find an unused unprivileged TCP port

netstat -atn | awk ' /tcp/ {printf("%s\n",substr($4,index($4,":")+1,length($4) )) }' | sed -e "s/://g" | sort -rnu | awk '{array [$1] = $1} END {i=32768; again=1; while (again == 1) {if (array[i] == i) {i=i+1} else {print i; again=0}}}'
Some commands (such as netcat) have a port option but how can you know which ports are unused?
Sample Output
$ netstat -atn | awk ' /tcp/ {printf("%s\n",substr($4,index($4,":")+1,length($4) )) }' | sed -e "s/://g" | sort -rnu | awk '{array [$1] = $1} END {i=32768; again=1; while (again == 1) {if (array[i] == i) {i=i+1} else {print i; again=0}}}'
32768

4
By: mpb
2009-03-27 20:38:43

5 Alternatives + Submit Alt

What Others Think

Not exactly easy to remember, is it?
atoponce · 478 weeks and 1 day ago
@atponce I am hoping that someone smarter than me will suggest a better way. In any case, it's easy to copy&paste or make a shell function with (perhaps: "freeport()"?).
mpb · 478 weeks and 1 day ago
Thanx - was looking for something like that!
bashrc · 388 weeks and 4 days ago

What do you think?

Any thoughts on this command? Does it work on your machine? Can you do the same thing with only 14 characters?

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