If or while loop inside case command positional parameters

Question

Being relatively new to anything other than bash scripting, I have created a script to

check if a process is running
output PID's to the shell
if not prompt user input and start etc/etc.

I've moved onto positional parameters and can't see where I'm going wrong:

if [ "$1" == "" ]; then
    proc_finder
elif [ $1 != "" ];then
    case $1 in
        -p | --process ) 
        shift
        z=$(ps aux |grep $1 |grep -v grep  > /dev/null)
        if [ ! -z "$z" ]; then
            echo "YES"
        else
            echo "NO"
        fi
    ;;
    * )
        echo "Usage -p (process)"
    esac
fi

This always seems to return yes even when putting in -p test for example. I know im doing something fundamentally wrong, looking at the verbose output the grep -v grep is being done last hence I believe it always returnes an exit state of 0.

ps aux | grep $1 | grep -v grep should be simplified to ps -C $1, if possible - at least Linux ps is able to do so. — user unknown
– user unknown, Commented Feb 19, 2012 at 10:30

torrential coding · Accepted Answer · 2012-02-18 00:42:43Z

1

Shouldn't that be if [ $? -eq 0 ]?

EDIT 1

You can try this:

z=`ps aux | grep $1 | grep -v grep > /dev/null`
if [ ! -z "$z" ]; then
    echo "YES"
else
    echo "NO"
fi

If $z is not empty (-z: test for zero-length string) this implies the process was found with the ps command.

EDIT 2

The ps ... grep ... grep is being redirect to /dev/null. That means z will contain nothing. remove the redirection and z should have some output.

z=`ps aux | grep $1 | grep -v grep`

EDIT 3

Alternatively, you can just do this:

ps aux | grep $1 | grep -v grep > /dev/null 2>&1
if [ $? -eq 0 ]; then
    echo "YES"
else
    echo "NO"
fi

In this case, you are not saving the grep output. That's good if you don't really need it.

edited Feb 18, 2012 at 0:42

answered Feb 17, 2012 at 23:06

torrential coding

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5 Comments

torrential coding Over a year ago

Why do you have a pipe | after the grep -v grep call? And what's the $z meant to be doing all by itself? That will always return 0

user1217286 Over a year ago

+ case $1 in + shift + echo test test ++ ps aux ++ grep test ++ grep -v grep + z= + '[' '!' -z == '' ']' + echo YES YES

user unknown Over a year ago

Backticks are deprecated. The $(...) construct, used by @user1217286 is much better: easy nestable, better readable.

torrential coding Over a year ago

@userunknown Thank you for pointing that out. Although I'm not sure how it would help readability, but I can appreciate the $(...) construct as far as nesting goes.

user unknown Over a year ago

Backticks get sometimes overseen, and some fonts make it hard to distinguish them from apostrophes.

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