What is the purpose of "-z" string checking?

Shell Programming and Scripting
1

what is the purpose of below specially "-z" string checking, how?

pid=`ps ax |grep java`
 
if [ ! -z $pid ]
2

It checks if the length of the string is zero or not.

In your case they want to perform some actions if PID is not forund for a java process

3 
if [ ... ]

is the same as

if test ...

So you can read about it in

man bash

or

man test

There are some glitches in your sample. The grep for processes can report the grep process itsself. A fix:

pid=`ps ax |grep '[j]ava'`

Certain values of pid can cause a syntax error. Fix:

if [ ! -z "$pid" ]
4

On many systems you'll have the much safer pgrep to check for the existence of a program, too.

if pgrep java >/dev/null
then
        echo "java is there"
else
        echo "java is not there"
fi