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find an unused unprivileged TCP port

Terminal - find an unused unprivileged TCP port
netstat -atn | perl -ane 'if ( $F[3] =~ /(\d+)$/ ) { $x{$1}=1 } END{ print( (grep {!$x{$_}} 32768..61000)[0] . "\n" )}'
2010-12-15 09:25:52
User: depesz
Functions: grep netstat perl
0
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Down
find an unused unprivileged TCP port

perl-based version

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Alternatives

There are 4 alternatives - vote for the best!

Terminal - Alternatives
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}}}'
2009-03-27 20:38:43
User: mpb
Functions: awk netstat sed sort
4
Up
Down
find an unused unprivileged TCP port

Some commands (such as netcat) have a port option but how can you know which ports are unused?

Show sample output | Comments (3) | Add to favourites | Report as malicious
(netstat -atn | awk '{printf "%s\n%s\n", $4, $4}' | grep -oE '[0-9]*$'; seq 32768 61000) | sort -n | uniq -u | head -n 1
2010-12-15 09:27:59
User: depesz
Functions: awk grep head netstat seq sort uniq
2
Up
Down
find an unused unprivileged TCP port

perl-less way

Comments (0) | Add to favourites | Report as malicious
netstat -atn | perl -0777 -ne '@ports = /tcp.*?\:(\d+)\s+/imsg ; for $port (32768..61000) {if(!grep(/^$port$/, @ports)) { print $port; last } }'
2010-12-14 20:44:37
User: bashrc
Functions: last netstat perl
1
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Down
find an unused unprivileged TCP port

Not really better - just different ;)

There's probably a really simple solution out there somewhere...

Show sample output | Comments (0) | Add to favourites | Report as malicious
netstat -tan | awk '$1 == "tcp" && $4 ~ /:/ { port=$4; sub(/^[^:]+:/, "", port); used[int(port)] = 1; } END { for (p = 32768; p <= 61000; ++p) if (! (p in used)) { print p; exit(0); }; exit(1); }'
2012-01-26 20:42:56
User: wirawan0
Functions: awk netstat
0
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Down
find an unused unprivileged TCP port

This is also perl-less, and only uses AWK as its postprocessor. Tested with GAWK and MAWK.

Comments (0) | Add to favourites | Report as malicious

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