The timeout command does not work when reading from /dev/tcp/$server/ssh, how can I make this timeout command work?

Question

I'm trying to write a bash function to test SSH connections :

$ echo $0
-bash
$ time timeout 20s cat < /dev/tcp/$server/ssh;test $? = 124 && echo "WARNING: Could not connect to $server on ssh."
-bash: connect: Connection timed out
-bash: /dev/tcp/X.Y.Z.T/ssh: Connection timed out

real    1m3.000s
user    0m0.000s
sys     0m0.000s
WARNING: Could not connect to X.Y.Z.T on ssh.

How can I make this timeout command work ?

@jhnc I had to leave the office in a hurry because of the heat wave we are currently having in France. Just added my question. — SebMa
– SebMa, Commented Jul 18, 2022 at 19:07
The reason it fails is that timeout is not buit in to bash. This means the redirection is set up before timeout even executes. — jhnc
– jhnc, Commented Jul 18, 2022 at 19:29
consider: set -vx; timeout 1 echo main 11< <(echo 1 >/dev/tty) 12< <(echo 2 >/dev/tty) </dev/tcp/$server/22 13< <(echo 3 >/dev/tty). Note that first and last echo don't happen because tcp redirect caused termination — jhnc
– jhnc, Commented Jul 18, 2022 at 19:35

ufopilot · Accepted Answer · 2022-07-18 19:50:51Z

1

$ server="google.com"; timeout=20; 

$ time timeout "$timeout" bash -c "</dev/tcp/${server}/22" || echo "WARNING: Could not connect to $server on ssh."

real    0m20.002s
user    0m0.003s
sys     0m0.000s
WARNING: Could not connect to google.com on ssh.

Using netcat (nc)

$ time nc -z -w "$timeout" "$server" 22 || echo "WARNING: Could not connect to $server on ssh."

real    0m20.059s
user    0m0.003s
sys     0m0.001s
WARNING: Could not connect to google.com on ssh.

edited Jul 18, 2022 at 19:50

answered Jul 18, 2022 at 19:36

ufopilot

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1 Comment

SebMa Over a year ago

Can you also explain in the beginning of your answer why my command didn't work ?

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