Hi,

I'm using Ubuntu Linux 11.04. Is there a way to find an unused port, starting from 4444 and gradually going upwards? I'm grateful for brevity on this one, since I'm using this port value in a Perl script, so I would like to call ...

my $openPort=`my_port_finding_command`;

Thanks, - Dave

I would guess the best way would be to use awk to parse netstat output.

Here is a quick pseudocode assume it is in php:
1.
$output = `netstat -l`; //execute netstat
2.
(filter lines containing only tcp/udp)
3.
then read this output line-by-line
4. per each line split or explode with the delimiter may be space/tab whatever.
5. parse the Nth item which is port and check it falls within the range.


Tip: you can use also a combination of sed, cut,echo statements with the first netstat -l via which you can greatly filter-out already what really you need. This reduces the parsing overheads.

For example netstat with sed: (multiple spaces are quickly converted to single spaces)
[root@koi ~]# netstat -l | sed 's/ / /g' | sed 's/ / /g' | sed 's/ / /g'| sed 's/ / /g'
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 *:mysql *:* LISTEN
tcp 0 0 *:sunrpc *:* LISTEN
tcp 0 0 *:35604 *:* LISTEN
tcp 0 0 192.168.122.1:domain *:* LISTEN
tcp 0 0 *:ssh *:* LISTEN
tcp 0 0 koi:smtp *:* LISTEN
tcp 0 0 *:55011 *:* LISTEN
tcp 0 0 *:sunrpc *:* LISTEN
tcp 0 0 *:http *:* LISTEN
tcp 0 0 *:8181 *:* LISTEN
tcp 0 0 *:ssh *:* LISTEN
tcp 0 0 *:https *:* LISTEN
udp 0 0 *:34292 *:*
udp 0 0 192.168.122.1:domain *:*
udp 0 0 *:bootps *:*
udp 0 0 *:sunrpc *:*
udp 0 0 *:ipp *:*
udp 0 0 115.184.76.140:ntp *:*
udp 0 0 192.168.122.1:ntp *:*
udp 0 0 koi:ntp *:*
udp 0 0 *:ntp *:*


Now with Sed and cut: (capture only the port numbers)
[root@koi ~]# netstat -l | sed 's/ / /g' | sed 's/ / /g' | sed 's/ / /g'| sed 's/ / /g' | cut -d ":" -f 2 | cut -d " " -f 1
Active
Proto
mysql
sunrpc
35604
domain
ssh
smtp
55011
sunrpc
http
8181
ssh
https
34292
domain
bootps
sunrpc
ipp
ntp
ntp
ntp
ntp
mdns
ns
783
52025
864
sunrpc
44667
ntp
ntp