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How do I tell a script to wait for a process to start accepting requests on a port?

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33

9

I need a command that will wait for a process to start accepting requests on a specific port.

Is there something in linux that does that?

while (checkAlive -host localhost -port 13000 == false)

  do some waiting



...

shell networking process open-files

share|improve this question

edited Jan 9 '11 at 17:44

Gilles

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asked Dec 31 '10 at 15:53

WillWill

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add a comment
 | 

33

9

I need a command that will wait for a process to start accepting requests on a specific port.

Is there something in linux that does that?

while (checkAlive -host localhost -port 13000 == false)

  do some waiting



...

shell networking process open-files

share|improve this question

edited Jan 9 '11 at 17:44

Gilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

asked Dec 31 '10 at 15:53

WillWill

28811 gold badge33 silver badges66 bronze badges

add a comment
 | 

I need a command that will wait for a process to start accepting requests on a specific port.

Is there something in linux that does that?

while (checkAlive -host localhost -port 13000 == false)

  do some waiting



...

shell networking process open-files

share|improve this question

edited Jan 9 '11 at 17:44

Gilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

asked Dec 31 '10 at 15:53

WillWill

28811 gold badge33 silver badges66 bronze badges

while (checkAlive -host localhost -port 13000 == false)

  do some waiting



...

share|improve this question

edited Jan 9 '11 at 17:44

Gilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

asked Dec 31 '10 at 15:53

WillWill

28811 gold badge33 silver badges66 bronze badges

share|improve this question

edited Jan 9 '11 at 17:44

Gilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

asked Dec 31 '10 at 15:53

WillWill

28811 gold badge33 silver badges66 bronze badges

edited Jan 9 '11 at 17:44

Gilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

edited Jan 9 '11 at 17:44

Gilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

asked Dec 31 '10 at 15:53

WillWill

28811 gold badge33 silver badges66 bronze badges

asked Dec 31 '10 at 15:53

WillWill

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3 Answers
3

active

oldest

votes

52

The best test to see if a server is accepting connections is to actually try connecting. Use a regular client for whatever protocol your server speaks and try a no-op command.

If you want a lightweight TCP or UDP client you can drive simply from the shell, use netcat. How to program a conversation depends on the protocol; many protocols have the server close the connection on a certain input, and netcat will then exit.

while ! echo exit | nc localhost 13000; do sleep 10; done

You can also tell netcat to exit after establishing the connection. It returns 1 if there's no connection and 0 if there is so we negate its output. Depending on your version of netcat, it may support one or both of the following commands:

while ! nc -z localhost 13000 </dev/null; do sleep 10; done

while ! nc -q 1 localhost 13000 </dev/null; do sleep 10; done

An alternative approach is to wait for the server process to open a listening socket.

while netstat -lnt | awk '$4 ~ /:13000$/ {exit 1}'; do sleep 10; done

If you are on Mac OS, netstat uses a slightly different output format, so you would want the following intead:

while netstat -lnt | awk '$4 ~ /.13000$/ {exit 1}'; do sleep 10; done

Or you might want to target a specific process ID:

while ! lsof -n -Fn -p $pid | grep -q '^n.*:13000$'; do sleep 10; done

I can't think of any way to react to the process starting to listen to the socket (which would avoid a polling approach) short of using ptrace.

share|improve this answer

edited 45 mins ago

Dwight Guth

311 bronze badge

answered Dec 31 '10 at 16:23

GillesGilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I think netcat is the answer, so thank you. To clarify, what I'm trying to do is write a script as part of a load balancing procedure. I need to start a process, wait for it to accept requests on the port and then shutdown the original. If there are better ways of doing this, rather than writing my own script, I'm all ears.
  
  – Will
  Dec 31 '10 at 16:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Will: That's a very different question! I've written a different answer.
  
  – Gilles
  Dec 31 '10 at 17:37
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I like netcat solution too. I have a script using nc -w 2 </dev/null >/dev/null — if the connection takes more than 2 seconds, it times out and fails — which is handy for my usage.
  
  – ephemient
  Jan 9 '11 at 23:10
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  FYI, I can't get the 'while nc -q 1 localhost 13000 </dev/null; do sleep 10; done' one to work. It just returns immediately. The first one works fine though. Thanks!
  
  – Ellis Percival
  Mar 19 '15 at 16:04
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Flyte nc -q 1 localhost 13000 </dev/null returns immediately if no server is listening, but it returns with an error code, so the loop makes it sleep and try again a few seconds later.
  
  – Gilles
  Mar 19 '15 at 17:16
  
  
  

  
  
  
  

 | 
show 4 more comments

17

If you have bash and coreutils (e.g. timeout, sleep), but not nc/lsof/netstat, you can use this solution which uses bash magic tcp sockets:

while ! timeout 1 bash -c "echo > /dev/tcp/localhost/13000"; do sleep 10; done

share|improve this answer

answered Aug 7 '14 at 17:31

kanakakanaka

27122 silver badges44 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  To elaborate, Bash has the optional feature of connecting to TCP sockets using "network redirections" -- gnu.org/software/bash/manual/html_node/…
  
  – johntellsall
  Dec 23 '14 at 17:25
  

  
  
  
  

add a comment
 | 

7

Following the previous example with bash tcp sockets magic, here is an enhanced version which waits for connection during limited amount of time.

timeout 15 bash -c 'until echo > /dev/tcp/localhost/13000; do sleep 0.5; done'

The difference is that if connection wasn't available during 15s, - it won't loop forever but exit with the error code.

This is useful in init scripts to wait for service readiness/availability after startup.

share|improve this answer

edited Apr 13 '17 at 12:37

Community♦

1

answered Mar 4 '17 at 15:10

armaarma

17111 silver badge33 bronze badges

add a comment
 | 

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52

The best test to see if a server is accepting connections is to actually try connecting. Use a regular client for whatever protocol your server speaks and try a no-op command.

If you want a lightweight TCP or UDP client you can drive simply from the shell, use netcat. How to program a conversation depends on the protocol; many protocols have the server close the connection on a certain input, and netcat will then exit.

while ! echo exit | nc localhost 13000; do sleep 10; done

You can also tell netcat to exit after establishing the connection. It returns 1 if there's no connection and 0 if there is so we negate its output. Depending on your version of netcat, it may support one or both of the following commands:

while ! nc -z localhost 13000 </dev/null; do sleep 10; done

while ! nc -q 1 localhost 13000 </dev/null; do sleep 10; done

An alternative approach is to wait for the server process to open a listening socket.

while netstat -lnt | awk '$4 ~ /:13000$/ {exit 1}'; do sleep 10; done

If you are on Mac OS, netstat uses a slightly different output format, so you would want the following intead:

while netstat -lnt | awk '$4 ~ /.13000$/ {exit 1}'; do sleep 10; done

Or you might want to target a specific process ID:

while ! lsof -n -Fn -p $pid | grep -q '^n.*:13000$'; do sleep 10; done

I can't think of any way to react to the process starting to listen to the socket (which would avoid a polling approach) short of using ptrace.

share|improve this answer

edited 45 mins ago

Dwight Guth

311 bronze badge

answered Dec 31 '10 at 16:23

GillesGilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I think netcat is the answer, so thank you. To clarify, what I'm trying to do is write a script as part of a load balancing procedure. I need to start a process, wait for it to accept requests on the port and then shutdown the original. If there are better ways of doing this, rather than writing my own script, I'm all ears.
  
  – Will
  Dec 31 '10 at 16:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Will: That's a very different question! I've written a different answer.
  
  – Gilles
  Dec 31 '10 at 17:37
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I like netcat solution too. I have a script using nc -w 2 </dev/null >/dev/null — if the connection takes more than 2 seconds, it times out and fails — which is handy for my usage.
  
  – ephemient
  Jan 9 '11 at 23:10
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  FYI, I can't get the 'while nc -q 1 localhost 13000 </dev/null; do sleep 10; done' one to work. It just returns immediately. The first one works fine though. Thanks!
  
  – Ellis Percival
  Mar 19 '15 at 16:04
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Flyte nc -q 1 localhost 13000 </dev/null returns immediately if no server is listening, but it returns with an error code, so the loop makes it sleep and try again a few seconds later.
  
  – Gilles
  Mar 19 '15 at 17:16
  
  
  

  
  
  
  

 | 
show 4 more comments

52

The best test to see if a server is accepting connections is to actually try connecting. Use a regular client for whatever protocol your server speaks and try a no-op command.

If you want a lightweight TCP or UDP client you can drive simply from the shell, use netcat. How to program a conversation depends on the protocol; many protocols have the server close the connection on a certain input, and netcat will then exit.

while ! echo exit | nc localhost 13000; do sleep 10; done

You can also tell netcat to exit after establishing the connection. It returns 1 if there's no connection and 0 if there is so we negate its output. Depending on your version of netcat, it may support one or both of the following commands:

while ! nc -z localhost 13000 </dev/null; do sleep 10; done

while ! nc -q 1 localhost 13000 </dev/null; do sleep 10; done

An alternative approach is to wait for the server process to open a listening socket.

while netstat -lnt | awk '$4 ~ /:13000$/ {exit 1}'; do sleep 10; done

If you are on Mac OS, netstat uses a slightly different output format, so you would want the following intead:

while netstat -lnt | awk '$4 ~ /.13000$/ {exit 1}'; do sleep 10; done

Or you might want to target a specific process ID:

while ! lsof -n -Fn -p $pid | grep -q '^n.*:13000$'; do sleep 10; done

I can't think of any way to react to the process starting to listen to the socket (which would avoid a polling approach) short of using ptrace.

share|improve this answer

edited 45 mins ago

Dwight Guth

311 bronze badge

answered Dec 31 '10 at 16:23

GillesGilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I think netcat is the answer, so thank you. To clarify, what I'm trying to do is write a script as part of a load balancing procedure. I need to start a process, wait for it to accept requests on the port and then shutdown the original. If there are better ways of doing this, rather than writing my own script, I'm all ears.
  
  – Will
  Dec 31 '10 at 16:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Will: That's a very different question! I've written a different answer.
  
  – Gilles
  Dec 31 '10 at 17:37
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I like netcat solution too. I have a script using nc -w 2 </dev/null >/dev/null — if the connection takes more than 2 seconds, it times out and fails — which is handy for my usage.
  
  – ephemient
  Jan 9 '11 at 23:10
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  FYI, I can't get the 'while nc -q 1 localhost 13000 </dev/null; do sleep 10; done' one to work. It just returns immediately. The first one works fine though. Thanks!
  
  – Ellis Percival
  Mar 19 '15 at 16:04
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Flyte nc -q 1 localhost 13000 </dev/null returns immediately if no server is listening, but it returns with an error code, so the loop makes it sleep and try again a few seconds later.
  
  – Gilles
  Mar 19 '15 at 17:16
  
  
  

  
  
  
  

 | 
show 4 more comments

The best test to see if a server is accepting connections is to actually try connecting. Use a regular client for whatever protocol your server speaks and try a no-op command.

If you want a lightweight TCP or UDP client you can drive simply from the shell, use netcat. How to program a conversation depends on the protocol; many protocols have the server close the connection on a certain input, and netcat will then exit.

while ! echo exit | nc localhost 13000; do sleep 10; done

You can also tell netcat to exit after establishing the connection. It returns 1 if there's no connection and 0 if there is so we negate its output. Depending on your version of netcat, it may support one or both of the following commands:

while ! nc -z localhost 13000 </dev/null; do sleep 10; done

while ! nc -q 1 localhost 13000 </dev/null; do sleep 10; done

An alternative approach is to wait for the server process to open a listening socket.

while netstat -lnt | awk '$4 ~ /:13000$/ {exit 1}'; do sleep 10; done

If you are on Mac OS, netstat uses a slightly different output format, so you would want the following intead:

while netstat -lnt | awk '$4 ~ /.13000$/ {exit 1}'; do sleep 10; done

Or you might want to target a specific process ID:

while ! lsof -n -Fn -p $pid | grep -q '^n.*:13000$'; do sleep 10; done

I can't think of any way to react to the process starting to listen to the socket (which would avoid a polling approach) short of using ptrace.

share|improve this answer

edited 45 mins ago

Dwight Guth

311 bronze badge

answered Dec 31 '10 at 16:23

GillesGilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

while ! echo exit | nc localhost 13000; do sleep 10; done

while ! nc -z localhost 13000 </dev/null; do sleep 10; done

while ! nc -q 1 localhost 13000 </dev/null; do sleep 10; done

while netstat -lnt | awk '$4 ~ /:13000$/ {exit 1}'; do sleep 10; done

while netstat -lnt | awk '$4 ~ /.13000$/ {exit 1}'; do sleep 10; done

while ! lsof -n -Fn -p $pid | grep -q '^n.*:13000$'; do sleep 10; done

share|improve this answer

edited 45 mins ago

Dwight Guth

311 bronze badge

answered Dec 31 '10 at 16:23

GillesGilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

edited 45 mins ago

Dwight Guth

311 bronze badge

edited 45 mins ago

Dwight Guth

311 bronze badge

answered Dec 31 '10 at 16:23

GillesGilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

answered Dec 31 '10 at 16:23

GillesGilles

575k140140 gold badges11881188 silver badges17001700 bronze badges

17

If you have bash and coreutils (e.g. timeout, sleep), but not nc/lsof/netstat, you can use this solution which uses bash magic tcp sockets:

while ! timeout 1 bash -c "echo > /dev/tcp/localhost/13000"; do sleep 10; done

share|improve this answer

answered Aug 7 '14 at 17:31

kanakakanaka

27122 silver badges44 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  To elaborate, Bash has the optional feature of connecting to TCP sockets using "network redirections" -- gnu.org/software/bash/manual/html_node/…
  
  – johntellsall
  Dec 23 '14 at 17:25
  

  
  
  
  

add a comment
 | 

17

If you have bash and coreutils (e.g. timeout, sleep), but not nc/lsof/netstat, you can use this solution which uses bash magic tcp sockets:

while ! timeout 1 bash -c "echo > /dev/tcp/localhost/13000"; do sleep 10; done

share|improve this answer

answered Aug 7 '14 at 17:31

kanakakanaka

27122 silver badges44 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  To elaborate, Bash has the optional feature of connecting to TCP sockets using "network redirections" -- gnu.org/software/bash/manual/html_node/…
  
  – johntellsall
  Dec 23 '14 at 17:25
  

  
  
  
  

add a comment
 | 

If you have bash and coreutils (e.g. timeout, sleep), but not nc/lsof/netstat, you can use this solution which uses bash magic tcp sockets:

while ! timeout 1 bash -c "echo > /dev/tcp/localhost/13000"; do sleep 10; done

share|improve this answer

answered Aug 7 '14 at 17:31

kanakakanaka

27122 silver badges44 bronze badges

while ! timeout 1 bash -c "echo > /dev/tcp/localhost/13000"; do sleep 10; done

share|improve this answer

answered Aug 7 '14 at 17:31

kanakakanaka

27122 silver badges44 bronze badges

answered Aug 7 '14 at 17:31

kanakakanaka

27122 silver badges44 bronze badges

answered Aug 7 '14 at 17:31

kanakakanaka

27122 silver badges44 bronze badges

7

Following the previous example with bash tcp sockets magic, here is an enhanced version which waits for connection during limited amount of time.

timeout 15 bash -c 'until echo > /dev/tcp/localhost/13000; do sleep 0.5; done'

The difference is that if connection wasn't available during 15s, - it won't loop forever but exit with the error code.

This is useful in init scripts to wait for service readiness/availability after startup.

share|improve this answer

edited Apr 13 '17 at 12:37

Community♦

1

answered Mar 4 '17 at 15:10

armaarma

17111 silver badge33 bronze badges

add a comment
 | 

7

Following the previous example with bash tcp sockets magic, here is an enhanced version which waits for connection during limited amount of time.

timeout 15 bash -c 'until echo > /dev/tcp/localhost/13000; do sleep 0.5; done'

The difference is that if connection wasn't available during 15s, - it won't loop forever but exit with the error code.

This is useful in init scripts to wait for service readiness/availability after startup.

share|improve this answer

edited Apr 13 '17 at 12:37

Community♦

1

answered Mar 4 '17 at 15:10

armaarma

17111 silver badge33 bronze badges

add a comment
 | 

Following the previous example with bash tcp sockets magic, here is an enhanced version which waits for connection during limited amount of time.

timeout 15 bash -c 'until echo > /dev/tcp/localhost/13000; do sleep 0.5; done'

The difference is that if connection wasn't available during 15s, - it won't loop forever but exit with the error code.

This is useful in init scripts to wait for service readiness/availability after startup.

share|improve this answer

edited Apr 13 '17 at 12:37

Community♦

1

answered Mar 4 '17 at 15:10

armaarma

17111 silver badge33 bronze badges

timeout 15 bash -c 'until echo > /dev/tcp/localhost/13000; do sleep 0.5; done'

share|improve this answer

edited Apr 13 '17 at 12:37

Community♦

1

answered Mar 4 '17 at 15:10

armaarma

17111 silver badge33 bronze badges

edited Apr 13 '17 at 12:37

Community♦

1

edited Apr 13 '17 at 12:37

Community♦

1

answered Mar 4 '17 at 15:10

armaarma

17111 silver badge33 bronze badges

answered Mar 4 '17 at 15:10

armaarma

17111 silver badge33 bronze badges