Wait until TCP port opened using bash and nc

Recently, I wrote a shell script which interacts with a TCP server. Since the script took charge of launching the server, sometimes my script fails because the server did not open the socket yet.

To come up with this problem, I used to wait certain amount of time using sleep(1) like this:

# launch the server which automatically daemonizes itself.
start-server 

# one second is enough for my system.
sleep 1

client do something

It was fine for my system, until I realized this method would not work on slow machine. Worse, when my system in a heavy-load, my system also failed to run above script successfully.

I would solve this problem by raising the amount of seconds sleeping, but I do not want to wait more on my relatively fast system.

Then I realized that netcat (a.k.a. nc) provides such feature with the "-z" option.

To test whether www.cinsk.org (TCP port 80) is opened, I can launch

# In BSD-like (MacOS) system
$ nc -z www.cinsk.org 80
Connection to www.cinsk.org 80 port [tcp/http] succeeded!
$ _

# On my Linux system, no message but just return zero on success
$ nc -z www.cinsk.org 80
$ _

Then I found another problem. Suppose that the firewall drops all packets for that port. Since by default, netcat(nc) will wait permanently until the port is open, the script that uses netcat(nc) will not return.

Thanks to the rich interface of netcat, it provides another option "-w" to specify the amount of time it will wait. The problem is, MacOS nc (BSD) does not work with "-w" option if the firewall drops all packets. Installing GNU version will solve this problem. (by the command "brew install netcat")

Finally, I wrote wait4tcp.sh to lessen the burden for others.

# Wait until port 80 8080 5984 are opened
$ ./wait4tcp.sh HOSTNAME 80 8080 5984

# The same as above, except it will retry only 10 times per each port.
$ ./wait4tcp.sh -w 10 HOSTNAME 80 8080 5984

# Wait until port 80 is closed
$ ./wait4tcp.sh -c HOSTNAME 80

# The same as above, except it will retry permanently.
$ ./wait4tcp.sh -w -1 -c HOSTNAME 80

# With the bash brace expansion, wait for port range 6379..6383 are opened
$ ./wait4tcp.sh HOSTNAME {6379..6383}

By default, wait4tcp.sh will retry 100 times per each port. Retrying 100 times is done less than 1 second in my system. Using "-w -1" option will retry permanently.