How to disable terminal cache? [duplicate]

When using Terminal.app, you can clear the screen by using the shell builtin clear or by pressing ^+L (Control-L).

However, all this does is push the current screen content back one screen height and reset the cursor/input at the first line. Meaning you can still scroll back and see it.

What you are also able to do, is reset your entire scrollback by pressing ⌘+K (Command-K).

After you've done this, you cannot scroll back at all.

In certain situations (notably, before running screen or vim), I'd like to reset the scroll back before the command actually executes.

Is there a command (like clear) that is implemented in OS X that allows me to do this? Given the existence of pbcopy and pbpaste, I'm thinking something similar might exist that will allow me to do this.

Solution 1:

Terminal supports an extension of the ED (Erase in Display) escape sequence to erase the scroll-back. It is also supported by xterm. The ED command, described in the VT100 manual, accepts these values for the Ps parameter:

ESC [ Ps J

Parameter   Parameter Meaning

0           Erase from the active position to the end of the screen
1           Erase from start of the screen to the active position
2           Erase all of the display

Terminal (and xterm) adds:

3           Erase the scroll-back (aka “Saved Lines”)

Note that this only erases the scroll-back, not the screen. This allows you to erase one or the other, or both by sending two escape sequences.

For example, you can clear the screen and the scroll-back with the following shell command: clear && printf '\e[3J'

(The clear command looks up the appropriate sequence for clearing the screen for the current terminal, but the “erase scroll-back” escape sequence is custom and must be hard-coded. If you put this in a shell script that you don’t know for certain will only ever be run with Terminal, you should check that $TERM_PROGRAM is Apple_Terminal before sending it.)

Solution 2:

⌘K, shortcut to “View > Clear scrollback”.

UPDATE: This can be automated with AppleScript by the following command:

osascript -e 'tell application "System Events" to keystroke "k" using command down'

…which you can of course alias to whatever you want, or store in a function in your ~/.profile login script  :)

As a security though, to be able to use this even in background without risking to issue a keystroke to a wrong front app, I'd advise surrounding it with the following test:

if application "Terminal" is frontmost

…which gives us the following code:

osascript -e 'if application "Terminal" is frontmost then tell application "System Events" to keystroke "k" using command down'

…which in turns, properly escaped and aliased, ends in:

alias clear="osascript -e 'if application \"Terminal\" is frontmost then tell application \"System Events\" to keystroke \"k\" using command down'"

And here is your new clear!  :)

Solution 3:

Here's code that works for both macOS' Terminal, and iTerm2. It doesn't need the window to be in the foreground (unlike some AppleScript solutions), either.

printf '\e[2J\e[3J\e[H'

How it works

This uses a series of ANSI escape sequences. Each ANSI escape sequence starts with the "ESC" (escape) character, it's a non-printable ASCII character (meaning it has no visual glyph representation like 1 or a). printf can print an this character using \e (or by its octal value \033, or its hex value \x1B).

One kind of ANSI escape sequence are the "Control Sequence Introducer" commands. They all start with the form ESC CSI, where ESC is the escape character just mentioned, and CSI is the value [ (or 0x5B in hex)

I use the \e[ representation, because it's shorter than \x1B\x5B, and easier to read.

As we can see, there are two ANSII escape sequences here, each of which are the "control sequence introducer" form, called with different arguments. Knowing this, we can split up the string into its 3 parts:

1. \e[2J

   • The start of an escape sequence (\e), containing a control sequence ([) with argument 2J
   • This is an instance of the "ED – Erase in Display" command, which has the form CSI n J
   • The n value is set to 3 in this case, which invokes the second variant:

   If n is 2, clear entire screen (and moves cursor to upper left on DOS ANSI.SYS).

2. \e[3J

   • The start of an escape sequence (\e), containing a control sequence ([) with argument 3J

   • This is an instance of the "ED – Erase in Display" command, which has the form CSI n J

   • The n value is set to 3 in this case, which invokes the third variant:

     If n is 3, clear entire screen and delete all lines saved in the scrollback buffer (this feature was added for xterm and is supported by other terminal applications)."

   • Despite the description on Wikipedia, it appears that this isn't actually clearing the screen in iTerm and the default Terminal, only the scrollback buffer was cleared. Hence the need for the \e[2J command above.

3. \e[H:

   • The start of an escape sequence (\e), containing a control sequence ([) with argument H.

   • This is an instance of the "CUP - Cursor Position" command, which has the form CSI n ; m H,

     Moves the cursor to row n, column m. The values are 1-based, and default to 1 (top left corner) if omitted. A sequence such as CSI ;5H is a synonym for CSI 1;5H as well as CSI 17;H is the same as CSI 17H and CSI 17;1H

   • This could have been more explicitly written as \e[1;1H

Fun fact

These control sequences are actually how the built-in clear (/usr/bin/clear) command works. All it seems to do is product some output, which we can inspect with xxd:

$ clear | xxd
00000000: 1b5b 481b 5b32 4a                        .[H.[2J

We can re-create the same output with printf '\e[H \e2J':

$ printf '\e[H \e2J' | xxd
00000000: 1b5b 4820 1b32 4a                        .[H .2J

Notice they don't use \e3J, hence why clear only clears the screen, but not the scrollback.