Shell 用于测试 tcp 服务器是否正在发送数据的脚本
Shell script to test if tcp server is sending data
我有以下问题：有一个 tcp 服务器正在发送传感器数据，我需要知道服务器是否仍在传输数据，或者服务器后面的传感器是否已停止，这需要采取一些措施。 tcp 服务器可以在远程机器或本地机器上。
我的第一个想法是编写一个 shell 小程序，它通过 netcat 打开与服务器的连接，读取行，当它停止读取行时它会响铃。基本上是这样的：
#!/bin/bash
set -uo pipefail
retry_counter=0
nc 127.0.0.1 10001 | \
while true; do
read -rt 1 input
if [[ -n $input ]]; then
retry_counter=0
continue
fi
sleep 5
retry_counter=$(( retry_counter+1 ))
if [[ $retry_counter -ge 5 ]]; then
echo "ALARM!"
break
fi
done
我现在的问题是：
在从 shell 的命令行启动的脚本中，这段代码可以完美运行，但是一旦我将它打包到 systemd 服务中或从另一个脚本启动它，nc 就会立即关闭建立连接后，脚本总是发出警报。
我正在 运行宁 ubuntu 18.04LTS
我做错了什么？
编辑：
我在过去几天做了一些挖掘，为自己创建了一个传感器到 tcp 网关的模拟器，并在意识到我实际上不需要整个读取行逻辑后简化了我的脚本：
watchdog_tmp.sh:
#!/bin/bash
set -u
retry_counter=0
reader=""
echo "Starting"
while [[ $retry_counter -le 5 ]]; do
reader="$(ncat -vvv -i 10 127.0.0.1 10001 2>&1 > /dev/null | tee >(cat >&2) )"
if [[ "$reader" =~ "Connected" ]]; then
echo "First failure after success resetting counter"
retry_counter=0
else
(( retry_counter++ ))
fi
echo "($retry_counter) ALARM!"
sleep 5
done
问题没有改变，如果我从 shell 的命令行 运行 我的 watch_dog（当传感器是 运行ning 时），我得到以下输出：
Starting
Ncat: Version 7.01 ( https://nmap.org/ncat )
NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 8 [127.0.0.1:10001]
Ncat: Connected to 127.0.0.1:10001.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #2)
libnsock nsock_read(): Read request from IOD #1 [127.0.0.1:10001] (timeout: -1ms) EID 18
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #2 [peer unspecified] EID 26
libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 36
libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 18 [127.0.0.1:10001] (2200 bytes)
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #1 [127.0.0.1:10001] EID 42
libnsock nsock_event_cancel(): Event #36 (type TIMER) cancelled
libnsock nevent_delete(): nevent_delete on event #36 (type TIMER)
libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 52
libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 42 [127.0.0.1:10001] (43 bytes): $KRPHT,41.8800000,9.9003000,17.0000000*57..
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #1 [127.0.0.1:10001] EID 58
libnsock nsock_event_cancel(): Event #52 (type TIMER) cancelled
libnsock nevent_delete(): nevent_delete on event #52 (type TIMER)
libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 68
libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 58 [127.0.0.1:10001] (57 bytes): 0.000000 M/SEC 9.876518 DBAR 0.0 C 0.0 MS/CM.
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #1 [127.0.0.1:10001] EID 74
libnsock nsock_event_cancel(): Event #68 (type TIMER) cancelled
libnsock nevent_delete(): nevent_delete on event #68 (type TIMER)
libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 84
(...)
libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 234 [127.0.0.1:10001] (43 bytes): $KRPHT,41.8800000,9.9003000,17.0000000*57..
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #1 [127.0.0.1:10001] EID 250
libnsock nsock_event_cancel(): Event #244 (type TIMER) cancelled
libnsock nevent_delete(): nevent_delete on event #244 (type TIMER)
libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 260
libnsock nsock_trace_handler_callback(): Callback: READ EOF for EID 250 [127.0.0.1:10001]
libnsock nsock_trace_handler_callback(): Callback: TIMER SUCCESS for EID 260
Ncat: Idle timeout expired (10000 ms).
First failure after success resetting counter
(0) ALARM!
Ncat: Version 7.01 ( https://nmap.org/ncat )
NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT ERROR [Connection refused (111)] for EID 8 [127.0.0.1:10001]
Ncat: Connection refused.
(1) ALARM!
Ncat: Version 7.01 ( https://nmap.org/ncat )
NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT ERROR [Connection refused (111)] for EID 8 [127.0.0.1:10001]
Ncat: Connection refused.
(2) ALARM!
Ncat: Version 7.01 ( https://nmap.org/ncat )
NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 8 [127.0.0.1:10001]
Ncat: Connected to 127.0.0.1:10001.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #2)
libnsock nsock_read(): Read request from IOD #1 [127.0.0.1:10001] (timeout: -1ms) EID 18
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #2 [peer unspecified] EID 26
libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 36
(...)
Ncat: Idle timeout expired (10000 ms).
First failure after success resetting counter
(0) ALARM!
Ncat: Version 7.01 ( https://nmap.org/ncat )
NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT ERROR [Connection refused (111)] for EID 8 [127.0.0.1:10001]
Ncat: Connection refused.
(1) ALARM!
Ncat: Version 7.01 ( https://nmap.org/ncat )
NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT ERROR [Connection refused (111)] for EID 8 [127.0.0.1:10001]
Ncat: Connection refused.
(2) ALARM!
因此，我使用以下文件切换回 systemd 服务：
[Unit]
Description=Watchdog for the Artemis Proxy Service
After=network.target
[Service]
ExecStart=/tmp/artemis_proxy/bin/watchdog_tmp.sh
User=kraken
Group=kraken
KillMode=process
Restart=on-failure
RestartSec=30s
[Install]
WantedBy=default.target
不行，连接打开后立即关闭。这是日志的输出：
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: Starting
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: Ncat: Version 7.01 ( https://nmap.org/ncat )
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 8 [127.0.0.1:10001]
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: Ncat: Connected to 127.0.0.1:10001.
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_iod_new2(): nsock_iod_new (IOD #2)
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_read(): Read request from IOD #1 [127.0.0.1:10001] (timeout: -1ms) EID 18
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_readbytes(): Read request for 0 bytes from IOD #2 [peer unspecified] EID 26
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 36
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: READ EOF for EID 26 [peer unspecified]
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 18 [127.0.0.1:10001] (600 bytes)
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_readbytes(): Read request for 0 bytes from IOD #1 [127.0.0.1:10001] EID 42
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_event_cancel(): Event #36 (type TIMER) cancelled
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nevent_delete(): nevent_delete on event #36 (type TIMER)
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 52
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: READ EOF for EID 42 [127.0.0.1:10001]
Jul 29 21:31:35 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: TIMER SUCCESS for EID 52
Jul 29 21:31:35 kraken-zbox1 watchdog_tmp.sh[31551]: Ncat: Idle timeout expired (10000 ms).
Jul 29 21:31:35 kraken-zbox1 watchdog_tmp.sh[31551]: First failure after success resetting counter
Jul 29 21:31:35 kraken-zbox1 watchdog_tmp.sh[31551]: (0) ALARM!
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: Ncat: Version 7.01 ( https://nmap.org/ncat )
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 8 [127.0.0.1:10001]
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: Ncat: Connected to 127.0.0.1:10001.
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_iod_new2(): nsock_iod_new (IOD #2)
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_read(): Read request from IOD #1 [127.0.0.1:10001] (timeout: -1ms) EID 18
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_readbytes(): Read request for 0 bytes from IOD #2 [peer unspecified] EID 26
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 36
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: READ EOF for EID 26 [peer unspecified]
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 18 [127.0.0.1:10001] (743 bytes)
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_readbytes(): Read request for 0 bytes from IOD #1 [127.0.0.1:10001] EID 42
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_event_cancel(): Event #36 (type TIMER) cancelled
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nevent_delete(): nevent_delete on event #36 (type TIMER)
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 52
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: READ EOF for EID 42 [127.0.0.1:10001]
Jul 29 21:31:50 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: TIMER SUCCESS for EID 52
Jul 29 21:31:50 kraken-zbox1 watchdog_tmp.sh[31551]: Ncat: Idle timeout expired (10000 ms).
Jul 29 21:31:50 kraken-zbox1 watchdog_tmp.sh[31551]: First failure after success resetting counter
Jul 29 21:31:50 kraken-zbox1 watchdog_tmp.sh[31551]: (0) ALARM!
我还在想我做错了什么...
经过一番搜索和与其他人交谈后，我终于找到了我的问题：
ncat，当启动时期望通过网络连接接收 stdin 上的输入并将传入的数据发送到 stdout。当你从 shell 启动我的脚本时，stdin 存在并正确地通过管道传输到 ncat，每个人都很高兴。另一方面，systemd 不为其服务提供标准输入，因此没有标准输入会导致 ncat 终止，因为它无法发送任何内容。
netcat 命令的简单 --recv-only 解决了问题，因此正确的脚本是：
#!/bin/bash
set -u
retry_counter=0
reader=""
echo "Starting"
while [[ $retry_counter -le 5 ]]; do
# start bcat in reciever mode only!
reader="$(ncat -v --recv-only -i 10 127.0.0.1 10001 2>&1 > /dev/null | tee >(cat >&2) )"
if [[ "$reader" =~ "Connected" ]]; then
echo "First failure after success resetting counter"
retry_counter=0
else
(( retry_counter++ ))
fi
echo "($retry_counter) ALARM!"
sleep 5
done
我有以下问题：有一个 tcp 服务器正在发送传感器数据，我需要知道服务器是否仍在传输数据，或者服务器后面的传感器是否已停止，这需要采取一些措施。 tcp 服务器可以在远程机器或本地机器上。
我的第一个想法是编写一个 shell 小程序，它通过 netcat 打开与服务器的连接，读取行，当它停止读取行时它会响铃。基本上是这样的：
#!/bin/bash
set -uo pipefail
retry_counter=0
nc 127.0.0.1 10001 | \
while true; do
read -rt 1 input
if [[ -n $input ]]; then
retry_counter=0
continue
fi
sleep 5
retry_counter=$(( retry_counter+1 ))
if [[ $retry_counter -ge 5 ]]; then
echo "ALARM!"
break
fi
done
我现在的问题是：
在从 shell 的命令行启动的脚本中，这段代码可以完美运行，但是一旦我将它打包到 systemd 服务中或从另一个脚本启动它，nc 就会立即关闭建立连接后，脚本总是发出警报。
我正在 运行宁 ubuntu 18.04LTS
我做错了什么？
编辑：
我在过去几天做了一些挖掘，为自己创建了一个传感器到 tcp 网关的模拟器，并在意识到我实际上不需要整个读取行逻辑后简化了我的脚本：
watchdog_tmp.sh:
#!/bin/bash
set -u
retry_counter=0
reader=""
echo "Starting"
while [[ $retry_counter -le 5 ]]; do
reader="$(ncat -vvv -i 10 127.0.0.1 10001 2>&1 > /dev/null | tee >(cat >&2) )"
if [[ "$reader" =~ "Connected" ]]; then
echo "First failure after success resetting counter"
retry_counter=0
else
(( retry_counter++ ))
fi
echo "($retry_counter) ALARM!"
sleep 5
done
问题没有改变，如果我从 shell 的命令行 运行 我的 watch_dog（当传感器是 运行ning 时），我得到以下输出：
Starting
Ncat: Version 7.01 ( https://nmap.org/ncat )
NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 8 [127.0.0.1:10001]
Ncat: Connected to 127.0.0.1:10001.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #2)
libnsock nsock_read(): Read request from IOD #1 [127.0.0.1:10001] (timeout: -1ms) EID 18
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #2 [peer unspecified] EID 26
libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 36
libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 18 [127.0.0.1:10001] (2200 bytes)
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #1 [127.0.0.1:10001] EID 42
libnsock nsock_event_cancel(): Event #36 (type TIMER) cancelled
libnsock nevent_delete(): nevent_delete on event #36 (type TIMER)
libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 52
libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 42 [127.0.0.1:10001] (43 bytes): $KRPHT,41.8800000,9.9003000,17.0000000*57..
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #1 [127.0.0.1:10001] EID 58
libnsock nsock_event_cancel(): Event #52 (type TIMER) cancelled
libnsock nevent_delete(): nevent_delete on event #52 (type TIMER)
libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 68
libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 58 [127.0.0.1:10001] (57 bytes): 0.000000 M/SEC 9.876518 DBAR 0.0 C 0.0 MS/CM.
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #1 [127.0.0.1:10001] EID 74
libnsock nsock_event_cancel(): Event #68 (type TIMER) cancelled
libnsock nevent_delete(): nevent_delete on event #68 (type TIMER)
libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 84
(...)
libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 234 [127.0.0.1:10001] (43 bytes): $KRPHT,41.8800000,9.9003000,17.0000000*57..
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #1 [127.0.0.1:10001] EID 250
libnsock nsock_event_cancel(): Event #244 (type TIMER) cancelled
libnsock nevent_delete(): nevent_delete on event #244 (type TIMER)
libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 260
libnsock nsock_trace_handler_callback(): Callback: READ EOF for EID 250 [127.0.0.1:10001]
libnsock nsock_trace_handler_callback(): Callback: TIMER SUCCESS for EID 260
Ncat: Idle timeout expired (10000 ms).
First failure after success resetting counter
(0) ALARM!
Ncat: Version 7.01 ( https://nmap.org/ncat )
NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT ERROR [Connection refused (111)] for EID 8 [127.0.0.1:10001]
Ncat: Connection refused.
(1) ALARM!
Ncat: Version 7.01 ( https://nmap.org/ncat )
NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT ERROR [Connection refused (111)] for EID 8 [127.0.0.1:10001]
Ncat: Connection refused.
(2) ALARM!
Ncat: Version 7.01 ( https://nmap.org/ncat )
NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 8 [127.0.0.1:10001]
Ncat: Connected to 127.0.0.1:10001.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #2)
libnsock nsock_read(): Read request from IOD #1 [127.0.0.1:10001] (timeout: -1ms) EID 18
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #2 [peer unspecified] EID 26
libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 36
(...)
Ncat: Idle timeout expired (10000 ms).
First failure after success resetting counter
(0) ALARM!
Ncat: Version 7.01 ( https://nmap.org/ncat )
NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT ERROR [Connection refused (111)] for EID 8 [127.0.0.1:10001]
Ncat: Connection refused.
(1) ALARM!
Ncat: Version 7.01 ( https://nmap.org/ncat )
NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT ERROR [Connection refused (111)] for EID 8 [127.0.0.1:10001]
Ncat: Connection refused.
(2) ALARM!
因此，我使用以下文件切换回 systemd 服务：
[Unit]
Description=Watchdog for the Artemis Proxy Service
After=network.target
[Service]
ExecStart=/tmp/artemis_proxy/bin/watchdog_tmp.sh
User=kraken
Group=kraken
KillMode=process
Restart=on-failure
RestartSec=30s
[Install]
WantedBy=default.target
不行，连接打开后立即关闭。这是日志的输出：
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: Starting
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: Ncat: Version 7.01 ( https://nmap.org/ncat )
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 8 [127.0.0.1:10001]
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: Ncat: Connected to 127.0.0.1:10001.
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_iod_new2(): nsock_iod_new (IOD #2)
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_read(): Read request from IOD #1 [127.0.0.1:10001] (timeout: -1ms) EID 18
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_readbytes(): Read request for 0 bytes from IOD #2 [peer unspecified] EID 26
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 36
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: READ EOF for EID 26 [peer unspecified]
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 18 [127.0.0.1:10001] (600 bytes)
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_readbytes(): Read request for 0 bytes from IOD #1 [127.0.0.1:10001] EID 42
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_event_cancel(): Event #36 (type TIMER) cancelled
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nevent_delete(): nevent_delete on event #36 (type TIMER)
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 52
Jul 29 21:31:25 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: READ EOF for EID 42 [127.0.0.1:10001]
Jul 29 21:31:35 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: TIMER SUCCESS for EID 52
Jul 29 21:31:35 kraken-zbox1 watchdog_tmp.sh[31551]: Ncat: Idle timeout expired (10000 ms).
Jul 29 21:31:35 kraken-zbox1 watchdog_tmp.sh[31551]: First failure after success resetting counter
Jul 29 21:31:35 kraken-zbox1 watchdog_tmp.sh[31551]: (0) ALARM!
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: Ncat: Version 7.01 ( https://nmap.org/ncat )
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: NCAT DEBUG: Using system default trusted CA certificates and those in /etc/ssl/certs/ca-certificates.crt.
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:10001 (IOD #1) EID 8
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 8 [127.0.0.1:10001]
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: Ncat: Connected to 127.0.0.1:10001.
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_iod_new2(): nsock_iod_new (IOD #2)
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_read(): Read request from IOD #1 [127.0.0.1:10001] (timeout: -1ms) EID 18
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_readbytes(): Read request for 0 bytes from IOD #2 [peer unspecified] EID 26
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 36
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: READ EOF for EID 26 [peer unspecified]
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 18 [127.0.0.1:10001] (743 bytes)
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_readbytes(): Read request for 0 bytes from IOD #1 [127.0.0.1:10001] EID 42
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_event_cancel(): Event #36 (type TIMER) cancelled
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nevent_delete(): nevent_delete on event #36 (type TIMER)
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_timer_create(): Timer created - 10000ms from now. EID 52
Jul 29 21:31:40 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: READ EOF for EID 42 [127.0.0.1:10001]
Jul 29 21:31:50 kraken-zbox1 watchdog_tmp.sh[31551]: libnsock nsock_trace_handler_callback(): Callback: TIMER SUCCESS for EID 52
Jul 29 21:31:50 kraken-zbox1 watchdog_tmp.sh[31551]: Ncat: Idle timeout expired (10000 ms).
Jul 29 21:31:50 kraken-zbox1 watchdog_tmp.sh[31551]: First failure after success resetting counter
Jul 29 21:31:50 kraken-zbox1 watchdog_tmp.sh[31551]: (0) ALARM!
我还在想我做错了什么...
经过一番搜索和与其他人交谈后，我终于找到了我的问题：
ncat，当启动时期望通过网络连接接收 stdin 上的输入并将传入的数据发送到 stdout。当你从 shell 启动我的脚本时，stdin 存在并正确地通过管道传输到 ncat，每个人都很高兴。另一方面，systemd 不为其服务提供标准输入，因此没有标准输入会导致 ncat 终止，因为它无法发送任何内容。
netcat 命令的简单 --recv-only 解决了问题，因此正确的脚本是：
#!/bin/bash
set -u
retry_counter=0
reader=""
echo "Starting"
while [[ $retry_counter -le 5 ]]; do
# start bcat in reciever mode only!
reader="$(ncat -v --recv-only -i 10 127.0.0.1 10001 2>&1 > /dev/null | tee >(cat >&2) )"
if [[ "$reader" =~ "Connected" ]]; then
echo "First failure after success resetting counter"
retry_counter=0
else
(( retry_counter++ ))
fi
echo "($retry_counter) ALARM!"
sleep 5
done