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I have output files from split called

shivi11aa
shivi11ab

(etc.)

How may I rename these existing files as

output_1
output_2

(etc.)

If I have 1000 files, the last one should be output_1000.

n=0
for filename in shivi11*; do
 n=$(( n + 1 ))
 mv -i "$filename" "output_$n"
done

... where shivi11* is a pattern that must match all files that you'd like to rename (and nothing else).

Back up your data and test this.

Answer to the original question (before editing it):

If you by "split" mean the split utility, then you may invoke the utility like this to get what you want (this assumes GNU split and that you'd like to split the file file into 20 KB bits):

split -b 20k -a 1 --numeric-suffixes=1 file output_

This creates output_1, output_2 etc.

Note that since we restrict the suffix length to a single character with -a 1, this would not be able to split into more than 9 files. Using e.g. -a 2 would create files named output_01, output_02 etc. up to and including output_99, but not output_100.

The --numeric-suffixes=1 option is a GNU split-specific option that selects numeric suffixes starting at the given number (instead of at 0 as using -d would do). Normally, split creates files with alphabetic suffixes.