What is wrong with following bash script?

Two things:

1. There can be no spaces between name of the variable, the = signal and the value of the variable

2. If you want to put the results of a command or series of commands on a variable, you must enclose it on $(command) or old style backticks. Both lines below are correct and do the same thing:

   a=`commands`
   a=$(commands)
   

So your line must be

a=$(grep -n mark /etc/samba/smb.conf |cut -d: f1)

There are some differences on how to concatenate the use of backticks or new style $(), check this question on SO to see a good response.

a=`grep -n mark /etc/samba/smb.conf |cut -d: -f1`

Try this instead

a=$(grep -n mark /etc/samba/smb.conf |cut -d: f1)

Or instead of the $() you could use backticks (which here turn on the nice code look)

Should be a=$( ... ) or

a=` … `

Maybe by trying to use cut like this:

a=`grep -n mark /etc/samba/smb.conf |cut -d: -f1`