Adapting getPID for BSD UNIX

Recall the getPID function introduced in Chapter 21:

getPID() {

    if [ $# -lt 1 ] ; then
        echo "ERROR: Insufficient Arguments." >&2
        return 1
    fi

    PSOPTS="-ef"

    /bin/ps $PSOPTS | grep "$1" | grep -v grep | awk '{ print $2; }'
}

Remember that it works correctly only on systems where the command

ps –ef

produces a listing of all running processes. This is not the case on Linux and BSD systems. On BSD systems, we need to use the command

ps –auwx

to get the correct output. This works on older Linux systems, but on newer Linux systems an error message similar to the following is generated:

warning: '-' deprecated; use 'ps auwx', not 'ps -auwx'

By using the getOSName function given earlier in this chapter, we can adapt the getPID function to work with both the BSD and System V versions of ps. The modified version of getPID is as follows:

getPID() {

    if [ $# -lt 1 ] ; then
        echo "ERROR: Insufficient Arguments." >&2
        return 1
    fi

    case `getOSName` in
        bsd|sunos|linux)
            PSOPTS="-auwx" ;;
        *)
            PSOPTS="-ef" ;;
    esac

    /bin/ps $PSOPTS 2> /dev/null | grep "$1" | grep -v grep | awk '{
    ⇒print $2; }'
}

The two main changes are

•  A case statement sets the variable PSOPTS based on the operating system name.

•  The STDERR of ps is redirected to /dev/null in order to discard the warning message generated on newer versions of Linux.

Summary

In this chapter, you learned how to determine which version of UNIX you are running by using the uname command. In addition, you developed the getOSName and isOS functions to help you adapt your shell scripts to multiple versions of UNIX.

You also looked at the following techniques for improving the portability of shell scripts:

•  Conditional execution

•  Abstraction

In conditional execution, you modify the flow of your script depending on the version of UNIX being used. In abstraction, you change the implementation of your functions to account for the differences between the versions of UNIX. Here, the flow of your script remains the same.

Using the techniques and tips in this chapter, you can port your shell script across different versions of UNIX.

Questions

1.  Write a function called getCharCount that prints the number of characters in a file. Use wc to obtain the character count.
On Linux, FreeBSD, and SunOS (not Solaris), use the -c option for wc,. On other versions of UNIX, use the -m option instead. You can use the function getOSName to get the name of the operating system.

Terms

Conditional Execution

Conditional execution alters the execution of a script based on the system type. A script that uses conditional execution usually contains an if statement at the beginning that sets variables to indicate the commands to use on a particular platform.

Abstraction

Scripts that use abstraction retain the same basic flow by placing the conditional execution statements within functions. When a function is called, it makes a decision as to what commands execute for a given platform.