Monitor Linux Processes with a Simple Shell Script

In daily operations, ensuring that critical services stay alive is essential; when a process exits, it often signals a problem that must be addressed immediately. This article provides a practical shell script to continuously check whether a specific process is running.

The core function GetPID accepts two arguments—the username and the process name—and returns the PID of the matching process. It uses ps -u $PsUser to list processes for the user, pipes the output through several grep filters to exclude irrelevant entries (such as the grep command itself, editors, or auxiliary scripts), then extracts the first PID with sed -n 1p and awk '{print $1}'.

function GetPID #User #Name {
  PsUser=$1
  PsName=$2
  pid=`ps -u $PsUser | grep $PsName | grep -v grep | grep -v vi | grep -v dbx \
       | grep -v tail | grep -v start | grep -v stop | sed -n 1p | awk '{print $1}'`
  echo $pid
}

Example usage:

PID=`GetPID root TestApp`
echo $PID

The command prints the PID (e.g., 11426) of the TestApp process running under the root user. If the process is not found, the function returns an empty string, which can be checked as follows:

if [ "-$PID" == "-" ]; then
  echo "The process does not exist."
fi

Command breakdown:

ps : displays a snapshot of current processes. Option -u limits output to a specific user.

grep : filters lines containing the target process name; -v inverts the match to exclude unwanted lines.

sed : a stream editor; -n 1p prints only the first matching line.

awk : processes text columns; {print $1} extracts the first field, which is the PID.

By integrating this function into monitoring scripts or cron jobs, administrators can automatically detect when a service stops and trigger alerts or restart procedures.