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Terminal detection (color) fails in sshx #140

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Terminal detection (color) fails in sshx#140

Description

@hellt
hellt
opened on Sep 26, 2025

Hi @ekzhang

I am a longtime user of sshx, and I noticed that CLI apps that try to detect the CLI color capabilities fail to do so in sshx session.

When launching sshx without any flags, I get a session with xterm terminal:

❯ echo $TERM
xterm-256color

Then when I run my CLI tool that uses terminal detection to understand the color theme and color support it hangs for two 5s periods on pselect6

❯ strace -tt -f -e trace=write,pselect6 containerlab version
strace: Process 1782530 attached
strace: Process 1782531 attached
strace: Process 1782532 attached
strace: Process 1782533 attached
strace: Process 1782534 attached
strace: Process 1782535 attached
[pid 1782533] 14:13:15.589627 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
strace: Process 1782536 attached
[pid 1782529] 14:13:15.589780 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782536] 14:13:15.590628 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782536] 14:13:15.594521 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
strace: Process 1782537 attached
[pid 1782529] 14:13:15.595111 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782535] 14:13:15.595581 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782529] 14:13:15.595757 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782536] 14:13:15.596008 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782535] 14:13:15.596767 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782529] 14:13:15.596997 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
strace: Process 1782538 attached
[pid 1782536] 14:13:15.604097 write(2, "\33]10;?\33\\", 8) = 8
[pid 1782536] 14:13:15.604285 write(2, "\33[6n", 4) = 4
[pid 1782536] 14:13:15.604349 pselect6(3, [2], NULL, NULL, {tv_sec=5, tv_nsec=0}, NULL) = 0 (Timeout)
[pid 1782536] 14:13:20.613026 write(2, "\33]11;?\33\\", 8) = 8
[pid 1782536] 14:13:20.613094 write(2, "\33[6n", 4) = 4
[pid 1782536] 14:13:20.613108 pselect6(3, [2], NULL, NULL, {tv_sec=5, tv_nsec=0}, NULL) = 0 (Timeout)
[pid 1782536] 14:13:25.623182 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782538] 14:13:25.624041 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782537] 14:13:25.624122 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782536] 14:13:25.624144 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782535] 14:13:25.624165 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782534] 14:13:25.624228 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782531] 14:13:25.624309 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782532] 14:13:25.624374 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782529] 14:13:25.624439 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782530] 14:13:25.624460 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782531] 14:13:25.624665 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782538] 14:13:25.624866 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782537] 14:13:25.625009 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782536] 14:13:25.625076 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782535] 14:13:25.625235 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782534] 14:13:25.625308 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782532] 14:13:25.625429 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782531] 14:13:25.625589 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782530] 14:13:25.625716 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782529] 14:13:25.625889 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1782529, si_uid=501} ---
[pid 1782533] 14:13:25.626138 write(1, "  ____ ___  _   _ _____  _    __"..., 441  ____ ___  _   _ _____  _    ___ _   _ _____ ____  _       _     
 / ___/ _ \| \ | |_   _|/ \  |_ _| \ | | ____|  _ \| | __ _| |__  
| |  | | | |  \| | | | / _ \  | ||  \| |  _| | |_) | |/ _` | '_ \ 
| |__| |_| | |\  | | |/ ___ \ | || |\  | |___|  _ <| | (_| | |_) |
 \____\___/|_| \_| |_/_/   \_\___|_| \_|_____|_| \_\_|\__,_|_.__/ 

) = 441
[pid 1782533] 14:13:25.626492 write(1, "    version: 0.70.2\n", 20    version: 0.70.2
) = 20
[pid 1782533] 14:13:25.626699 write(1, "     commit: 4c7cab2c\n", 22     commit: 4c7cab2c
) = 22
[pid 1782533] 14:13:25.626908 write(1, "       date: 2025-09-25T12:23:38"..., 34       date: 2025-09-25T12:23:38Z
) = 34
[pid 1782533] 14:13:25.626992 write(1, "     source: https://github.com/"..., 54     source: https://github.com/srl-labs/containerlab
) = 54
[pid 1782533] 14:13:25.627099 write(1, " rel. notes: https://containerla"..., 52 rel. notes: https://containerlab.dev/rn/0.70/#0702
) = 52
[pid 1782538] 14:13:25.627399 +++ exited with 0 +++
[pid 1782536] 14:13:25.627418 +++ exited with 0 +++
[pid 1782537] 14:13:25.627442 +++ exited with 0 +++
[pid 1782531] 14:13:25.627445 +++ exited with 0 +++
[pid 1782530] 14:13:25.627446 +++ exited with 0 +++
[pid 1782535] 14:13:25.627451 +++ exited with 0 +++
[pid 1782534] 14:13:25.627453 +++ exited with 0 +++
[pid 1782532] 14:13:25.627472 +++ exited with 0 +++
[pid 1782533] 14:13:25.628904 +++ exited with 0 +++
14:13:25.628934 +++ exited with 0 +++

If I set the TERM=dumb, it works:

❯ TERM=dumb strace -tt -f -e trace=write,pselect6 containerlab version
strace: Process 1783699 attached
strace: Process 1783700 attached
strace: Process 1783701 attached
strace: Process 1783702 attached
strace: Process 1783703 attached
strace: Process 1783704 attached
strace: Process 1783705 attached
[pid 1783698] 14:16:02.631074 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783701] 14:16:02.631175 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783698] 14:16:02.631279 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783704] 14:16:02.631565 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783704] 14:16:02.631850 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783698] 14:16:02.632041 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783698] 14:16:02.632531 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783698] 14:16:02.632945 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783698] 14:16:02.636692 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783703] 14:16:02.636828 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783705] 14:16:02.636856 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
strace: Process 1783706 attached
[pid 1783698] 14:16:02.637943 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783704] 14:16:02.638134 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783698] 14:16:02.638227 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783701] 14:16:02.641022 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783698] 14:16:02.641169 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783700] 14:16:02.641412 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
strace: Process 1783707 attached
[pid 1783698] 14:16:02.641442 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783700] 14:16:02.641712 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
strace: Process 1783708 attached
[pid 1783698] 14:16:02.641929 --- SIGURG {si_signo=SIGURG, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783708] 14:16:02.643489 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783707] 14:16:02.643740 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783706] 14:16:02.643874 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783705] 14:16:02.644187 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783704] 14:16:02.644331 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783703] 14:16:02.644418 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783702] 14:16:02.644555 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783701] 14:16:02.644711 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783699] 14:16:02.644881 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783698] 14:16:02.644995 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783708] 14:16:02.645562 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783707] 14:16:02.645692 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783706] 14:16:02.645903 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783705] 14:16:02.646096 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783704] 14:16:02.646340 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783703] 14:16:02.646530 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783702] 14:16:02.646764 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783701] 14:16:02.646904 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783699] 14:16:02.647149 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783698] 14:16:02.647314 --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=1783698, si_uid=501} ---
[pid 1783700] 14:16:02.647561 write(1, "  ____ ___  _   _ _____  _    __"..., 441  ____ ___  _   _ _____  _    ___ _   _ _____ ____  _       _     
 / ___/ _ \| \ | |_   _|/ \  |_ _| \ | | ____|  _ \| | __ _| |__  
| |  | | | |  \| | | | / _ \  | ||  \| |  _| | |_) | |/ _` | '_ \ 
| |__| |_| | |\  | | |/ ___ \ | || |\  | |___|  _ <| | (_| | |_) |
 \____\___/|_| \_| |_/_/   \_\___|_| \_|_____|_| \_\_|\__,_|_.__/ 

) = 441
[pid 1783700] 14:16:02.647638 write(1, "    version: 0.70.2\n", 20    version: 0.70.2
) = 20
[pid 1783700] 14:16:02.647664 write(1, "     commit: 4c7cab2c\n", 22     commit: 4c7cab2c
) = 22
[pid 1783700] 14:16:02.647709 write(1, "       date: 2025-09-25T12:23:38"..., 34       date: 2025-09-25T12:23:38Z
) = 34
[pid 1783700] 14:16:02.647720 write(1, "     source: https://github.com/"..., 54     source: https://github.com/srl-labs/containerlab
) = 54
[pid 1783700] 14:16:02.647745 write(1, " rel. notes: https://containerla"..., 52 rel. notes: https://containerlab.dev/rn/0.70/#0702
) = 52
[pid 1783708] 14:16:02.647949 +++ exited with 0 +++
[pid 1783706] 14:16:02.647964 +++ exited with 0 +++
[pid 1783701] 14:16:02.647966 +++ exited with 0 +++
[pid 1783705] 14:16:02.647967 +++ exited with 0 +++
[pid 1783700] 14:16:02.647968 +++ exited with 0 +++
[pid 1783704] 14:16:02.647969 +++ exited with 0 +++
[pid 1783707] 14:16:02.647974 +++ exited with 0 +++
[pid 1783703] 14:16:02.647975 +++ exited with 0 +++
[pid 1783702] 14:16:02.647977 +++ exited with 0 +++
[pid 1783699] 14:16:02.648860 +++ exited with 0 +++
14:16:02.648874 +++ exited with 0 +++

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