如何從“>/dev/tcp/ip/port”中刪除“連接被拒絕”行

打開埠

user@linux:~$ >/dev/tcp/127.0.0.1/8080 && echo open || echo closed
open
user@linux:~$ 

封閉港口

user@linux:~$ >/dev/tcp/127.0.0.1/8088 && echo open || echo closed
bash: connect: Connection refused
bash: /dev/tcp/127.0.0.1/8088: Connection refused
closed
user@linux:~$ 

將 stderr 重定向到 /dev/null 不起作用

user@linux:~$ >/dev/tcp/127.0.0.1/8088 2>/dev/nul
bash: connect: Connection refused
bash: /dev/tcp/127.0.0.1/8088: Connection refused
user@linux:~$ 

期望的輸出

user@linux:~$ >/dev/tcp/127.0.0.1/8088 && echo open || echo closed
closed
user@linux:~$ 

顛倒重定向的順序，讓我們bash知道 stderr 在 stdin 之前應該是什麼

$ >/dev/tcp/127.0.0.1/8088 2>/dev/nul
bash: connect: Connection refused
bash: /dev/tcp/127.0.0.1/8088: Connection refused
$ 2>/dev/null >/dev/tcp/127.0.0.1/8088 && echo open || echo closed
closed
$ 

---

第一個版本失敗的原因是重定向是按照它們出現的順序處理的，所以在>/dev/tcp/127.0.0.1/8088 2>/dev/null重定向到埠時，stderr 流還沒有被重定向。您可以使用實用程序在診斷輸出中清楚地看到相應的系統呼叫dup2()以重定向 stderr（或在命令失敗的情況下缺少）strace

$ strace -e dup2,connect  -f bash -c '>/dev/tcp/127.0.0.1/8088 2>/dev/null'
connect(3, {sa_family=AF_INET, sin_port=htons(8088), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 ECONNREFUSED (Connection refused)
bash: connect: Connection refused
bash: /dev/tcp/127.0.0.1/8088: Connection refused
+++ exited with 1 +++
$ strace -e dup2,connect  -f bash -c '2>/dev/null >/dev/tcp/127.0.0.1/8088 '
dup2(3, 2)                              = 2
connect(3, {sa_family=AF_INET, sin_port=htons(8088), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 ECONNREFUSED (Connection refused)
dup2(10, 2)                             = 2
+++ exited with 1 +++
$ 

引用自：https://unix.stackexchange.com/questions/588446