPUT=$OPTARG    # copy argument's option into $USER_INPUT

        else

            exitError

        fi;;



    p) if [ "$PID_INPUT" = "" ]

        then

            PID_INPUT=$OPTARG

        else

            exitError

        fi;;



    c) NONZERO_FLAG=1;;

  \?) exitError;;

  esac

done





# Call the 'master controller', checks what arguments 

# are set and launches appropriate reg-exp on it

psController

This is all starting to work nicely for me after being at school till 3am last night, but I have a problem. The psController function thats defined in a seperate file does some outputting before it checks the values of the arguments. Thats fine, if I can do what I would do in C++ (C++/pseudocode below) Before my function call to psController, I would like to first do a check that see's if any options were set. A simple if statement with some logical &&'s thrown in there should do it. Thats how I'd do it in C++ at least. If no options are set (ie $USER_INPUT = "", $PID_INPUT = "", NONZERO_FLAG=0) I'd like to simply perform an exit 1 inside the if statement. See code:

Code:

#include <cstdlib>



// ...

if(USER_INPUT == "" && PID_INPUT == "" && NONZERO_FLAG == 0)

    exit 1;



// call function psController - pretend i'm passing 3 vars by reference

psController(USER_INPUT, PID_INPUT, NONZERO_FLAG);

Now in C++ I'd probably use a bool instead of a 1/0 flag, but thats besides the point. I just want to understand how to write those logical expressions using AND's and OR's, because I use them quite alot in C++ and java when I program in java.

BIG thanks for any help, I'll be working on other parts of the assignment until then. I can finish it properly without worrying about the situation I mentioned, but thats just my most recent example of something I'm trying to accomplish. For our exam we have to write bash scripts on the exam paper, so I need to code and practice now on a computer while I can so I do alrite on the exam.