How to do an if statement from the result of an executed command

Question

I am trying to do an IF statement from the output of an executed commmand. Here is how I am trying to do it, but it doesn't work. Does anyone know the right way to do this?

if [ "`netstat -lnp | grep ':8080'`" == *java* ]; then
  echo "Found a Tomcat!"
fi

Answer 1

Use the bash [[ conditional construct and prefer the $(<command>) command substitution convention. Additionally, [[ prevents word splitting of variable values therefore there is no need to quote the command substitution bit..

if [[ $(netstat -lnp | grep ':8080') = *java* ]]; then
  echo "Found a Tomcat!"
fi

Answer 2

Another alternative is to simply test the exit status of grep itself, which will return false (1) if there was no match and true (0) if there was one, by not using the [ command.

if netstat -lntp | grep ':8080.*java' > /dev/null; then
    echo "Found a Tomcat!"
fi

The redirection to /dev/null is to prevent it from also printing the found line to the screen.

Answer 3

Even more simple,

netstat -lntp | grep ':8080.*java' > /dev/null && command

If you just want to do one thing.

Answer 4

You can do more precise, yet simple, matching with awk.

if netstat -lnp | awk '$4 ~ /:8080$/ && $7 ~ /java/ {exit(0)} END {exit(1)}'; then …

To match the structure of your command more closely, the portable way of doing wildcard matching on a string in a shell is with the case construct.

case "$(netstat -lnp | grep ':8080')" in
  *java*)  echo "Found a Tomcat!";;
esac

Answer 5

Another real-world example :)

if [[ $(HEAD mycompany-intranet.com | grep '200\ OK' | wc -l) = "1" ]];  then
    echo doing some intranet settings (proxy, etc)
else
    echo doing some work-at-home settings (proxy, etc)
fi

Answer 6

Here's how I did it in a script that determines whether my music player is open or not, and if a song is playing if it is open.

rbstatus=$(rhythmbox-client --print-playing)

pgrep -x rhythmbox >/dev/null && if [ "$rbstatus" = "-" ]; then
   echo "No music playing"
else
   rhythmbox-client --print-playing
fi || echo "Music player is Closed."