How to split a file using a numeric suffix

(From manpage, GNU coreutils 8.21) What you need seems to be -a/--suffix-length=N (generate suffixes of length N (default 2)), not -n/--number=CHUNKS (generate CHUNKS output files)

split -d -l 50000 -a 4 domains.xml domains_

and you should get: domains_0000, domains_0001...

I would use awk. It gives you finer control over your output files and filenames. It should be just ask quick too. Here's how to split a 100 line file into 20 line blocks:

awk 'NR%20==1 { file = FILENAME "_" sprintf("%04d", NR+19) } { print > file }' domains.xml

This should create some files like:

file_0020
file_0040
file_0060
file_0080
file_0100

Adjust accordingly. HTH.

Although you haven't asked for it but I suppose you'd want a proper extension to the resultant files (let us say xml) :

split -d -l 50000 -a 4 --additional-suffix=.xml domains.xml domains_

The --additional-suffix=.xml will make file names of type domains_0000.xml, domains_1453.xml etc.

Since the primary help from GNU split says:

Usage: /usr/gnu/bin/split [OPTION]... [INPUT [PREFIX]]
Output fixed-size pieces of INPUT to PREFIXaa, PREFIXab, ...; default
size is 1000 lines, and default PREFIX is 'x'.  With no INPUT, or when INPUT
is -, read standard input.

Mandatory arguments to long options are mandatory for short options too.
  -a, --suffix-length=N   generate suffixes of length N (default 2)
      --additional-suffix=SUFFIX  append an additional SUFFIX to file names.
  -b, --bytes=SIZE        put SIZE bytes per output file
  -C, --line-bytes=SIZE   put at most SIZE bytes of lines per output file
  -d, --numeric-suffixes[=FROM]  use numeric suffixes instead of alphabetic.
                                   FROM changes the start value (default 0).
  -e, --elide-empty-files  do not generate empty output files with '-n'
      --filter=COMMAND    write to shell COMMAND; file name is $FILE
  -l, --lines=NUMBER      put NUMBER lines per output file
  -n, --number=CHUNKS     generate CHUNKS output files.  See below
  -u, --unbuffered        immediately copy input to output with '-n r/...'
      --verbose           print a diagnostic just before each
                            output file is opened
      --help     display this help and exit
      --version  output version information and exit

It looks to me like you need to reorganize your options a bit:

split -a 4 -d -l 50000 domains.xml domains_