Linux – Wait until a specific pattern is written on log using tail and sed

In the Ant build.xml, i need to make sure the JBoss server is started before running the test cases. There are 2 possible approaches.

• Using twiddle.sh to check the status
• Monitoring server.log until “Started in” is written on it

I took the 2nd approach. Here is an example output log when the JBoss server is started successfully.

• 2013-10-07 10:06:33,657 INFO [org.jboss.bootstrap.microcontainer.ServerImpl] (main) JBoss (Microcontainer) [5.1.0 (build: SVNTag=JBPAPP_5_1_0 date=201009150028)] Started in 39s:984ms

 

In the build.xml, the sshexec task is used and keep checking the remote server log.

<sshexec
  trust="true"
  host="<host>"
  username="<ssh user>" keyfile="<key file>"
  command="<command to be run on the remote server>" />

I got this working at first

<!-- command: tail -n 0 -f <server.log> | sed --quiet '/Started in/ q' -->
<sshexec
  trust="true"
  host="<host>"
  username="<ssh user>" keyfile="<key file>"
  command="tail -n 0 -f <server.log> | sed --quiet '/Started in/ q'" />

But it only quits the tail command until a new line is written to the log.

Then i come up with this solution. Thanks StackOverflow!

<!-- command: sed --quiet '/Started in/ q' <(tail -n 0 -f <server.log>) -->
<sshexec
  trust="true"
  host="<host>"
  username="<ssh user>" keyfile="<key file>"
  command="sed --quiet '/Started in/ q' &lt;(tail -n 0 -f <server.log>)" />

Unfortunately, it doesn’t work in my case. I have to wait for a few JBoss server to complete the startup in Ant script but it turns out the above approach only works for the first JBoss server. For the others, it only exits the tail command when a new line is written to the server.log.

Finally i got another solution which will write the log file once the string is found.

<!-- command: sed --quiet '/Started in/ q' <(tail -n 0 -f <server.log>) -->
<sshexec
  trust="true"
  host="<host>"
  username="<ssh user>" keyfile="<key file>"
  command="tail -f <server.log> | grep -m 1 'Started in' | xargs echo '' >> <server.log>" />

There are other suggested solutions. You can refer to the references below.

Done =)

Reference:

• bash – Monitoring a file until a string is found
• StackOverflow – Do a tail -F until matching a pattern
• `tail -f` until text is seen
• StackOverflow – Linux: Block until a string is matched in a file (“tail + grep with blocking”)