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

EDIT: I wonder if there is a way to do it by capturing exit code?

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 (non-zero) if there was no match and true (zero) if there was one, by not using the [ command.

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

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

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."

Answer 6

A simple and logic way:

netcat -zw3 localhost 8000 && echo 'tomcat is up'