Error in checking string is null in shell script?

Question

I am new to Shell Scripts and not aware of the syntax.

I have a shell script that I am pushing to my android device, and then converting it with 'busybox dos2unix' :

adb shell busybox dos2unix /data/local/myShellScript.sh

In the script I am trying to check a string whether it is empty or not ? But I am getting error : 'unexpected operator/operand' .

Below is my code :

echo "Getting a PID (Process ID) from a file" ;
pid=`cat  /sdcard/PIDofTask.txt` ;
echo "PID of task is : " $pid ;
echo "Checking if the pid exists, to verify the process is running" ;    
pidString=$(ps | grep ${pid}) ;
echo "PID string : " $pidString ;

if [ $pidString ] ;
then
    echo "pid String is not empty" ;
else 
    echo "pid String is empty" ;
fi ;

Output :

Getting a PID (Process ID) from a file
PID of task is : 11571
Checking if the pid exists, to verify the process is running
PID string : root 11571 8082 4180 1828 poll_sched 7faa42dcdc S sh
/data/local/myShellScript.sh[2]: [: 11571: unexpected operator/operand
pid String is empty

I have also tried [ -n $pidString ] and also [ -z $pidString ] options. But both are giving errors.

Where am I doing a mistake? Really appreciate help...

Answer 1

To check for a empty string, you need to use -z.

Example:

if [ -z "$str" ] ;

Answer 2

To check for a empty string.

Example:

line="hello welcome"
if [ -z "$line" ] ; then
echo "String null"
fi