listing files in a directory without listing subdirectories and their contents in that directory

Find-based solution:

find . -maxdepth 1 -type f -printf '%f\n'

Bash-based solution:

for f in *; do [[ -d "$f" ]] || echo "$f"; done
##  or, if you want coloured output:
for f in *; do [[ -d "$f" ]] || ls -- "$f"; done

The bash-based solution will get you everything that isn't a directory; it will include things like named pipes (you probably want this). If you specifically want just files, either use the find command or one of these:

for f in *; do [[ -f "$f" ]] && echo "$f"; done
##  or, if you want coloured output:
for f in *; do [[ -f "$f" ]] && ls -- "$f"; done

If you're going to be using this regularly, you can of course put this into an alias somewhere in your ~/.bashrc:

alias lsfiles='for f in *; do [[ -f "$f" ]] && ls -- "$f"; done'

Since you noted in the comments that you're actually on OSX rather than Ubuntu, I would suggest that next time you direct questions to the Apple or more general Unix & Linux Stack Exchange sites.

List filenames only:

 1. ls -p | grep -v /                                   (without hidden files)
 2. ls -l | grep ^- | tr -s ' ' | cut -d ' ' -f 9       (without hidden files)
  
 a) ls -pa | grep -v /                                  (with hidden files)
 b) ls -la | grep ^- | tr -s ' ' | cut -d ' ' -f        (with hidden files)

List directories only:

 1. ls -p | grep /                                      (without hidden)
 2. ls -l | grep ^d | tr -s ' ' | cut -d ' ' -f 9       (without hidden) 

  a) ls -pa | grep /                                     (with hidden)
 b) ls -l | grep ^d | tr -s ' ' | cut -d ' ' -f 9       (with hidden)

grep -v -e ^$ is to remove blank lines from the result.

More details:
ls -p flag is to put '/' at the end of the directory name, -R flag is for recursive search, -l for listing with info, -a for listing all(including hidden files) with info, grep -v flag is for result invertion and -e flag for regex matching.

To list regular files only:

ls -al | grep ^-

With symbolic links included:

ls -al | grep ^[-l]

Where the first character of the list describes the type of file, so - means that it's a regular file, for symbolic link is l.

Debian/Ubuntu

Print the names of the all matching files (including links):

run-parts --list --regex . .

With absolute paths:

run-parts --list --regex . $PWD

Print the names of all files in /etc that start with p and end with d:

run-parts --list --regex '^p.*d$' /etc

Yet another solution, a naively short one that worked for me:

ls -la | grep -E '^[^d]' > files