how to check if stream is empty?

Question

For example I want to execute :

ps x | grep something | grep -v grep | somecommand

I want to execute somecommand only if there is something running. How to do that ?

Answer 1

You can use if. Also, to grep for a command, but to exclude the grep itself, you can use the following trick:

if ps x | grep '[s]omething' ; then
    echo Running.
else
    echo Not running.
fi

Answer 2

This is simple:

pgrep something && somecommand

pgrep is like ps | grep something | grep -v grep, terser but with less options to control output. However, you don't have to worry that you filter out by chance something that actually is needed (e.g. pgrep agreppina will work, but ps x | grep agreppina | grep -v grep will not).

command1 && command2 is somewhat like any conjunction in a programming language, command2 will be executed only if first command was successful (actually, if it returned a status of zero).

Answer 3

Here is the solution

ps x | grep something | [[-n $(grep -v grep) ]] && somecommand

• -n will check whether string is null or not
• && is and operator