How to use “less -F” without “-X”, but still display output if only one page

Actually it uses whatever is specified in the MANPAGER or the PAGER environment variable.

Depending on your man implementation and version there could be also a command line switch to specify the pager.

With the man-db implementation I use all the below ways work:

MANPAGER=cat man man

PAGER=cat man man

MANOPT='-P cat' man man

man -P cat man

To set it permanently, just add it to your ~/.bashrc (or other initialization file used by your shell):

export MANPAGER=cat

That works with some older man implementations too, while MANOPT is man-db specific:

export MANOPT='-P cat'

(Better do not set PAGER that way. That one is used by many other applications too.)

There could be also a global configuration file. man-db has /etc/man_db.conf or /etc/manpath.config. There you can set:

DEFINE    pager    cat

But unfortunately that is taken in consideration only if neither MANPAGER nor PAGER is set.

Since less 530 (released in December 2017), less --quit-if-one-screen does not switch to the alternate screen if it reads less than one screenful. So you won't have this problem if your version of less is recent enough.

In earlier versions, less has to decide whether to use the alternate screen when it starts. You can't defer that choice to when it terminates.

You could call less, let it use the alternate screen, and cat the content onto the primary screen if less terminates automatically. However I don't know of a way to detect automatic termination.

On the other hand, it isn't that difficult to call cat for short inputs and less for larger inputs, even preserving buffering so that you don't have to wait for the whole input to start seeing stuff in less (the buffer may be slightly larger — you won't see anything until you have at least one screenful of data — but not much more).

#!/bin/sh
n=3  # number of screen lines that should remain visible in addition to the content
lines=
newline='
'
case $LINES in
  ''|*[!0-9]*) exec less;;
esac
while [ $n -lt $LINES ] && IFS= read -r line; do
  lines="$lines$newline$line"
done
if [ $n -eq $LINES ]; then
  { printf %s "$lines"; exec cat; } | exec less
else
  printf %s "$lines"
fi

You might prefer to see the lines on the main screen as they come in, and switch to the alternate screen if the lines would cause scrolling.

#!/bin/sh
n=3  # number of screen lines that should remain visible in addition to the content
beginning=
newline='
'
# If we can't determine the terminal height, execute less directly
[ -n "$LINES" ] || LINES=$(tput lines) 2>/dev/null
case $LINES in
  ''|*[!0-9]*) exec less "$@";;
esac
# Read and display enough lines to fill most of the terminal
while [ $n -lt $LINES ] && IFS= read -r line; do
  beginning="$beginning$newline$line"
  printf '%s\n' -- "$line"
  n=$((n + 1))
done
# If the input is longer, run the pager
if [ $n -eq $LINES ]; then
  { printf %s "$beginning"; exec cat; } | exec less "$@"
fi