Modification of data group in CLI mode

If you want to update from a file just keys without values try this...

 tmsh list ltm data-group internal dg1
ltm data-group internal dg1 {
    records {
        a { }
        b { }
    }
    type string
}

 more updates.txt 
c
d
e
f

 for a in `cat updates.txt`; do tmsh modify ltm data-group internal dg1 records add { $a } ; done

 tmsh list ltm data-group internal dg1
ltm data-group internal dg1 {
    records {
        a { }
        b { }
        c { }
        d { }
        e { }
        f { }
    }
    type string
}