Efficiently test if a port is open on Linux?

Question

A surprise I found out recently is that Bash natively supports tcp connections as file descriptors. To use:

exec 6<>/dev/tcp/ip.addr.of.server/445
echo -e "GET / HTTP/1.0\n" >&6
cat <&6

I'm using 6 as the file descriptor because 0,1,2 are stdin, stdout, and stderr. 5 is sometimes used by Bash for child processes, so 3,4,6,7,8, and 9 should be safe.

As per the comment below, to test for listening on a local server in a script:

exec 6<>/dev/tcp/127.0.0.1/445 || echo "No one is listening!"
exec 6>&- # close output connection
exec 6<&- # close input connection

To determine if someone is listening, attempt to connect by loopback. If it fails, then the port is closed or we aren't allowed access. Afterwards, close the connection.

Modify this for your use case, such as sending an email, exiting the script on failure, or starting the required service.

There's a very short with "fast answer" here : How to test if remote TCP port is opened from Shell script?

nc -z <host> <port>; echo $?

I use it with 127.0.0.1 as "remote" address.

this returns "0" if the port is open and "1" if the port is closed

e.g.

nc -z 127.0.0.1 80; echo $?

-z Specifies that nc should just scan for listening daemons, without sending any data to them. It is an error to use this option in conjunc- tion with the -l option.

You can use netstat this way for much faster results:

On Linux:

netstat -lnt | awk '$6 == "LISTEN" && $4 ~ /\.445$/'

On Mac:

netstat -anp tcp | awk '$6 == "LISTEN" && $4 ~ /\.445$/'

This will output a list of processes listening on the port (445 in this example) or it will output nothing if the port is free.

You can use netcat for this.

nc ip port < /dev/null

connects to the server and directly closes the connection again. If netcat is not able to connect, it returns a non-zero exit code. The exit code is stored in the variable $?. As an example,

nc ip port < /dev/null; echo $?

will return 0 if and only if netcat could successfully connect to the port.

Based on Spencer Rathbun's answer, using bash:

true &>/dev/null </dev/tcp/127.0.0.1/$PORT && echo open || echo closed

they're listed in /proc/net/tcp.

it's the second column, after the ":", in hex:

> cat /proc/net/tcp
  sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode                                                     
   0: 00000000:0016 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 10863 1 ffff88020c785400 99 0 0 10 -1                     
   1: 0100007F:0277 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 7983 1 ffff88020eb7b3c0 99 0 0 10 -1                      
   2: 0500010A:948F 0900010A:2328 01 00000000:00000000 02:00000576 00000000  1000        0 10562454 2 ffff88010040f7c0 22 3 30 5 3                   
   3: 0500010A:E077 5F2F7D4A:0050 01 00000000:00000000 02:00000176 00000000  1000        0 10701021 2 ffff880100474080 41 3 22 10 -1                 
   4: 0500010A:8773 16EC97D1:0050 01 00000000:00000000 02:00000BDC 00000000  1000        0 10700849 2 ffff880104335440 57 3 18 10 -1                 
   5: 0500010A:8772 16EC97D1:0050 01 00000000:00000000 02:00000BF5 00000000  1000        0 10698952 2 ffff88010040e440 46 3 0 10 -1                  
   6: 0500010A:DD2C 0900010A:0016 01 00000000:00000000 02:0006E764 00000000  1000        0 9562907 2 ffff880104334740 22 3 30 5 4                    
   7: 0500010A:AAA4 6A717D4A:0050 08 00000000:00000001 02:00000929 00000000  1000        0 10696677 2 ffff880106cc77c0 45 3 0 10 -1  

so i guess one of those :50 in the third column must be stackoverflow :o)

look in man 5 proc for more details. and picking that apart with sed etc is left as an exercise for the gentle reader...