Bash : Check if PID exists with further logic

Question

Question: How do you check if a PID exists and use the result within an IF statement in bash?

Things I've tried

if [ "$(ps -p $pid)" -eq 0 ]; then 
 echo "Running"
else
 echo "Not Running"
fi

---

if [ "$(kill -0 $pid)" -eq 0 ]; then 
 echo "Running"
else
 echo "Not Running"
fi

Neither of these evaluate correctly no matter how I redirect STDOUT/STDER

Answer 1

How do you check if a PID exists and use the result within an if statement?

You can capture the output in a variable and then check the exit status:

output=$(ps -p "$pid")
if [ "$?" -eq 0 ]; then
  echo "Found"
  echo "$output"
fi

Just remember that $? is getting reset every time you run a command, so something like the following wont work:

output=$(ps -p "$pid")
echo "$output"
# Now $? will be refering to the exit status of echo
if [ "$?" -eq 0 ]; then
  echo "Found"
fi

One can also stick everything together in the if statement:

if output=$(ps -p "$pid")
then
  echo "Found: $output"
fi