Shell command/script to see if a host is alive?

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I'm trying to figure out more ways to see if a given host is up, solely using shell commands (primarily bash). Ideally, it would be able to work with both hostnames and IP addresses. Right now the only native way I know of is ping, perhaps integrated into a script as described here. Any other ideas?










share|improve this question



























    up vote
    9
    down vote

    favorite
    2












    I'm trying to figure out more ways to see if a given host is up, solely using shell commands (primarily bash). Ideally, it would be able to work with both hostnames and IP addresses. Right now the only native way I know of is ping, perhaps integrated into a script as described here. Any other ideas?










    share|improve this question

























      up vote
      9
      down vote

      favorite
      2









      up vote
      9
      down vote

      favorite
      2






      2





      I'm trying to figure out more ways to see if a given host is up, solely using shell commands (primarily bash). Ideally, it would be able to work with both hostnames and IP addresses. Right now the only native way I know of is ping, perhaps integrated into a script as described here. Any other ideas?










      share|improve this question















      I'm trying to figure out more ways to see if a given host is up, solely using shell commands (primarily bash). Ideally, it would be able to work with both hostnames and IP addresses. Right now the only native way I know of is ping, perhaps integrated into a script as described here. Any other ideas?







      shell-script hostname ping






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited Mar 14 '15 at 16:02









      Gilles

      516k12210291557




      516k12210291557










      asked Mar 14 '15 at 15:52









      user67459

      202127




      202127




















          2 Answers
          2






          active

          oldest

          votes

















          up vote
          6
          down vote



          accepted










          ping is the way to test whether a host is alive and connected. (If a host is alive but disconnected or slow to respond, you can't distinguish that from its being dead.)



          Options supported by the ping command vary from system to system. You'll want to ensure that it doesn't loop forever but returns after a few seconds if it didn't receive a reply.



          With FreeBSD and Linux iputils, ping -c 1 -W 1 >/dev/null sends a single ping and wait 1 second. You don't need to parse the output: the command returns 0 if it received a ping back and nonzero otherwise (unknown host name, no route to host, no reply). Some implementations may need different flags (e.g. -w instead of -W on FreeBSD), check the manual on your system.



          if ping -c 1 -W 1 "$hostname_or_ip_address"; then
          echo "$hostname_or_ip_address is alive"
          else
          echo "$hostname_or_ip_address is pining for the fjords"
          fi





          share|improve this answer




















          • I know it's the way, I was just hoping it wasn't the only way, even if the other ways are strange or fickle or what have you. Oh well!
            – user67459
            Mar 18 '15 at 19:25






          • 1




            This answer should NOT have been marked as the answered answer. The OP specifically asked for more ways to see if a given host is up, other than with ping, which this answer does not supply.
            – Yokai
            Jan 21 '17 at 9:21











          • From 'ping' man page: "Because of the load it can impose on the network, it is unwise to use ping during normal operations or from automated scripts."
            – 1111161171159459134
            18 hours ago










          • @1111161171159459134 That paragraph could have been worded better. It's overly alarmist. You shouldn't do flood pings or pings with a very high rate, but one ping packet now and then is negligible.
            – Gilles
            16 hours ago

















          up vote
          0
          down vote













          Ping is great to get a quick response about whether the host is connected to the network, but it often won't tell you whether the host is alive or not, or whether it's still operating as expected. This is because ping responses are usually handled by the kernel, so even if every application on the system has crashed (e.g. due to a disk failure or running out of memory), you'll often still get ping responses and may assume the machine is operating normally when the situation is quite the opposite.



          Checking services



          Usually you don't really care whether a host is still online or not, what you really care about is whether the machine is still performing some task. So if you can check the task directly then you'll know the host is both up and that the task is still running.



          For a remote host that runs a web server for example, you can do something like this:



          # Add the -f option to curl if server errors like HTTP 404 should fail too
          if curl -I "http://$TARGET"; then
          echo "$TARGET alive and web site is up"
          else
          echo "$TARGET offline or web server problem"
          fi


          If it runs SSH and you have keys set up for passwordless login, then you have a few more options, for example:



          if ssh "$TARGET" true; then
          echo "$TARGET alive and accessible via SSH"
          else
          echo "$TARGET offline or not accepting SSH logins"
          fi


          This works by SSH'ing into the host and running the true command and then closing the connection. The ssh command will only return success if that command could be run successfully.



          Remote tests via SSH



          You can extend this to check for specific processes, such as ensuring that mysqld is running on the machine:



          if ssh "$TARGET" bash -c 'ps aux | grep -q mysqld'; then
          echo "$TARGET alive and running MySQL"
          else
          echo "$TARGET offline or MySQL crashed"
          fi


          Of course in this case you'd be better off running something like monit on the target to ensure the service is kept running, but it's useful in scripts where you only want to perform some task on machine A as long as machine B is ready for it.



          This could be something like checking that the target machine has a certain filesystem mounted before performing an rsync to it, so that you don't accidentally fill up its main disk if a secondary filesystem didn't mount for some reason. For example this will make sure that /mnt/raid is mounted on the target machine before continuing.



          if ssh "$TARGET" bash -c 'mount | grep -q /mnt/raid'; then
          echo "$TARGET alive and filesystem ready to receive data"
          else
          echo "$TARGET offline or filesystem not mounted"
          fi


          Services with no client



          Sometimes there is no easy way to connect to the service and you just want to see whether it accepts incoming TCP connections, but when you telnet to the target on the port in question it just sits there and doesn't disconnect you, which means doing that in a script would cause it to hang.



          While not quite so clean, you can still do this with the help of the timeout and netcat programs. For example this checks to see whether the machine accepts SMB/CIFS connections on TCP port 445, so you can see whether it is running Windows file sharing even if you don't have a password to log in, or the CIFS client tools aren't installed:



          # Wait 1 second to connect (-w 1) and if the total time (DNS lookups + connect
          # time) reaches 5 seconds, assume the connection was successful and the remote
          # host is waiting for us to send data. Connecting on TCP port 445.
          if echo 'x' | timeout --preserve-status 5 nc -w 1 "$TARGET" 445; then
          echo "$TARGET alive and CIFS service available"
          else
          echo "$TARGET offline or CIFS unavailable"
          fi




          share




















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            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes








            up vote
            6
            down vote



            accepted










            ping is the way to test whether a host is alive and connected. (If a host is alive but disconnected or slow to respond, you can't distinguish that from its being dead.)



            Options supported by the ping command vary from system to system. You'll want to ensure that it doesn't loop forever but returns after a few seconds if it didn't receive a reply.



            With FreeBSD and Linux iputils, ping -c 1 -W 1 >/dev/null sends a single ping and wait 1 second. You don't need to parse the output: the command returns 0 if it received a ping back and nonzero otherwise (unknown host name, no route to host, no reply). Some implementations may need different flags (e.g. -w instead of -W on FreeBSD), check the manual on your system.



            if ping -c 1 -W 1 "$hostname_or_ip_address"; then
            echo "$hostname_or_ip_address is alive"
            else
            echo "$hostname_or_ip_address is pining for the fjords"
            fi





            share|improve this answer




















            • I know it's the way, I was just hoping it wasn't the only way, even if the other ways are strange or fickle or what have you. Oh well!
              – user67459
              Mar 18 '15 at 19:25






            • 1




              This answer should NOT have been marked as the answered answer. The OP specifically asked for more ways to see if a given host is up, other than with ping, which this answer does not supply.
              – Yokai
              Jan 21 '17 at 9:21











            • From 'ping' man page: "Because of the load it can impose on the network, it is unwise to use ping during normal operations or from automated scripts."
              – 1111161171159459134
              18 hours ago










            • @1111161171159459134 That paragraph could have been worded better. It's overly alarmist. You shouldn't do flood pings or pings with a very high rate, but one ping packet now and then is negligible.
              – Gilles
              16 hours ago














            up vote
            6
            down vote



            accepted










            ping is the way to test whether a host is alive and connected. (If a host is alive but disconnected or slow to respond, you can't distinguish that from its being dead.)



            Options supported by the ping command vary from system to system. You'll want to ensure that it doesn't loop forever but returns after a few seconds if it didn't receive a reply.



            With FreeBSD and Linux iputils, ping -c 1 -W 1 >/dev/null sends a single ping and wait 1 second. You don't need to parse the output: the command returns 0 if it received a ping back and nonzero otherwise (unknown host name, no route to host, no reply). Some implementations may need different flags (e.g. -w instead of -W on FreeBSD), check the manual on your system.



            if ping -c 1 -W 1 "$hostname_or_ip_address"; then
            echo "$hostname_or_ip_address is alive"
            else
            echo "$hostname_or_ip_address is pining for the fjords"
            fi





            share|improve this answer




















            • I know it's the way, I was just hoping it wasn't the only way, even if the other ways are strange or fickle or what have you. Oh well!
              – user67459
              Mar 18 '15 at 19:25






            • 1




              This answer should NOT have been marked as the answered answer. The OP specifically asked for more ways to see if a given host is up, other than with ping, which this answer does not supply.
              – Yokai
              Jan 21 '17 at 9:21











            • From 'ping' man page: "Because of the load it can impose on the network, it is unwise to use ping during normal operations or from automated scripts."
              – 1111161171159459134
              18 hours ago










            • @1111161171159459134 That paragraph could have been worded better. It's overly alarmist. You shouldn't do flood pings or pings with a very high rate, but one ping packet now and then is negligible.
              – Gilles
              16 hours ago












            up vote
            6
            down vote



            accepted







            up vote
            6
            down vote



            accepted






            ping is the way to test whether a host is alive and connected. (If a host is alive but disconnected or slow to respond, you can't distinguish that from its being dead.)



            Options supported by the ping command vary from system to system. You'll want to ensure that it doesn't loop forever but returns after a few seconds if it didn't receive a reply.



            With FreeBSD and Linux iputils, ping -c 1 -W 1 >/dev/null sends a single ping and wait 1 second. You don't need to parse the output: the command returns 0 if it received a ping back and nonzero otherwise (unknown host name, no route to host, no reply). Some implementations may need different flags (e.g. -w instead of -W on FreeBSD), check the manual on your system.



            if ping -c 1 -W 1 "$hostname_or_ip_address"; then
            echo "$hostname_or_ip_address is alive"
            else
            echo "$hostname_or_ip_address is pining for the fjords"
            fi





            share|improve this answer












            ping is the way to test whether a host is alive and connected. (If a host is alive but disconnected or slow to respond, you can't distinguish that from its being dead.)



            Options supported by the ping command vary from system to system. You'll want to ensure that it doesn't loop forever but returns after a few seconds if it didn't receive a reply.



            With FreeBSD and Linux iputils, ping -c 1 -W 1 >/dev/null sends a single ping and wait 1 second. You don't need to parse the output: the command returns 0 if it received a ping back and nonzero otherwise (unknown host name, no route to host, no reply). Some implementations may need different flags (e.g. -w instead of -W on FreeBSD), check the manual on your system.



            if ping -c 1 -W 1 "$hostname_or_ip_address"; then
            echo "$hostname_or_ip_address is alive"
            else
            echo "$hostname_or_ip_address is pining for the fjords"
            fi






            share|improve this answer












            share|improve this answer



            share|improve this answer










            answered Mar 14 '15 at 16:02









            Gilles

            516k12210291557




            516k12210291557











            • I know it's the way, I was just hoping it wasn't the only way, even if the other ways are strange or fickle or what have you. Oh well!
              – user67459
              Mar 18 '15 at 19:25






            • 1




              This answer should NOT have been marked as the answered answer. The OP specifically asked for more ways to see if a given host is up, other than with ping, which this answer does not supply.
              – Yokai
              Jan 21 '17 at 9:21











            • From 'ping' man page: "Because of the load it can impose on the network, it is unwise to use ping during normal operations or from automated scripts."
              – 1111161171159459134
              18 hours ago










            • @1111161171159459134 That paragraph could have been worded better. It's overly alarmist. You shouldn't do flood pings or pings with a very high rate, but one ping packet now and then is negligible.
              – Gilles
              16 hours ago
















            • I know it's the way, I was just hoping it wasn't the only way, even if the other ways are strange or fickle or what have you. Oh well!
              – user67459
              Mar 18 '15 at 19:25






            • 1




              This answer should NOT have been marked as the answered answer. The OP specifically asked for more ways to see if a given host is up, other than with ping, which this answer does not supply.
              – Yokai
              Jan 21 '17 at 9:21











            • From 'ping' man page: "Because of the load it can impose on the network, it is unwise to use ping during normal operations or from automated scripts."
              – 1111161171159459134
              18 hours ago










            • @1111161171159459134 That paragraph could have been worded better. It's overly alarmist. You shouldn't do flood pings or pings with a very high rate, but one ping packet now and then is negligible.
              – Gilles
              16 hours ago















            I know it's the way, I was just hoping it wasn't the only way, even if the other ways are strange or fickle or what have you. Oh well!
            – user67459
            Mar 18 '15 at 19:25




            I know it's the way, I was just hoping it wasn't the only way, even if the other ways are strange or fickle or what have you. Oh well!
            – user67459
            Mar 18 '15 at 19:25




            1




            1




            This answer should NOT have been marked as the answered answer. The OP specifically asked for more ways to see if a given host is up, other than with ping, which this answer does not supply.
            – Yokai
            Jan 21 '17 at 9:21





            This answer should NOT have been marked as the answered answer. The OP specifically asked for more ways to see if a given host is up, other than with ping, which this answer does not supply.
            – Yokai
            Jan 21 '17 at 9:21













            From 'ping' man page: "Because of the load it can impose on the network, it is unwise to use ping during normal operations or from automated scripts."
            – 1111161171159459134
            18 hours ago




            From 'ping' man page: "Because of the load it can impose on the network, it is unwise to use ping during normal operations or from automated scripts."
            – 1111161171159459134
            18 hours ago












            @1111161171159459134 That paragraph could have been worded better. It's overly alarmist. You shouldn't do flood pings or pings with a very high rate, but one ping packet now and then is negligible.
            – Gilles
            16 hours ago




            @1111161171159459134 That paragraph could have been worded better. It's overly alarmist. You shouldn't do flood pings or pings with a very high rate, but one ping packet now and then is negligible.
            – Gilles
            16 hours ago












            up vote
            0
            down vote













            Ping is great to get a quick response about whether the host is connected to the network, but it often won't tell you whether the host is alive or not, or whether it's still operating as expected. This is because ping responses are usually handled by the kernel, so even if every application on the system has crashed (e.g. due to a disk failure or running out of memory), you'll often still get ping responses and may assume the machine is operating normally when the situation is quite the opposite.



            Checking services



            Usually you don't really care whether a host is still online or not, what you really care about is whether the machine is still performing some task. So if you can check the task directly then you'll know the host is both up and that the task is still running.



            For a remote host that runs a web server for example, you can do something like this:



            # Add the -f option to curl if server errors like HTTP 404 should fail too
            if curl -I "http://$TARGET"; then
            echo "$TARGET alive and web site is up"
            else
            echo "$TARGET offline or web server problem"
            fi


            If it runs SSH and you have keys set up for passwordless login, then you have a few more options, for example:



            if ssh "$TARGET" true; then
            echo "$TARGET alive and accessible via SSH"
            else
            echo "$TARGET offline or not accepting SSH logins"
            fi


            This works by SSH'ing into the host and running the true command and then closing the connection. The ssh command will only return success if that command could be run successfully.



            Remote tests via SSH



            You can extend this to check for specific processes, such as ensuring that mysqld is running on the machine:



            if ssh "$TARGET" bash -c 'ps aux | grep -q mysqld'; then
            echo "$TARGET alive and running MySQL"
            else
            echo "$TARGET offline or MySQL crashed"
            fi


            Of course in this case you'd be better off running something like monit on the target to ensure the service is kept running, but it's useful in scripts where you only want to perform some task on machine A as long as machine B is ready for it.



            This could be something like checking that the target machine has a certain filesystem mounted before performing an rsync to it, so that you don't accidentally fill up its main disk if a secondary filesystem didn't mount for some reason. For example this will make sure that /mnt/raid is mounted on the target machine before continuing.



            if ssh "$TARGET" bash -c 'mount | grep -q /mnt/raid'; then
            echo "$TARGET alive and filesystem ready to receive data"
            else
            echo "$TARGET offline or filesystem not mounted"
            fi


            Services with no client



            Sometimes there is no easy way to connect to the service and you just want to see whether it accepts incoming TCP connections, but when you telnet to the target on the port in question it just sits there and doesn't disconnect you, which means doing that in a script would cause it to hang.



            While not quite so clean, you can still do this with the help of the timeout and netcat programs. For example this checks to see whether the machine accepts SMB/CIFS connections on TCP port 445, so you can see whether it is running Windows file sharing even if you don't have a password to log in, or the CIFS client tools aren't installed:



            # Wait 1 second to connect (-w 1) and if the total time (DNS lookups + connect
            # time) reaches 5 seconds, assume the connection was successful and the remote
            # host is waiting for us to send data. Connecting on TCP port 445.
            if echo 'x' | timeout --preserve-status 5 nc -w 1 "$TARGET" 445; then
            echo "$TARGET alive and CIFS service available"
            else
            echo "$TARGET offline or CIFS unavailable"
            fi




            share
























              up vote
              0
              down vote













              Ping is great to get a quick response about whether the host is connected to the network, but it often won't tell you whether the host is alive or not, or whether it's still operating as expected. This is because ping responses are usually handled by the kernel, so even if every application on the system has crashed (e.g. due to a disk failure or running out of memory), you'll often still get ping responses and may assume the machine is operating normally when the situation is quite the opposite.



              Checking services



              Usually you don't really care whether a host is still online or not, what you really care about is whether the machine is still performing some task. So if you can check the task directly then you'll know the host is both up and that the task is still running.



              For a remote host that runs a web server for example, you can do something like this:



              # Add the -f option to curl if server errors like HTTP 404 should fail too
              if curl -I "http://$TARGET"; then
              echo "$TARGET alive and web site is up"
              else
              echo "$TARGET offline or web server problem"
              fi


              If it runs SSH and you have keys set up for passwordless login, then you have a few more options, for example:



              if ssh "$TARGET" true; then
              echo "$TARGET alive and accessible via SSH"
              else
              echo "$TARGET offline or not accepting SSH logins"
              fi


              This works by SSH'ing into the host and running the true command and then closing the connection. The ssh command will only return success if that command could be run successfully.



              Remote tests via SSH



              You can extend this to check for specific processes, such as ensuring that mysqld is running on the machine:



              if ssh "$TARGET" bash -c 'ps aux | grep -q mysqld'; then
              echo "$TARGET alive and running MySQL"
              else
              echo "$TARGET offline or MySQL crashed"
              fi


              Of course in this case you'd be better off running something like monit on the target to ensure the service is kept running, but it's useful in scripts where you only want to perform some task on machine A as long as machine B is ready for it.



              This could be something like checking that the target machine has a certain filesystem mounted before performing an rsync to it, so that you don't accidentally fill up its main disk if a secondary filesystem didn't mount for some reason. For example this will make sure that /mnt/raid is mounted on the target machine before continuing.



              if ssh "$TARGET" bash -c 'mount | grep -q /mnt/raid'; then
              echo "$TARGET alive and filesystem ready to receive data"
              else
              echo "$TARGET offline or filesystem not mounted"
              fi


              Services with no client



              Sometimes there is no easy way to connect to the service and you just want to see whether it accepts incoming TCP connections, but when you telnet to the target on the port in question it just sits there and doesn't disconnect you, which means doing that in a script would cause it to hang.



              While not quite so clean, you can still do this with the help of the timeout and netcat programs. For example this checks to see whether the machine accepts SMB/CIFS connections on TCP port 445, so you can see whether it is running Windows file sharing even if you don't have a password to log in, or the CIFS client tools aren't installed:



              # Wait 1 second to connect (-w 1) and if the total time (DNS lookups + connect
              # time) reaches 5 seconds, assume the connection was successful and the remote
              # host is waiting for us to send data. Connecting on TCP port 445.
              if echo 'x' | timeout --preserve-status 5 nc -w 1 "$TARGET" 445; then
              echo "$TARGET alive and CIFS service available"
              else
              echo "$TARGET offline or CIFS unavailable"
              fi




              share






















                up vote
                0
                down vote










                up vote
                0
                down vote









                Ping is great to get a quick response about whether the host is connected to the network, but it often won't tell you whether the host is alive or not, or whether it's still operating as expected. This is because ping responses are usually handled by the kernel, so even if every application on the system has crashed (e.g. due to a disk failure or running out of memory), you'll often still get ping responses and may assume the machine is operating normally when the situation is quite the opposite.



                Checking services



                Usually you don't really care whether a host is still online or not, what you really care about is whether the machine is still performing some task. So if you can check the task directly then you'll know the host is both up and that the task is still running.



                For a remote host that runs a web server for example, you can do something like this:



                # Add the -f option to curl if server errors like HTTP 404 should fail too
                if curl -I "http://$TARGET"; then
                echo "$TARGET alive and web site is up"
                else
                echo "$TARGET offline or web server problem"
                fi


                If it runs SSH and you have keys set up for passwordless login, then you have a few more options, for example:



                if ssh "$TARGET" true; then
                echo "$TARGET alive and accessible via SSH"
                else
                echo "$TARGET offline or not accepting SSH logins"
                fi


                This works by SSH'ing into the host and running the true command and then closing the connection. The ssh command will only return success if that command could be run successfully.



                Remote tests via SSH



                You can extend this to check for specific processes, such as ensuring that mysqld is running on the machine:



                if ssh "$TARGET" bash -c 'ps aux | grep -q mysqld'; then
                echo "$TARGET alive and running MySQL"
                else
                echo "$TARGET offline or MySQL crashed"
                fi


                Of course in this case you'd be better off running something like monit on the target to ensure the service is kept running, but it's useful in scripts where you only want to perform some task on machine A as long as machine B is ready for it.



                This could be something like checking that the target machine has a certain filesystem mounted before performing an rsync to it, so that you don't accidentally fill up its main disk if a secondary filesystem didn't mount for some reason. For example this will make sure that /mnt/raid is mounted on the target machine before continuing.



                if ssh "$TARGET" bash -c 'mount | grep -q /mnt/raid'; then
                echo "$TARGET alive and filesystem ready to receive data"
                else
                echo "$TARGET offline or filesystem not mounted"
                fi


                Services with no client



                Sometimes there is no easy way to connect to the service and you just want to see whether it accepts incoming TCP connections, but when you telnet to the target on the port in question it just sits there and doesn't disconnect you, which means doing that in a script would cause it to hang.



                While not quite so clean, you can still do this with the help of the timeout and netcat programs. For example this checks to see whether the machine accepts SMB/CIFS connections on TCP port 445, so you can see whether it is running Windows file sharing even if you don't have a password to log in, or the CIFS client tools aren't installed:



                # Wait 1 second to connect (-w 1) and if the total time (DNS lookups + connect
                # time) reaches 5 seconds, assume the connection was successful and the remote
                # host is waiting for us to send data. Connecting on TCP port 445.
                if echo 'x' | timeout --preserve-status 5 nc -w 1 "$TARGET" 445; then
                echo "$TARGET alive and CIFS service available"
                else
                echo "$TARGET offline or CIFS unavailable"
                fi




                share












                Ping is great to get a quick response about whether the host is connected to the network, but it often won't tell you whether the host is alive or not, or whether it's still operating as expected. This is because ping responses are usually handled by the kernel, so even if every application on the system has crashed (e.g. due to a disk failure or running out of memory), you'll often still get ping responses and may assume the machine is operating normally when the situation is quite the opposite.



                Checking services



                Usually you don't really care whether a host is still online or not, what you really care about is whether the machine is still performing some task. So if you can check the task directly then you'll know the host is both up and that the task is still running.



                For a remote host that runs a web server for example, you can do something like this:



                # Add the -f option to curl if server errors like HTTP 404 should fail too
                if curl -I "http://$TARGET"; then
                echo "$TARGET alive and web site is up"
                else
                echo "$TARGET offline or web server problem"
                fi


                If it runs SSH and you have keys set up for passwordless login, then you have a few more options, for example:



                if ssh "$TARGET" true; then
                echo "$TARGET alive and accessible via SSH"
                else
                echo "$TARGET offline or not accepting SSH logins"
                fi


                This works by SSH'ing into the host and running the true command and then closing the connection. The ssh command will only return success if that command could be run successfully.



                Remote tests via SSH



                You can extend this to check for specific processes, such as ensuring that mysqld is running on the machine:



                if ssh "$TARGET" bash -c 'ps aux | grep -q mysqld'; then
                echo "$TARGET alive and running MySQL"
                else
                echo "$TARGET offline or MySQL crashed"
                fi


                Of course in this case you'd be better off running something like monit on the target to ensure the service is kept running, but it's useful in scripts where you only want to perform some task on machine A as long as machine B is ready for it.



                This could be something like checking that the target machine has a certain filesystem mounted before performing an rsync to it, so that you don't accidentally fill up its main disk if a secondary filesystem didn't mount for some reason. For example this will make sure that /mnt/raid is mounted on the target machine before continuing.



                if ssh "$TARGET" bash -c 'mount | grep -q /mnt/raid'; then
                echo "$TARGET alive and filesystem ready to receive data"
                else
                echo "$TARGET offline or filesystem not mounted"
                fi


                Services with no client



                Sometimes there is no easy way to connect to the service and you just want to see whether it accepts incoming TCP connections, but when you telnet to the target on the port in question it just sits there and doesn't disconnect you, which means doing that in a script would cause it to hang.



                While not quite so clean, you can still do this with the help of the timeout and netcat programs. For example this checks to see whether the machine accepts SMB/CIFS connections on TCP port 445, so you can see whether it is running Windows file sharing even if you don't have a password to log in, or the CIFS client tools aren't installed:



                # Wait 1 second to connect (-w 1) and if the total time (DNS lookups + connect
                # time) reaches 5 seconds, assume the connection was successful and the remote
                # host is waiting for us to send data. Connecting on TCP port 445.
                if echo 'x' | timeout --preserve-status 5 nc -w 1 "$TARGET" 445; then
                echo "$TARGET alive and CIFS service available"
                else
                echo "$TARGET offline or CIFS unavailable"
                fi





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                answered 3 mins ago









                Malvineous

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