What is pid in Linux?

In the Linux operating system, PID stands for "Process IDentifier." It is a unique numerical identifier assigned to each running process on the system. To answer the question what is pid in Linux in simple words, whenever you start a new process, the Linux kernel assigns a PID to it, allowing the system to keep track of the process and manage it effectively. Understanding what is pid in Linux is crucial for monitoring, troubleshooting, and managing processes efficiently in a Linux environment.

How PIDs are Assigned?

When a process is created, the Linux kernel assigns a unique PID to it from the pool of available IDs. The PID is an integer value and is automatically incremented for each new process. The PID 1 is reserved for the init process, which is the first process started during system boot and is the ancestor of all other processes.

The PID assignment is not random; it follows a sequential order. When the PID reaches its maximum limit, it wraps back to lower values and starts reassigning PIDs to new processes from there. Therefore, PIDs can be reused, but only after a process with that specific PID has terminated.

PID Usage in Process Management

Now that we have understood what is pid in Linux, let us look at the crucial role of PIDs in various aspects of process management in Linux. Some of the common uses of PIDs are:

• Process Identification:

  PIDs uniquely identify each process, making it easy for users and the system to reference a specific process.

• Killing Processes:

  You can terminate a process using the kill command, which requires the PID of the target process. For example, to terminate a process with PID 1234, you can use the command: kill 1234.

• Monitoring Processes:

  System administrators and users can monitor the behavior of processes and resource usage by querying their PIDs through various tools like ps, top, and htop.

• Parent-Child Relationship:

  PIDs also establish parent-child relationships among processes. When a new process is created, it inherits the PID of its parent process, becoming a child of that process. This forms a hierarchical structure in process management.

• Signals:

  Processes can communicate with each other using signals. Signals are numeric values sent by one process to another to request specific actions, such as termination, interruption, or custom actions. PIDs are used to specify the target process for sending signals.

• Process Priority:

  The Linux scheduler uses PIDs to manage the priority and order in which processes are executed on the CPU.

Creating a .pid File

In addition to the PID managed by the Linux kernel, many applications create a special file called the ".pid file" to store their own PID. The .pid file is typically created in the /var/run or /run directory and contains the PID of the associated running process.

The purpose of the .pid file is to allow other applications or scripts to identify if the specific process is running, and if so, what its PID is. This is useful, for example, when you want to avoid starting multiple instances of the same application or when you need to stop a daemon gracefully by sending a signal to the correct process.

Location of the .pid File

The location of the .pid file can vary depending on the application and its configuration. However, there are some common locations where you can find these files:

1. /var/run:

   Historically, this directory was used to store PID files. However, in modern Linux distributions, it is more common to use the /run directory, which is a tmpfs filesystem and is cleared on every boot.

2. /run:

   As mentioned earlier, this directory is commonly used to store PID files in modern Linux distributions. The advantage of using /run is that it is a tmpfs filesystem, which means it resides in memory, avoiding unnecessary disk writes and reducing wear on storage devices.

3. /var/run/<app_name>:

   Some applications create a subdirectory in /var/run to store their PID file. The <app_name> will be replaced by the actual name of the application.

4. /var/run/<app_name>.pid or /run/<app_name>.pid:

   Another common convention is to name the PID file directly after the application, with a ".pid" extension, and place it in either /var/run or /run directory.

When dealing with applications that use PID files, it is essential to check the application's documentation or configuration files to determine the exact location of its .pid file.

How to Find the PID of a Process?

In Linux, there are various ways to find the PID of a specific process. This can be useful when you want to monitor a process or send signals to it. Here are some common methods to find the PID:

1. Using the ps Command:

   The ps command is used to provide information about the running processes. By default, it shows the processes running in the current terminal session. To find the PID of a specific process, you can use the following command:

   Replace <process_name> with the name of the process you want to find. The grep command helps to filter out the specific process from the list.

2. Using the pgrep Command:

   The pgrep command is a handy utility that directly finds the PID of a process based on its name. Simply use the command as follows:

   It will display the PID of the process with the provided name.

3. Using the .pid File:

   As mentioned earlier, many applications create a .pid file to store their PID. You can check the content of the .pid file to find the PID of the associated process. For example:

   Replace <app_name> with the name of the application you want to check.

4. Using the top or htop command:

   Both top and htop are interactive system monitoring tools. When you run top or htop, you'll see a list of running processes along with their PIDs, CPU usage, memory usage, and more.

Tracking Processes with PIDs: Practical Example

In Linux, PIDs are used to track and manage processes. Let's consider a web server running on your Linux machine. The web server process is identified by its PID, obtained using the ps command or checking the .pid file.

If the web server becomes unresponsive, you can diagnose the issue using PIDs. Find the PID, monitor resource usage with tools like top or htop, and analyze log files for errors. To resolve the problem, gracefully terminate the process with the SIGTERM signal. If needed, use SIGKILL for forceful termination.

After resolving the issue, restart the web server using its PID from the .pid file or its start-up script.