Check for process and kill if running

Question

I am trying to find if a process is running or not, and if it is runing then I want to call a script or print something. I tried with the below few ways, but it gives me syntax error or command not found error.

APP_ID = ps -eaf | grep -i `whoami` | grep -i <process_name> | grep -i java | awk '{print$2}'

if[''!= '${APP_ID}'] then
    echo "Stopping instance $APP_ID"

I get the result as below:

test.sh: line 15: APP_ID: command not found
test.sh: line 17: if[!= ${APP_ID}]: command not found
Stopping instance 

What is wrong in the above script? And if it has some errors, then why does it print the echo in the if condition?

Also the below script fails saying test.sh: line 3: [ps: command not found

if [ps -eaf | grep -i `whoami` | grep -i <process_name> | grep -i java]; then
    echo 'stop'
fi

Answer 1

Your syntax has many problems:

• remove spaces around "=" when setting a variable

  wrong:

  APP_ID = value
  

  right:

  APP_ID=value
  

• to run a program and put its output into a variable, you need $(...) (preferred in bash, not available in sh) or backticks `command` (supported in bash and sh). (and remember that when assigning a variable, quotes are not needed, but in other cases they are important: command "$(command)")

  wrong:

  APP_ID=command
  

  right:

  APP_ID=$(command)
  

• add spaces around everything when using "[", and you need a semicolon or newline after the "]". This is because "[" is a bash builtin command, and the non-builtin one is also known as "test" (see man test), which just like other commands, takes its arguments separated by space:

  wrong:

  if[x!= y] then echo hello world fi
  

  right:

  if [ x != y ]; then echo hello world; fi
  

• use double quotes, not single quotes when you want to expand a variable

  wrong:

  if [ '' != '${APP_ID}' ]; then
  

  right:

  if [ '' != "${APP_ID}" ]; then
  

• Also for the above example, you can use -n (non-empty) instead of comparing to empty string

  if [ -n "${APP_ID}" ]; then
  

• And for the ps example, you don't need grep and awk:

  APP_ID=$(ps -C whoami --no-header --format 'pid')
  

And so here is the fixed up script:

APP_ID=$(ps -C whoami --no-header --format 'pid')

if [ -n "${APP_ID}" ]; then
    echo "Stopping instance $APP_ID"
fi

Answer 2

This is easiest done with pgrep and pkill:

if pgrep process_name 2>/dev/null; then
  echo "Terminating process_name"
  pkill process_name
fi

As user @OrangeDog points out in the comments, there is the theoretical possibility of this shell code failing to terminate the process process_name if the process spawns inbetween the call to pgrep and pkill. In this case, pgrep would not detect the process and pkill would never execute. How much of a problem this is, I don't know. If the process spawns just after the statement, what can you do?

Just do

echo "terminating process_name (if it is running)"
pkill process_name

to be rid of the pesky race condition in any case.

Answer 3

I am not sure why but the above script was not giving any syntax issues but was not giving the process id also though the process was running.

I finally changed it as below and it works -

APP_ID=$(ps -ef | grep <app_name> | grep java | awk '{print $2}')

echo "Instance $APP_ID"
if [ -n "${APP_ID}" ]; then
    echo "Stopping instance $APP_ID"
fi

Thanks once again for the explanation and pointers.

@Peter I will check again the script you posted on why it is not working for me.

Answer 4

The command above will also catch the grep it self, so the command, in case of java process, should be

APP_ID=$(ps -ef | grep <app_name> | grep -v grep | awk '{print $2}')

echo "Instance $APP_ID"
if [ -n "${APP_ID}" ]; then
    echo "Stopping instance $APP_ID"
    kill $APP_ID   (or kill -9 $APP_ID)
fi

hope this helps.