abc.out
def.out

# input_file:

abc.out abc.out abc.out
def.out def.out
def.out

sed -i 's/.out//g' input_file

var1="abc.out"

var1=${var1%.out}
echo "$var1"

abc

$ foo=abc.def.out
$ echo ${foo%.out}
abc.def

$ echo ${foo%.*}
abc.def

$ echo ${foo%%.*}
abc

$ basename a.out .out

a

echo ${str%.*}

echo $str | grep -o '^[^.]*'

echo $str | sed -E 's/(.*?)..*/1/'

echo $str | awk -F. '{print $1}'

abc