Need help understanding the command

Question

I found this command at Quick way to find if a port is open on Linux.

exec 6<>/dev/tcp/127.0.0.1/445 || echo "No one is listening!"

I am a newbie to shell scripting. Could anyone please explain me in detail the syntax of this command? Especially exec 6<>/dev/tcp/127.0.0.1/445. Also is exec going to return a bool value or something (since || is being used?)

Thanks a lot.

i didnot understand the use of <>. Also can we use exec 6<>/dev/tcp/127.0.0.1/445 in an if statement somehow? — aniztar, Nov 28 '14 at 08:55
http://www.tldp.org/LDP/abs/html/io-redirection.html and yes the `||` is effectively an `else` as it short circuits if the first command is successful. — , Nov 28 '14 at 08:56
You mean we can write something like if [exec 6<>/dev/tcp/127.0.0.1/445] ? — aniztar, Nov 28 '14 at 09:02
rather: `if exec 6<>/dev/tcp/127.0.0.1/445; then echo port open; fi` — sjas, Oct 15 '18 at 04:06

score 2 · Accepted Answer · edited May 23 '17 at 10:26

2

<>

does I/O redirection for both reading and writing. See I/O Redirection

The 6 means we assign it to file descriptor six. What are file descriptors?

|| is in bash executed when the lefthand-side command failed. In other words, this in an else

edited May 23 '17 at 10:26

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answered Nov 28 '14 at 09:03

timbru31

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Need help understanding the command

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