I found an odd shell bug & I'm not entirely sure who's at fault.

[hanno@mastodon.social]

I found an odd shell bug & I'm not entirely sure who's at fault. Some interaction between screen, ANSI colors & grep seems to make a char disappear.
Reproducer: run this in screen on a terminal with 80 chars width:
echo -e 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx12345'|grep 234
The "4" disappears mysteriously... Seems to be caused by escape code '[K' from grep. I'd appreciate explanations+whether I should report a bug to screen, grep or elsewhere.

[hanno@mastodon.social]

Possibly related, but slightly different issue: if I do this in xterm, but not in screen, the 4 also disappears, but it's not replaced by empty space, but by the 5 (which should end up on the next line)

[uint8_t@chaos.social]

@hanno what if you pipe to grep 4 the output? is the 4 in the pipe or already eaten by grep?

[hanno@mastodon.social]

@uint8_t same result with only grepping 4. 4 is in the output. Minimal ANSI escape code PoC without grep looks like this:
echo -e 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx1234\033[K5'

[qbi@freie-re.de]

@hanno If I do this in tmux in xterm, the 4 "reappears".

[jmorahan@mastodon.ie]

@hanno https://debbugs.gnu.org/cgi/bugreport.cgi?bug=15444

[wr@infosec.exchange]

@hanno Here are a few other examples of disappearing text, the resulting effect depends on the terminal emulator you use. Here on xfce4-terminal (and bash):

$ echo -e "blabla\033\0135blibli\n\n\n" | hexdump -C
00000000  62 6c 61 62 6c 61 1b 5d  62 6c 69 62 6c 69 0a 0a  |blabla.]blibli..|
00000010  0a 0a                                             |..|
00000012
$ echo -e "blabla\033\0135blibli\n\n\n"
blabla$ echo -e "blabla\033\0135blibli\n\n\n" | grep bli
blabla$ 
$ 
$ echo -e "blabla\n\033\0135blibli\n\n\n" | hexdump -C
00000000  62 6c 61 62 6c 61 0a 1b  5d 62 6c 69 62 6c 69 0a  |blabla..]blibli.|
00000010  0a 0a 0a                                          |...|
00000013
$ echo -e "blabla\n\033\0135blibli\n\n\n"
blabla
$ echo -e "blabla\n\033\0135blibli\n\n\n" | grep bli
$ 
$ echo -e "blabla\033\0135\nblibli\n\n\n" | hexdump -C
00000000  62 6c 61 62 6c 61 1b 5d  0a 62 6c 69 62 6c 69 0a  |blabla.].blibli.|
00000010  0a 0a 0a                                          |...|
00000013
$ echo -e "blabla\033\0135\nblibli\n\n\n"
blabla$ echo -e "blabla\033\0135\nblibli\n\n\n" | grep bli
blibli
$ 
$ echo -e "blabla\033\n\0135blibli\n\n\n" | hexdump -C
00000000  62 6c 61 62 6c 61 1b 0a  5d 62 6c 69 62 6c 69 0a  |blabla..]blibli.|
00000010  0a 0a 0a                                          |...|
00000013
$ echo -e "blabla\033\n\0135blibli\n\n\n"
blabla
$ echo -e "blabla\033\n\0135blibli\n\n\n" | grep bli
]blibli
$

That's the trick I used for https://github.com/fuxx/GPG-ANSI-Art-Injector/blob/main/designs/fake_key_summary.py and https://codeberg.org/wllm-rbnt/hacklu-2025-CLI-ambush/