The alternate screen terminal emulator plague
There is one bit of behavior in modern X environments that drives me
up the wall: the use of 'alternate screens' in terminal emulators (and
in programs). That's probably obscure to most people, so let me put it
the other way; what drives me up the wall is when I edit a file in
or view it in a pager like
less, quit, and all of the text that I
was just looking at instantly disappears in favour of what was on the
screen before I started whatever it was.
(Having this happen in
less is especially infuriating, since
entire purpose is showing me information. But the moment I quit, I can't
What is happening is not the fault of
less, and so
on, or at least not exactly. Unix systems have terminfo, which is essentially a big
database of terminals and escape sequences that programs can send to
make them do things; two of the defined terminfo capabilities are escape
sequences that you are supposed to send when your full screen program
starts and stops. Many terminal emulators (and a certain number of
real terminals) support an alternate screen, and the people who wrote
terminfo database entries for them decided that full screen programs
should always use this alternate screen so they put the escape sequences
for 'switch to alternate screen' and 'switch to main screen' into the
initialization and de-initialization sequences. When programs like
less dutifully send the escape sequences that the
terminfo database tells them to, they shoot your foot off.
My personal opinion is that this is an unfortunate example of the slow
fossilization of X terminal emulators.
xterm probably does this
because some real terminal had its terminfo set up this way in the
depths of time, and everyone else has probably slavishly copied how
xterm works. No one has stopped to ask if the end result makes sense
and is usable, because if they had a great many people would have told
them that it doesn't and isn't.
(Xterm gets partial credit because it has a way to turn this off, so
at least the
xterm people recognize that it's a terrible idea even
if they feel constrained by backwards compatibility to not fix the
Unfortunately there is no general fix for this. Some programs can be told to not send the terminfo initialization and de-initialization strings; some terminal emulators can be told to ignore them. Sadly, sending the strings and paying attention to them is the default behavior; this leads to fixing a lot of programs on a lot of systems, one by one.
(For extra fun, some Unixes do things right to start with. For example,
Solaris has never put the alternate screen escape sequences into their
terminfo entries for
Sidebar: the quick fixes
If you use
xterm, set the
XTerm*titeInhibit resource to
to make it ignore the alternate screen escape sequences. As usual,
gnome-terminal has no way of controlling this.
less, do '
export LESS=X' or otherwise give it the
vi, add '
set t_ti= t_te=' to your
Comments on this page:Written on 29 November 2011.