CODEWITHSUNDEEP
bashif-statementgrep
Danny Watson
Danny Watson

Reputation: 165

Bash is redirecting output from command only after script has finished

Context

Got a daft script that checks a process is running on a group of hosts, like a watchdog, as I say it's a daft script so bear in mind it isn't 'perfect' by scripting standards

Problem

I've ran bash -x and can see that the script finishes its first check without actually redirecting the output of the command to the file which is very frustrating, it means each host is actually being evaluated to the last hosts output

Code

#!/bin/bash
FILE='OUTPUT'
for host in $(cat /etc/hosts | grep webserver.[2][1-2][0-2][0-9] | awk {' print $2 ' })

do ssh -n -f $host -i <sshkey> 'ps ax | grep myprocess | wc -l' > $FILE 2> /dev/null
cat $FILE
if grep '1' $FILE ; then
 echo "Process is NOT running on $host"
 cat $FILE
else
 cat $FILE
 echo "ALL OK on $host"
fi
cat $FILE
done

Script traceback

++ cat /etc/hosts
++ awk '{ print $2 }'
++ grep 'webserver.[2][1-2][0-2][0-9]'
+ for host in '$(cat /etc/hosts | grep webserver.[2][1-2][0-2][0-9] | awk {'\'' print $2 '\''})'
+ ssh -n -f webserver.2100 -i <omitted> 'ps ax | grep myprocess | wc -l'
+ cat OUTPUT
+ grep 1 OUTPUT
+ cat OUTPUT
+ echo 'ALL OK on webserver.2100'
ALL OK on webserver.2100
+ cat OUTPUT
+ printf 'webserver.2100 checked \n'
webserver.2100 checked 
+ for host in '$(cat /etc/hosts | grep webserver.[2][1-2][0-2][0-9] | awk {'\'' print $2 '\''})'
+ ssh -n -f webserver.2101 -i <omitted> 'ps ax | grep myprocess | wc -l'
+ cat OUTPUT
2
+ grep 1 OUTPUT
+ cat OUTPUT
2
+ echo 'ALL OK on webserver.2101'
ALL OK on webserver.2101
+ cat OUTPUT
2
+ printf 'webserver.2101 checked \n'
webserver.2101 checked

Issue

As you can see, it's registering nothing for the first host, then after it is done, it's piping the data into the file, then the second host is being evaluated for the previous hosts data...

I suspect its to do with redirection, but in my eyes this should work, it doesn't so it's frustrating.

Upvotes: 1

Views: 362

Answers (1)

glenn jackman
glenn jackman

Reputation: 246754

I think you're assuming that ps ax | grep myprocess will always return at least one line (the grep process). I'm not sure that's true. I'd rewrite that like this:

awk '/webserver.[2][1-2][0-2][0-9]/ {print $2}' /etc/hosts | while IFS= read -r host; do
    output=$( ssh -n -f "$host" -i "$sshkey" 'ps ax | grep "[m]yprocess"' )
    if [[ -z "$output" ]]; then
        echo "Process is NOT running on $host"
    else
        echo "ALL OK on $host"
    fi
done

This trick ps ax | grep "[m]yprocess" effectively removes the grep process from the ps output:

  • the string "myprocess" matches the regular expression "[m]yprocess" (that's the running "myprocess" process), but
  • the string "[m]yprocess" does not match the regular expression "[m]yprocess" (that's the running "grep" process)

Upvotes: 2

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