How can I make the watch command interpret vt100 sequences?

Question

Consider this simple example (which displays in red):

echo -e "\033[31mHello World\033[0m"

It displays on the terminal correctly in red. Now consider:

watch echo -e "\033[31mHello World\033[0m"

It does not display the color.

Note: I am aware that it is easy to write a loop that mimics the basic behavior by clearing and rerunning. However, the clear operation causes the screen to flash, which does not happen under watch

EDIT: Originally this question specified escape sequences rather than vt100 sequences, but that is not really what I am after, and was solved with single quotes.

Answer 1

From man watch of watch 0.3.0 on Ubuntu 11.10:

By default watch will normally not pass escape characters, however if you use the --c or --color option, then watch will interpret ANSI color sequences for the foreground.

It doesn't seem to work with your literal string on my terminal, but these work:

watch --color 'tput setaf 1; echo foo'
watch --color ls -l --color

Answer 2

Edit:

More recent versions of watch support color. You will need to use an extra level of quoting to preserve the quotes and escapes in the particular situation of the example in the question:

watch 'echo -e "\033[31mHello World\033[0m"'

From man watch:

  -c, --color
          Interpret ANSI color sequences.

Previously:

From man watch:

Non-printing characters are stripped from program output. Use "cat -v" as part of the command pipeline if you want to see them.

But they don't get interpreted, so I don't think there's any way.

Answer 3

You can try single quoting your command :

watch 'echo -e "\tHello World"'

On my machine this leaves me with a -e as first character, and a correctly tabbed hello world. It seems -e is the default for my version of echo. Still, it is a progress toward a correctly tabbed hello world

What happens is a double unquoting :
what watch see

echo -e "\033[31mHello World\033[0m"

what the shell called by watch see :

echo -e \033[31mHello World\033[0m

And then the backslash come into play, even when quoted, and it becomes a quoting nightmare.