More bash scripting

        substring="trajectory"
        ##================================================
        index="$(echo $f | grep -b -o $substring | sed 's/:.*$//')" # redirecting the output of echo into a variable with "$(...)"
        #echo $index
        model_name=${f:0:$((index-1))} # taking the substring 0-(index-1)
        echo "model_name : "$model_name
           
        ##=================================================
        # find the 'trajectory_id'
        #expr index "$f" trajectory
        #echo $index
        trajectory_id=${f#*$substring} # find the occurrence of $substring in $f
        #echo $f
        #echo $trajectory_id
        len=$((${#trajectory_id}-4)) # arithmatic operation with $((.)) excluding the .txt extension
        #echo "length is "$len       
        trajectory_id=${trajectory_id:0:$len}
        echo "trajectory_id : "$trajectory_id


Reference
Reference