Shell Programming and Scripting

Hi,

I am trying to edit the values in a file. For example i am trying to edit the value of "ABC" in a file by executing shell script.

Please Note that ABC value can be there mulitple times or it may not be there in the file
Conditions for it is

1. If ABC is less than 123 then it should change the ABC value to 123
2. If ABC is greater than 123 then it should leave the ABC value to existing value only as it is greater than 123
3. If ABC is not there is file then it should add ABC=123 in file.

Code i am trying is not working. PFB

Code:

     if [ $ABC -lt 123 ]
     then
        sed -i "s|\("ABC" *= *\).*|\1123|" file.con
     fi

    if [ $ABC -gt 123 ]
     then

    "ABC=$ABC" >> file.con  

     else

     echo "kernel.shmall=" >> /mqseries1/sysctl.conf
     sed -i "s|\("kernel.shmall" *= *\).*|\1123|" file.con
     
     fi

Experts can you please help me over here?