some questions about dump command

To whom it may concern,
Here are some of my in.file:
clear
variable i loop 120
label loop

Why don't you put the dump outside the loop?

or use:
dump 3 total custom 20 file id type x y z

(instead of:
dump 3 total custom 20 file.$i id type x y z)

--- Alternately ---
I'm under the impression that the dump file format is simple enough
that you can concatenate all of the individual dump files together to
create a long dump file. Try entering this (into the bash shell) with
your existing dump files (file.1, file.2, file.3, ..., file.120):

for ((i=1; i<=120; ++i)); do
    cat file.$i >> new_file.lammpstrj
done

Cheers
Andrew

Thanks Andrew- I forgot about the filename.

To be clear, unless there's also a "dump_modify 3 append yes" command, the dump still needs to go outside the loop (the default behaviour is to overwrite dump files).