File descriptors to serial ports (/dev/ttyUSB*) not properly closed after bash command execution

Description

When Claude Code executes bash commands that access serial ports (e.g., /dev/ttyUSB0), the file descriptors are not properly closed after the command completes. This causes the Claude Code process to hang indefinitely, requiring a kill -9 to terminate.

Steps to reproduce

1. Connect an Arduino/ESP device to /dev/ttyUSB0
2. Ask Claude to read serial output using direct bash commands:

``bash
cat /dev/ttyUSB0
# or
stty -F /dev/ttyUSB0 115200 raw -echo
``

1. Even with timeout, the Claude process keeps the fd open and becomes unresponsive

Observed behavior

• The bash command completes (timeout works, no child process remains)
• But Claude Code keeps the file descriptor to /dev/ttyUSB0 open
• The process enters sleeping state (wait_woken) and never recovers
• lsof /dev/ttyUSB0 shows the Claude process still holding the fd
• Only kill -9 can terminate the session

Diagnostic info

$ cat /proc/<pid>/status | grep State
State: S (sleeping)

$ cat /proc/<pid>/wchan
wait_woken

$ lsof /dev/ttyUSB0
COMMAND   PID   USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
claude  <pid>  user   22r   CHR  188,0      0t0  868 /dev/ttyUSB0

Environment

• Claude Code version: 2.1.6
• OS: Linux (openSUSE Tumbleweed)
• Device: ESP8266 on /dev/ttyUSB0

Workaround

Avoid direct bash access to serial ports. Use arduino-cli monitor with timeout instead, which seems to work correctly:

timeout 30 arduino-cli monitor -p /dev/ttyUSB0 -c baudrate=115200