How to make bash put prompt on a new line after cat command?

Most unix tools are designed to work well with text files. A text file consists of a sequence of lines. A line consists of a sequence of printable characters ending with a newline character. In particular, the last character of a non-empty text file is always a newline character. Evidently, example.txt contains only some text with no final newline, so it is not a text file.

cat does a simple job; turning arbitrary files into text files isn't part of that job. Some other tools always turn their input into text files; if you aren't sure the file you're displaying ends with a newline, try running awk 1 instead of cat.

You can make the bash display its prompt on the next line if the previous command left the cursor somewhere other than the last margin. Put this in your .bashrc (variation by GetFree of a proposal by Dennis Williamson):

shopt -s promptvars
PS1='$(printf "%$((COLUMNS-1))s\r")'$PS1

I prefer the following method...

cat example.txt ; echo

This doesn't doesn't evaluate the contents of example.txt or occasionally add a newline. It just echos a newline once the cat is done, is easy to remember, and no one is thinking about whether they're using strong or weak quoting correctly.

The only downside, really, is that you'll get an extra newline if the file has its own trailing newline.

I started using @Gilles's answer, but found that if the terminal changed the number of columns the prompt would no longer be at the start of a line as expected. This can happen for a variety of reasons, including tmux/screen splits, manual resizing of a GUI container, font changes, etc.

What I really wanted was something that would add a newline if the terminal would start printing its prompt at something other than the first column. To do this I needed to figure out how to get the current column, which I used this answer to get. The final working prompt configuration is below:

###
# Configure PS1 by using the old value but ensuring it starts on a new line.
###
__configure_prompt() {
  PS1=""

  if [ "$(__get_terminal_column)" != 0 ]; then
    PS1="\n"
  fi

  PS1+="$PS1_WITHOUT_PREPENDED_NEWLINE"
}

###
# Get the current terminal column value.
#
# From https://stackoverflow.com/a/2575525/549363.
###
__get_terminal_column() {
  exec < /dev/tty
  local oldstty=$(stty -g)
  stty raw -echo min 0
  echo -en "\033[6n" > /dev/tty
  local pos
  IFS=';' read -r -d R -a pos
  stty $oldstty
  echo "$((${pos[1]} - 1))"
}

# Save the current PS1 for later.
PS1_WITHOUT_PREPENDED_NEWLINE="$PS1"

# Use our prompt configuration function, preserving whatever existing
# PROMPT_COMMAND might be configured.
PROMPT_COMMAND="__configure_prompt;$PROMPT_COMMAND"