In linux, how to delete all files EXCEPT the pattern *.txt?

You can use find:

find . -type f ! -name '*.txt' -delete

Or bash's extended globbing features:

shopt -s extglob
rm *.!(txt)

Or in zsh:

setopt extendedglob
rm *~*.txt(.)
#  ||     ^^^ Only plain files
#  ||^^^^^ files ending in ".txt"
#  | \Except
#   \Everything

If you just want to delete all files except '*.txt' then you can use the following command:

$ find . -type f ! -name "*.txt" -exec rm -rf {} \;

but if you also want to delete directories along with the files then you can use this:

$ find . ! -name "*.txt" -exec rm -r {} \;

there are many ways could do it. but the most simple way would be (bash):

shopt -s extglob
rm !(*.txt)